THE INFINITE VARIETY: THE BEGINNING OF LIFE
Foreword
This material was originally prepared for a first year tutorial system at the Zoology
Department and is built around the David Attenborough film series “Life on Earth”. At
the time of development there was not intention to develop it as eLearning content. With
the introduction of the Ecological Informatics courses at the University of the Western
Cape, it became necessary to provide a basic review of how Biodiversity evolved? In
updating extensive use of hypertext is used so you can dip and out at various points of the
material and get additional information. This material should be used in conjunction with
viewing of the material “Life on Earth” by David Attenborough, but we also encourage
you to follow the hyperlinks and examine the classifications provided.
For both updating and providing supplementary information I have used the public
domain Wikipedia Encyclopaedia, and unless otherwise stated all images and
nomenclature/classifications were sourced from Wikipedia and Wikispecies.
This resource was developed using standard html and ccs and should work under all
platforms and browser configurations, but extensive use of pop-up is made which means
that you must enable pop-ups and your browser is java-enabled.
For registered UWC students assessment criteria will be provided separately through
continuous assessment (using electronic quizzes), discussion forum and a course-project.
Good Luck with your Biodiversity studies.
Richard Knight
Coordinator: National Information Society Learnership- Ecological Informatics
c/o Department of Biodiversity and Conservation Biology
University of the Western Cape
Private Bag X17
Bellville 7535
South Africa
Phone: 27 + 21 + 959 3940
Email: rknight@uwc.ac.za
Darwin and the Giant Tortoises
The world is rich in animals and plants, some of which still remain to be discovered. A
small area of the Tropical Forests of South America will still yield insects that have never
been described, the difficulty is finding a specialist whose is able to classify them. The
understanding of such biodiversity would have been almost impossible, if it had not been
for Charles Darwin and his trip around the world. For example Darwin described the
adaptations of the Giant Tortoises (Geochelone nigra) that occur on the Galapagos
Islands in the South Pacific.
Tortoises occurring on the well-watered islands, with short, cropped vegetation had
gently curved front edges to their shell.
An example of dome-shell Galapagos Tortoise that occurs on the well-watered
parts of the islands.
Tortoises occurring on more arid islands had to stretch their necks to reach branches of
cactus and other vegetation. Consequently, these later individuals had longer necks and a
high peak to the front edges of their shells, which enabled them to stretch their heads
almost vertically.
A “saddle-back” Galapagos Tortoise that inhabits drier areas of the islands and
has a longer neck and a high peak to the front edge of its shell, this enables it to
stretch it neck further out and obtain food higher up off the ground.
Observations such as these were the foundations for the theory of evolution, which
suggests that species were not fixed for ever but changed with time and thereby
contribute to the immense diversity of life.
Geochelone nigra
Galápagos Tortoise
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Testudines
Family: Testudinidae
Genus: Geochelone
Binomial name Geochelone nigra (Quoy & Gaimard, 1824)
<size>scrolling</size>
Darwin's argument for the evolution of different necks in these tortoises was as follows:all individuals of the same species are not identical. In a single clutch of eggs there will
be some hatchlings, which, because of their genetic constitution, will develop longer
necks than others. In times of drought such individuals will be able to reach leaves
higher off the ground than their siblings and therefore will survive. The brothers and
sisters in the clutch who possessed shorter necks would be unable to stretch and reach
food and therefore would starve to death. Since this time natural selection has been
debated and tested, refined, quantified and elaborated. Later discoveries about genetics,
molecular biology, population dynamics and animal behaviour have developed the theory
of natural selection still further. It remains the key to our understanding of the natural
world and it enables us to recognize that life has a long and continuous history during
which organisms, both plants and animals, have changed, generation by generation, as
they colonized all parts of the world.
Evidence of Evolution in the Rocks
Occasionally some animals after dying may be covered in mud, where their bones can be
preserved. Dead plant material may also accumulate and is turned to peat, in time peat is
compressed and turned to coal. Great pressure from overlying sediments and mineralrich solutions that circulate through them cause chemical changes in the calcium
phosphate of the bones. Eventually they are turned to stone giving an accurate
representation of the original bones. This process is called fossilization.
A fossil Ammonite
The most suitable sites for fossilization are in seas and lakes were sedimentary deposits
like sandstone and limestone are slowly accumulated. Fossils are exposed when such
deposits erode away. Fossils can often be dated with the discovery of radioactivity in the
surrounding rocks. Some chemicals in rocks decay with time producing radioactivity, for
example potassium turns to argon, uranium to lead and rubidium to strontium. The
amount of change from one chemical to the other depends on the amount of elapsed time.
Consequently the proportion of the second element to the first can be used to calculate
the time when the rocks were first laid down around the fossil.
Layers of Rocks give us clues to their age
When rocks occur as undisturbed layers, we find that the lowest layers will be the oldest
and topmost layers will be the youngest. Frequently rivers cut incisions into the earths's
surface and expose such layers. The Grand Canyon in the U.S.A. is the deepest cleft on
the earth's surface.
The Grand Canyon, Western United States of America
The upper rocks of this canyon are about 200 million years old and contain traces of
reptiles, impressions of fern leaves and wings of insects. Halfway down the canyon you
find limestone of about 400 million years old which contains the remains of primitive
armoured fish. Further down the canyon there are no traces of vertebrates. Threequarters way down there are no apparent traces of life. Close to the bottom of the canyon
the rocks are more than 2 000 million years old.
MEDIA:
A model of Dunkleosteus telleri a highly evolved Placoderm which were
armoured and jawed fish. Instead of actual teeth, Dunkleosteus possessed two
long, bony blades that could slice through flesh and snap and crush bones and
almost anything else. It was a vicious hunter, and probably ate whatever it could
find, including sharks.
Dunkleosteus
Dunkleosteus
Conservation status: Fossil
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Class: Placodermi
Order: Arthrodira
Family: Dinichthyidae
Genus: Dunkleosteus
Species: D. telleri
<size>scrolling</size>
Rocks as old as those of the bottom of the Grand Canyon have been found to contain a
fine-grain flint-like substance called chert. Contained in this chert are simple organisms
some of which resemble algae filaments others resemble bacteria.
Chert similar to that found at the bottom of the Grand Canyon
These were thought to be until recently the earliest known organism (see further down
and for a time-line of life go here) and are referred to as cyanobacteria or blue-greens.
These organisms are able to extract hydrogen from water and thereby produce oxygen
which is essential for other organisms to survive. The chemical agent responsible for this
process is called chlorophyll and process is called photosynthesis, and occurs in true
algae and higher plants.
The Anabaena is a genus of filamentous-cyanobacteria, or blue-green algae,
found as plankton. It is known for its nitrogen fixing abilities, and they form
Anabaena
Anabaena
Anabaena sphaerica (Nostocales)
Scientific classification
Kingdom: Bacteria
Division: Cyanobacteria
Class: Cyanophyceae
Order: Nostocales
Family: Nostocaceae
Genus: Anabaena
<size>scrolling</size>
Pre-Cambrian-stromatolites in the Siyeh Formation, Glacier National Park. In
2002, William Schopf of UCLA published a controversial paper in the scientific
journal:Nature arguing that geological formations such as this possess 3.5 billion
year old fossilized-algae microbes.
Recent News: Original article
Minik & Frei (2004) wrote a paper that concluded that “Planktonic organisms
lived in the Isuan oceans where they produced large amounts of highly 13Cdepleted organic matter. The aquatic environment of these organisms comprised
relatively oxidized compartments, which allowed solute transport of U. The high
biomass productivity of planktonic organisms, the strongly 13C-depleted carbon
isotopic signature and the evidence for the presence of oxidized aquatic
environments all suggest that oxygenic photosynthesis had developed before
3700 Ma.”
How life started?
How did life begin? Even before these blue-greens existed organic molecules must have
evolved. The original atmosphere (see separate page) of the earth was very thin and
contained hydrogen, carbon-monoxide, ammonia and methane, but no oxygen. This
chemical mixture, together with ultra-violet radiation and frequent electrical discharges
causing lightening was simulated in the Miller Urey experiment in the 1950s.
This experiment used water (H2O), methane (CH4), ammonia (NH3) and hydrogen (H2).
The chemicals were all sealed inside a sterile array of glass tubes and flasks connected
together in a loop, with one flask half-full of liquid water and another flask containing a
pair of electrodes. The liquid water was heated to induce evaporation, sparks were fired
through the atmosphere and water vapor to simulate lightning, and then the atmosphere
was cooled again so that the water could condense and trickle back into the first flask in a
continuous cycle.
At the end of one week of continuous operation, Miller and Urey observed that as much
as 10-15% of the carbon within the system was now in the form of organic compounds.
Two percent of the carbon had formed amino acids, including 13 of the 21 that are used
to make proteins in living cells, with glycine as the most abundant.
Miller-Urey
Miller-Urey
The original Miller-Urey experiment that recreate the chemical conditions of the
primitive Earth in the laboratory, and synthesized some of the building blocks of
life.
<size>scrolling</size>
A diagramatic representation of the Miller-Urey experiment which attempted to
synthesized some of the building blocks of life based on our understanding of the
earths first environmental conditions.
Archives: Original article
Miller S. L., (1953). Production of Amino Acids Under Possible Primitive Earth
Conditions, Science, 117: 528.
Interpretation of the Miller-Urey Experiment
The molecules produced form this experiment were relatively simple organic molecules,
far from a complete living biochemical system, but the experiment established that
natural processes could produce the building blocks of life without requiring life to
synthesize them in the first place. With time these substances probably increased and
interacted with each other to form more complex molecules. Eventually one substance
essential to life as we know it appeared. This substance was called deoxyribonucleic acid
or DNA.
This experiment inspired many experiments in a similar vein. In 1961, Joan Oro found
that amino acids could be made from hydrogen cyanide (HCN) and ammonia in a water
solution. He also found that his experiment produced a large amount of the nucleotide
base adenine. Experiments conducted later showed that the other RNA and DNA bases
could be obtained through simulated prebiotic chemistry with a reducing atmosphere.
Conditions similar to those of the Miller-Urey experiments are present in other
regions of the solar system, often substituting ultraviolet light for lightning as the
driving force for chemical reactions. On September 28, 1969, a meteorite that fell
over Murchison, Victoria, Australia was found to contain over 90 different amino
acids, nineteen of which are found in Earth life. Comets and other icy outer-solarsystem bodies are thought to contain large amounts of complex carbon
compounds (such as tholins) formed by these processes, in some cases so
much so that the surfaces of these bodies are turned dark red or as black as
asphalt. The early Earth was bombarded heavily by comets, possibly providing a
large supply of complex organic molecules along with the water and other
volatiles they contributed. (This could also imply an origin of life outside of Earth,
which then migrated here. See: Panspermia)
How valid was the Miller Urey Experiment?
There have been a number of objections to the implications derived from these
experiments. The following are extracts from Wikipedia:
Originally it was thought that the primitive secondary atmosphere
contained mostly NH3 and CH4. However, it is likely that most of the
atmospheric carbon was CO2 with perhaps some CO and the nitrogen
mostly N2. The reasons for this are (a) volcanic gas has more CO2,
CO and N2 than CH4 and NH3 and (b) UV radiation destroys NH3 and
CH4 so that these molecules would have been short-lived. UV light
photolyses H2O to H· and ·OH radicals. These then attack methane,
giving eventually CO2 and releasing H2 which would be lost into space.
In practice gas mixtures containing CO, CO2, N2, etc. give much the
same products as those containing CH4 and NH3 so long as there is
no O2. The H atoms come mostly from water vapor. In fact, in order to
generate aromatic amino acids under primitive earth conditions it is
necessary to use less hydrogen-rich gaseous mixtures. Most of the
natural amino acids, hydroxyacids, purines, pyrimidines, and sugars
have been produced in variants of the Miller experiment.
Off the Scientific Press
More recent results may have called this into question, however. Simulations
done at the University of Waterloo and University of Colorado in 2005 indicated
that the early atmosphere of Earth could have contained up to 40% hydrogen,
implying a much more hospitable environment for the formation of prebiotic
organic molecules. The escape of hydrogen from Earth's atmosphere into space
may have occurred at only 1% of the rate previously believed based on revised
estimates of the upper atmosphere's temperature. One of the authors, Prof.
Owen Toon notes: "In this new scenario, organics can be produced efficiently in
the early atmosphere, leading us back to the organic-rich soup-in-the-ocean
concept... I think this study makes the experiments by Miller and others relevant
again." Outgassing calculations using a chondritic model for the early earth,
(Washington University, September 2005) complement the Waterloo/Colorado
results in re-establishing the importance of the Miller-Urey experiment.
Other views
Although lightning storms are thought to have been very common in the primordial
atmosphere, they are not thought to have been as common as the amount of electricity
used by the Miller-Urey experiment may imply. These factors suggest that much lower
concentrations of biochemicals would have been produced on Earth than was originally
predicted (although the time scale would be 100 million years instead of a week). Similar
experiments, both with different sources of energy and with different mixtures of gases,
have resulted in amino and hydroxy acids being produced; it is likely that at least some
organic compounds would have been generated on the early Earth.
However, as soon as oxygen gas is added to the mixture, no organic molecules are
formed. Recent research has been seized upon by opponents of Urey-Miller hypothesis
which shows the presence of uranium in sediments dated to 3.7 Ga and indicates it was
transported in solution by oxygenated water (otherwise it would have precipitated out)
(Rosing & Frei 2004). It is wrongly argued by some, in an attempt to invalidate the
hypothesis of abiogenesis, that this presence of oxygen precludes the formation of
prebiotic molecules via a Miller-Urey-like scenario. However, the authors of the paper
are arguing that the oxygen is evidence merely of the existence of photosynthetic
organisms 3.7 Ga ago (a value about 200 Ma earlier than current values), a conclusion
which would possibly have the effect of pushing back the time frame in which MillerUrey reactions and abiogenesis could potentially have occurred, it would not preclude
them in any way. Though there is somewhat controversial evidence for very small (less
than 0.1%) amounts of oxygen in the atmosphere almost as old as Earth's oldest rocks the
authors are not in any way arguing for the existence of a strongly oxygen containing
atmosphere occurring any earlier than previously thought, and they state:"..In fact most
evidence suggests that oxygenic photosynthesis was present during time periods from
which there is evidence for a non-oxygenic atmosphere".
http://biology.clc.uc.edu/courses/bio106/origins.htm (requires Netscape to do
interactive parts)
DNA the blueprint for life
This molecule can act as a blueprint for the manufacture of amino acids and has the
capacity to replicate itself. Such properties occur in all life as we know it including the
simplest forms such as bacteria. DNA's ability to replicate itself is due to its double helix
structure. During cell division, the DNA molecule splits longitudinally, and each side
acts as template to which simpler molecules become attached until each half has once
more become a double helix. The simple molecules from which DNA is built are of four
kinds and are grouped in trios, and these can be abbreviated A, T, C, and G representing
Adenine, Thymine, Cytosine and Guanine respectively. These arranged in particular and
significant orders. Each base can only "pair up" with one single predetermined other
base: A+T, T+A, C+G and G+C are the only possible combinations; that is, an "A" on
one strand of double-stranded DNA will "mate" properly only with a "T" on the other,
complementary strand. Because each strand of DNA has a directionality, the sequence
order does matter: A+T is not the same as T+A, just as C+G is not the same as G+C; For
each given base, there is just one possible complementary base, so naming the bases on
the conventionally chosen side of the strand is enough to describe the entire doublestrand sequence. These sequences of amino acids on the immensely long DNA molecule
specifies how various amino acids are arranged in a protein, and how much protein is to
be synthesized. A length of DNA bearing the information for an unbroken sequence of
manufacture is called a gene.
Occasionally, the DNA copying process goes wrong. A mistake may be made at a single
point on the length of the DNA and a particular molecule may become temporarily
dislocated and be re-inserted in the wrong place. The copy is then imperfect and the
protein that it synthesizes will be different. Such mistakes are sources of variation from
which natural selection can produce evolutionary change. We now know that
photosynthesising organisms had evolved as long ago as 3700 million years.
Schematic representation of the DNA which illustrates its double helix structure
Oxygenating the World
The arrival of blue-greens dictated the rest of the development of life. The oxygen they
produced accumulated and created the atmosphere as we know it today. Atmospheric
oxygen and ozone forms the screen which filters ultra-violet rays which provided the
original energy to synthesize the first amino-acids and sugars. From primitive bluegreens the first single-celled organisms evolved (Eukaryots). Such organisms are called
protista. Each celled organism is more complex than any bacteria and includes a DNA
filled nucleus and elongated bodies called mitochondria which provide energy from
burning oxygen. Some of these unicellular organisms have tail or flagellum which
resemble the filamentous bacterium called a spirochaetae. These unicellular organisms
may also contain chloroplasts (packets of chlorophyll which like blue-greens use energy
from sunlight to assemble complex molecules as food for the cell). Consequently each of
these tiny unicellular organisms appear to be a committee of simpler organisms. It is
even possible that the first cells engulfed and incorporated bacteria and blue-greens to
form a communal life (Endosymbiosis). Cells of this complexity first appeared about
1200 million years ago.
One of the best examples of a protista is Parmecium.
Paramecium
Paramecium
Paramecium aurelia
Scientific classification
Kingdom: Protista
Phylum: Ciliophora
Class: Oligohymenophorea
Order: Peniculida
Family: Parameciidae
Genus: Paramecium Müller, 1773
<size>scrolling</size>
Protista – basic unicellular organisms
These protistans and bacteria can reproduce by binary fission, but since their internal
organization is more complex, the division process is more complex and includes the
division of the separate structures within the cell. The division of mitochondria and
chloroplast (each with their own DNA) may be independent of division of the main cell.
Binary fission begins when the DNA of the cell is replicated. Each circular strand
of DNA then attaches to the plasma membrane. The cell elongates, causing the
two chromosomes to separate. The plasma membrane then invaginates (grows
inwards) and splits the cell into two daughter cells through a process called
cytokinesis.
There are, however, other means of reproduction which involves the exchange of genetic
material when two individual cells conjugate. Some protistans contain two complete sets
of genes which after exchange of genetic material divide to make new cells with only one
set of genes. These cells are of two types, a large and comparatively immobile one and a
smaller active one that possesses a flagellum and are referred to as egg and sperm cells.
When the two types unite in a new amalgamated cell the genes are once again in two sets
but with new combinations of genes that occur from two parent sources. This sexual
reproduction increases the possibilities for genetic variation and an accelerated rate of
evolution.
Protista Diversity
There are thousands of species of protistans, some possessing cilia or flagellum, whereas
others use pseudopodium for locomotion. Some protistans secrete shells of silica or lime,
whereas others have combined individual cells to produce a colony (eg Volvox). The
constituent cells of Volvox, however, are co-ordinated, for all the flagellum around the
sphere beat in an organized way and give direction to locomotion.
Volvox is one of the best known genera of green algae, and is the culmination of
the evolution of spherical colonies. Each Volvox is composed of on the order of a
thousand cells, each a bi-flagellate similar to Chlamydomonas, interconnected
and arranged in a hollow sphere (a Coenobia), with a distinct anterior and
posterior. Asexual colonies consist of somatic or vegetative cells, which do not
reproduce, and gonidia, which reproduce, the reproduction being a process of
longitudinal division. Sexual or oogamous colonies contain, as well as somatic
cells, ova (non-motile female cells) or spermatozoa (small, motile male cells) or a
mixture of the two. These cells, near the back of the colony, develop into new
colonies, initially with the flagella directed inwards and held within the parent.
Eventually the parent bursts and the daughter colonies evert.
Volvox
Volvox
Volvox aureus
Scientific classification
Kingdom: Plantae
Phylum: Chlorophyta
Class: Chlorophyceae
Order: Volvocales
Family: Volvocaceae
Genus: Volvox
Species
Volvox aureus
Volvox carteri (V. nagariensis)
Volvox globactor
Volvox dissipatrix
Volvox tertius
<size>scrolling</size>
The first Multicellular Organisms?
Increased co-ordination between colonial cells appeared with the evolution of the
sponges (Porifera). Sponges may be formless lumps on the sea floor reaching two metres
in size. Their surfaces are covered with tiny pores through which water is drawn into the
body by flagella and then expelled through larger vents. The sponges feed by filtering
particles from this stream of water passing through its body. Some sponges produce a
soft flexible silica-based substance which supports the whole organism, whereas other
sponges secrete lime or silica to create a hard "skeleton" for support. Despite the
elaborate skeletons that some sponges are able to produce they cannot be considered as
an integrated multi-cellular animals since they have no nervous systems nor muscle
fibres.
Sponges are primitive, sessile, mostly marine, waterdwelling filter feeders that
pump water through their matrix to filter out particulates of food matter. Sponges
are among the simplest of animals, with partially differentiated tissues but without
muscles, nerves, or internal organs. In some ways they are closer to being cellcolonies than multicellular organisms. There are over 5,000 modern species of
sponges known, and they can be found attached to surfaces anywhere from the
intertidal zone to as deep as 8,500 m. Though the fossil record of sponges dates
back to the Precambrian era, new species are still commonly discovered.
The structure of a sponge is simple: it is shaped like a tube, with one end stuck to a rock
or other object and an open end, the osculum, open to the environment. The spongocoel,
or interior of the sponge, is composed of walls perforated with microscopic pores that
allow water to flow through the spongocoel.
Sponges
Sponges
Scientific classification
Kingdom: Animalia
Phylum: Porifera
Grant in Todd, 1836
Classes
Calcarea
Hexactinellida
Demospongiae
Sclerospongiae
<size>scrolling</size>
Ctenophores and Cnidarians – first organism with real structure
The simplest organisms to possess such structures are the Ctenophores which include
comb jellies, "sea gooseberries", "sea walnuts" and the "Venus' girdles" and the
Cnidarians which are represented by the Anthozoa which are corals and sea anemones,
Scyphozoa which are jellyfish, the Cubozoa represented by the box jellyfish (sea wasps)
and the and Hydrozoa which includes the Hydroids, hydra-like animals. Chironex
fleckeri is a highly venomous species of box jellyfish that inhabits Australian coast and
is a very fast swimmer and has very sophisticated eyes.
Leidy's comb jelly was introduced to the Black Sea in the early 1980s from the
United States. The absence of competitors and predators allowed this
ctenophore to flourish with a total biomass of about 1,000,000,000 tons. As a
result of the huge amount of food consumed by the exploding comb jelly
population, many fish fry starved. Along with over-fishing and pollution, the
introduction of Leidy's comb jelly has been cited as an important factor in the
collapse of commercial fisheries in the Black Sea in the 1990's.
Image source:
http://www.enature.com/fieldguides/detail.asp?recnum=SC0127
A Jellyfish that has turned itself upside down
Image Source
http://www.bio.umass.edu/biology/troptrip2/Fish.html
An orange Hydra
Image Source
http://www.eeob.iastate.edu/faculty/DrewesC/htdocs/invert-thumbs.htm
Comb jellies
Comb jellies
Scientific classification
Kingdom:
Animalia
Phylum:
Ctenophora
Eschscholtz, 1829
Classes
Tentaculata
Nuda
<size>scrolling</size>
Sea Gooseberries
Sea Gooseberries
Image Source:
http://www.imagequest3d.com/catalogue/ctenophores/pages/h091_jpg.htm
<size>scrolling</size>
Sea Walnuts
Sea Walnuts
Mnemiopsis mccradyi
Image Source:
http://faculty.shc.edu/cchester/Bio499/ctenophora.htm
<size>scrolling</size>
Venus's Girdle
Venus's Girdle
Pleurobrachia
Image Source:
http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopyuk.org.uk/mag/artmay04/wavenus.html
<size>scrolling</size>
One best known Ctenophores are the Comb jellies are voracious marine predators,
feeding mostly on plankton. Ctenophores are mainly composed of inert mesoglea, which
causes them to have a low rate of metabolism. Many species are bioluminescent. The
name comb jelly comes from eight "comb rows" of fused cilia, called ctenes, which are
arranged laterally along the sides of the animal and used primarily for locomotion. The
ctenes of the ctenophores gives rise to a rainbow-like effect that is caused by scattering of
light due to the beating of cilia, not because of bioluminescence. The ctenophores are
hermaphroditic, and some species can reproduce asexually. Most ctenophores have two
long tentacles, but some lack tentacles completely. The tentacles have adhesive structures
called colloblasts, or lasso cells. These cells burst open when prey comes in contact with
the tentacle. Sticky threads released from each of the colloblasts will then capture the
food. Some species have their entire body surface covered with sticky mucus that
captures prey. There are about 100 modern species of these marine animals. One of the
most familiar genera of ctenophore is Mnemiopsis. Due to their soft and fragile bodies,
the fossil record for comb jellies is poor. One possible ctenophore is known from the
Middle Cambrian period.
Coral
Anthozoa
Actinodiscus sp.
Scientific classification
Kingdom: Animalia
Phylum: Cnidaria
Class: Anthozoa
Ehrenberg, 1831
Orders
Subclass Alcyonaria (Octocorallia)
Alcyonacea - Soft corals
Gorgonacea - sea fan,sea feather
Helioporacea
Pennatulacea - sea pen, sea pansy
Stolonifera
Telestacea
Subclass Ceriantipatharia
Antipatharia - black coral, thorny coral
Ceriantharia - tube-dwelling anemone
Subclass Hexacorallia
Actiniaria - Sea anemone
Scleractinia - stony coral
Subclass Zoantharia
Corallimorpharia
Ptychodactiaria
Rugosa†
Zoanthidea - zoanthid
† Extinct
<size>scrolling</size>
Corals1
Corals
Brain Coral (Diploria labyrinthiformis)
Scientific classification
Kingdom: Animalia
Phylum: Cnidaria
Class: Anthozoa
(Corals and sea anemones)
Orders
Scleractinia
<size>scrolling</size>
Sea Anemones
Sea Anemones
Giant Green Anemone, Southern California
Scientific classification
Kingdom: Animalia
Phylum: Cnidaria
Class: Anthozoa
Subclass: Hexacorallia
Order: Actiniaria
Families
Suborder Endocoelantheae
Family Actinernidae
Family Halcuriidae
Suborder Nyantheae
Infraorder Athenaria
Family Andresiidae
Family Andwakiidae
Family Edwardsiidae
Family Galatheanthemidae
Family Halcampidae
Family Halcampoididae
Family Haliactiidae
Family Haloclavidae
Family Ilyanthidae
Family Limnactiniidae
Family Octineonidae
Infraorder Boloceroidaria
Family Boloceroididae
Family Nevadneidae
Infraorder Thenaria
Family Acontiophoridae
Family Actiniidae
Family Actinodendronidae
Family Actinoscyphiidae
Family Actinostolidae
Family Aiptasiidae
Family Aiptasiomorphidae
Family Aliciidae
Family Aurelianidae
Family Bathyphelliidae
Family Condylanthidae
Family Diadumenidae
Family Discosomidae
Family Exocoelactiidae
Family Haliplanellidae
Family Hormathiidae
Family Iosactiidae
Family Isanthidae
Family Isophelliidae
Family Liponematidae
Family Metridiidae
Family Minyadidae
Family Nemanthidae
Family Paractidae
Family Phymanthidae
Family Sagartiidae
Family Sagartiomorphidae
Family Stichodactylidae
Family Thalassianthidae
Suborder Protantheae
Family Gonactiniidae
Suborder Ptychodacteae
Family Preactiidae
Family Ptychodactiidae
<size>scrolling</size>
Jellyfish
Jellyfish
Sea nettle, Chrysaora quinquecirrha
Scientific classification
Kingdom: Animalia
Phylum: Cnidaria
Class: Scyphozoa
Goette, 1887
Orders
Stauromedusae
Coronatae
Semaeostomeae - Disc jellyfish
Rhizostomae
<size>scrolling</size>
Box jellyfish
Box jellyfish
Scientific classification
Kingdom: Animalia
Phylum: Cnidaria
Class: Cubozoa Werner, 1975]
There are two main groups of Cubozoa, Chirodropidae and Carybdeidae
containing 20 species between them. A phylogenic analysis of their relationships
is yet to be published.
Taxon Chirodropidae
Chironex fleckeri
Chirosoides buitendijkl
Chirodropus gorilla
Chirodropus palmatus
Chiropsalmus zygonema
Chiropsalmus quadrigatus
Chiropsalmus quadrumanus
Taxon Carybdeidae
Carukia barnesi
Manokia stiasnyi
Tripedalia binata
Tripedalia cystophora
Tamoya haplonema
Tamoya gargantua
Carybdea alata
Carybdea xaymacana
Carybdea sivicksi
Carybdea rastonii
Carybdea marsupialis
Carybdea aurifera
<size>scrolling</size>
Chironex fleckeri
Chironex fleckeri
Chironex fleckeri is a highly venomous species of box jellyfish. For a jellyfish it is
a very fast swimmer and has very sophisticated eyes. Chironex fleckeri grow to
approximately the size of a basketball, is nearly transparent and has four clusters
of 15 tentacles. When the jellyfish are swimming the tentacles contract so they
are about 15cm long and as thick as bootlaces, when they are hunting the
tentacles are thinner and about three metres long. The tentacles are covered
with stinging cells or Nematocysts which are activated by pressure and a
chemical trigger: they react to proteinous chemicals.
The polyps are found in estuaries in northern Australia, the medusa is pelagic
and is found in the coastal waters of northern Australia and adjacent areas of the
tropical Indo-West Pacific, and are also found in southeastern Asia. They are not
usually found on the reef.
In common with other box jellyfish, Chironex fleckeri have four eye-clusters with
twenty-four eyes. Some of these eyes seem capable of forming images, but it is
debated whether they exhibit any object recognition or object tracking and it is
not known how they process information from their sense of touch and eye-like
light detecting structures. Chironex fleckeri live on a diet of prawns and small fish
and are themselves prey to turtles.
The Sting of Chironex fleckeri has killed about one hundred people in Australia
over the last one hundred years, making it possibly the most dangerous species
of jellyfish in the world.
Chironex flickeri appear to avoid human beings when they are close to them and
so can be said to avoid stinging humans. Their sting is incredibly powerful and
can be fatal. The sting produces instant excruciating pain accompanied by an
intense burning sensation, and the venom has multiple effects attacking the
nervous system, heart and skin at the same time. While an appreciable amount
of venom (about ten feet or three metres of tentacle) needs to be delivered in
order to have a fatal effect on an adult human, the potently neurotoxic venom is
extremely quick to act. Fatalities have been observed as little as four minutes
after envenomation, notably quicker than any snake, insect or spider and
prompting its description as the world's deadliest venomous animal. Although an
antivenom exists, treating a patient in time can be difficult or impossible. Dousing
a sting with vinegar immediately kills any venom which has not been activated,
while rubbing a sting exacerbates the problem.
<size>scrolling</size>
Hydrozoa
Hydrozoa
Scientific classification
Kingdom: Animalia
Subkingdom: Metazoa
Phylum: Cnidaria
Class: Hydrozoa
Owen, 1843
Orders
Actinulida
Capitata
Chondrophora
Filifera
Hydroida
Siphonophora
Trachylina
<size>scrolling</size>
The Cnidarians are more common and have bodies clearly divided into two cellular
layers, each layer one-cell thick. The outer layer of cells is the ectoderm whereas the
inner layer is the endoderm. The individual cells of the ectoderm are specialized for
various functions such as protection, secretion, defence and cell replacement whereas the
endoderm is specialized for digestion, absorption and assimilation of food. The stinging
cells (Cnidocytes) of the ectoderm are highly specialized and contain coiled threads
inside. When food or an enemy comes near, the cell discharges the thread which is
armed with spines like a miniature harpoon and often loaded with poison. These cells are
often concentrated at the ends of tentacles. Cnidarians reproduce by releasing eggs and
sperm into the sea. The fertilized egg first develops into a free swimming creature that is
quite different from its parents. It eventually settles down at the bottom of the sea and
develops into a tiny flower-like organism called a polyp which filter-feed with the aid of
tiny-beating cilia. Eventually, the polyp bud in a different way and produce miniature
medusae which detach themselves and once again become free-swimming. True jellyfish
spend most of their time as free-floating medusae with only the minimum period fixed to
the rocks as solitary polyp, whereas sea anemones do the reverse with most of their life
spent attached to rock as solitary polyp. Yet other coelenterates exist as colonies of
polyps which have given-up a sessile life and have become free-floating e.g. Portuguese
Man O'War (Physalia).
The Portuguese Man O' War (Physalia physalis), also known as the bluebottle,
is commonly thought of as a jellyfish but is actually a siphonophore—a colony of
four sorts of polyps.
Portuguese Man O' War
Portuguese Man O' War
Scientific classification
Kingdom: Animalia
Phylum: Cnidaria
Class: Hydrozoa
Order: Siphonophora
Family: Physaliidae
Genus: Physalia
Species: P. physalis
Binomial name
Physalia physalis
(Linnaeus, 1758)
<size>scrolling</size>
Cnidarians and the Fossil Record
Although Cnidarians are relatively simple organisms and appeared fairly early in the
history of life, fossil evidence for them was only recently found (1940's) in the Flinders
Range, southern Australia in rock strata that has been dated at about 650 million years.
Photograph of the fossil coral Heliophyllum taken by Dlloyd. Heliophyllum halli
from the Devonian. Locality - Arkona, Ontario, Canada. Complete matrix free
specimen which measures 4.3 cm across.
Coral Reefs under threat
Not all Cnidarians are soft-bodied, and some produce skeletons of limestone in a similar
way to the sea sponges and are better known as corals. These animals secrete their
skeletons from their base. Each polyp is connected with its neighbours by strands that
extend laterally. As the colony develops new polyps form, leaving a limestone skeleton
that is riddled with tiny cells were polyps once existed. Live polyp are restricted to a thin
surface layer. The size of these colonial polyps are enormous and create entire coral
islands called atolls and created the Great Barrier Reef running parallel to the east coast
of Australia. This coral reef extends for over a sixteen hundred kilometres and is the
greatest animal construction prior to man's artefacts.
Portion of a Pacific atoll showing two islets on the ribbon or barrier reef separated
by a deep pass betwen the ocean and the lagoon.
coral reef
coral reef
A coral reef is a type of biotic reef developing in tropical waters. Although corals
are major contributors to the overall framework and bulk material comprising a
coral reef, the organisms most responsible for reef growth against the constant
assault by ocean waves are calcareous algae, especially, although not entirely,
species of red algae.
Water temperature of 20–28 °C (68–82 °F) is an optimal range for proper growth
and health of coral reefs. Coral reefs are found in all oceans of the world, except
the Arctic Ocean, generally between the Tropic of Cancer and the Tropic of
Capricorn, because reef-building corals live in these waters. Reef-building corals
are found mainly in the photic zone (less than 50m), where the sunlight reaches
the ground and offers the corals enough energy. The corals themselves do not
photosynthesise, but they live in a symbiotic relationship with types of
microscopic algae that photosynthesise for them. Because of this, coral reefs
also grow much faster in clear water, which absorbs less light.
Such reefs take a variety of forms, defined as the following;
Apron reef — short reef resembling a fringing reef, but more sloped; extending
out and downward from a point or peninsular shore.
Fringing reef — reef extending directly out from a shoreline, and more or less
following the trend of the shore.
Barrier reef — reef separated from a mainland or island shore by a lagoon; see
Great Barrier Reef.
Patch reef — an isolated, often circular reef, usually within a lagoon or
embayment.
Ribbon reef — long, narrow, somewhat winding reef, usually associated with an
atoll lagoon.
Table reef — isolated reef, approaching an atoll type, but without a lagoon.
Atoll reef — a more or less circular or continuous barrier reef surrounding a
lagoon without a central island; see atoll.
<size>scrolling</size>
Satellite image of a part of the Great Barrier Reef, off the East coast of Australia.
Photo courtesy of NASA.
Humans continue to represent the single biggest threat to coral reefs. In particular, landbased pollution and over-fishing are the most serious threats to these ecosystems. The
live food fish trade has been implicated as one driver of decline due to the use of cyanide
in the capture of fish. Rising water temperatures produce toxins in the coral tissue, due to
bleaching.
High levels of land development have also been threatening the survival of coral reefs.
Within the last 20 years, the once thick mangrove forests, which absorb massive amounts
of nutrients from runoff caused by farming and the construction of roads, buildings, ports,
channels, and harbors, are being destroyed. Nutrient-rich water causes algae to thrive in
coastal areas in suffocating amounts, also known as algal blooms.
Assignments
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
Discuss the structure and significance of DNA to life as we know it.
Describe the process of fossilization and its significance in the interpretation of
evolutionary events.
Describe how cells have become specialized to perform different functions in a
multicellular organism.
Cnidarians and the Fossil Record
Although Cnidarians are relatively simple organisms and appeared fairly early in the
history of life, fossil evidence for them was only recently found (1940's) in the Flinders
Range, southern Australia in rock strata that has been dated at about 650 million years.
Not all Cnidarians are soft-bodied, and some produce skeletons of limestone in a similar
way to the sea sponges and are better known as corals. These animals secrete their
skeletons from their base. Each polyp is connected with its neighbours by strands that
extend laterally. As the colony develops new polyps form, leaving a limestone skeleton
that is riddled with tiny cells were polyps once existed. Live polyps are restricted to a
thin surface layer. The size of these colonial polyps are enormous and create entire coral
islands such as the Great Barrier Reef running parallel to the east coast of Australia. This
coral reef extends for over a sixteen hundred kilometres and is the greatest animal
construction prior to man's artifacts.
Assignments
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
Discuss the structure and significance of DNA to life as we know it.
Describe the process of fossilization and its significance in the interpretation of
evolutionary events.
Describe how cells have become specialized to perform different functions in a
multicellular organism.
BUILDING BODIES: INVERTEBRATES OF THE OCEANS
Great Barrier Reef - Australia
Living in association with the Great Barrier Reef is a multitude of higher animals which
include shelled animals of the phylum Mollusca (clams, cowries, mussels and sea-snails),
radially symmetrical creatures of the phylum Echinodermata and includes sea urchins and
starfish, elongated animals with segmented bodies occurring in the phylums Annelida and
Arthropoda which includes bristle worms, shrimps and crabs as well as the vertebrates
(phylum Chordata) which includes cartilaginous and bony fishes and marine mammals
such as dolphins and seals.
The giant clam Tridacna gigas and a Parrot Fish. The Giant Clam is the largest
living bivalve mollusc. One of a number of large clam species native to the
shallow coral reefs of the South Pacific and Indian oceans, they can weigh more
than 400 pounds and measure as much as 1.5 meters across.
Sessile in adulthood, the creature's mantle tissues act as a habitat for the
symbiotic single-celled dinoflagellate algae from which it gets it nutrition. By day,
the clam spreads out its mantle tissue so that the algae receive the sunlight they
need to photosynthesize.
Parrotfish are mostly tropical, perciform marine fish of the family Scaridae.
Abundant on shallow reefs of the Atlantic, Indian and Pacific Oceans, the
parrotfish family contains nine genera and about 80 species. Parrotfish are
named for their oral dentition: their numerous teeth are arranged in a tightly
packed mosaic on the external surface of the jaw bones, forming a parrot-like
beak which is used to rasp algae from coral and other rocky substrates. Many
species are also brightly coloured in shades of blue, green, red and yellow.
Although they are considered to be herbivores, parrotfish eat a wide variety of
organisms that live on coral reefs.
Image Source
http://www.richard-seaman.com/Underwater/Australia/Coral/
Tridacna gigas
Tridacna gigas
Giant Clam
Conservation status: Vulnerable
Scientific classification
Kingdom: Animalia
Phylum: Mollusca
Class: Bivalvia
Order: Veneroida
Family: Tridacnidae
Genus: Tridacna
Species: gigas
Binomial name
Tridacna gigas
Linnaeus, 1758
Parrotfish
Parrotfish
Parrotfish
Midnight parrotfish (Scarus coelestinus)
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Perciformes
Family: Scaridae
Genera
Bolbometopon
Calotomus
Cetoscarus
Chlorurus
Cryptotomus
Hipposcarus
Leptoscarus
Nicholsina
Scarus
Sparisoma
Fossil History of Marine Invertebrates
To trace the invertebrate lines we must also look for fossils where animals were deposited
continuously and the fossil remains to have survived in a relatively undistorted condition
such as has occurred in the Atlas Mountains of Morocco. From this fossil record and
from other sites scattered around the world there appears a clear dichotomy in the history
of earth where fossils are found and they are not found. This period of transition
corresponds with about 600 million years and records the first annuals which are
characterized by the presence of shells. It is conceivable that before this period the
animals were soft bodied and did not fossilize. It has also been suggested that seas were
not at the right temperature and or chemical composition to favour deposition of lime
from which most marine shells and skeletons are constructed.
Platyhelminthes: the building block for other invertebrates
Simpler animals than those first found in the fossil records still inhabit the earth and its
oceans and their ancestors may have represented the predecessors for the shelled
invertebrates that are found in the fossil records. These soft-bodied animals belong to the
phylum Platyhelminthes. The most basic of these animals is the flatworm, a flat-leaf
shaped worm which like jellyfish have a single opening to their gut through which food is
ingested and waste is ejected. Their bodies have differentiated into three layers, the
ectoderm, mesoderm and endoderm. Cells with a different structure and function have
aggregated to form a primitive system (eg nervous system which consists of a network of
nerve fibres). Nevertheless, they have no breathing system with oxygen diffusing
directly through the skin. Their undersides are covered with cilia which, by beating,
permits them to glide over surfaces. Their front end has a mouth on the under-surface
and a few light sensitive spots above.
Platyhelminthes: a surprisingly diverse group
There are some 3000 species varying in size from microscopic to 600 mm, and although
most are marine some species have managed to inhabit humid terrestrial environments
and move on a bed of mucus. Many species in this phylum have become parasitic and
live on the surface and inside bodies of other animals including man. Some of these
parasitic forms such as liver flukes still resemble a basic flatworm form whereas others
such as the tape worm have a highly modified morphology with hooks on their heads and
an ability to detach egg-bearing sections of their posterior body parts.
Annelids: the first segmented animals
It is hypothesized that the period between 600 and 1000 million years considerable
erosion of the continents was producing great expanses of mud and sand adjacent to the
continental shelf. This environment may have contained abundant quantities of organic
material. However, in order to give protection and concealment in this environment
burrowing would be a pre-requisite, and more tubular body plan would become
necessary. It is possible that under such conditions the segmented worms evolved. Some
of these animals became active burrowers who tunnelled through mud in search of food,
whereas others lay half-buried, with their mouthparts filtering food above the sediment.
Brachiopods: developing a bivalve shell
Some of these animals lived in secreted protective tubes, whereas others evolved two flat,
protective shells which represented the first Brachiopods descendants which exist belong
to the genus Lingula. Brachiopods had great variations in their design, including heavy
lime shells, and large tentacles contained inside, whereas others developed a hole at the
hinge end of one of the valves through which a stalk emerged and fastened the animal
onto the ground.
The first Molluscs
Other kinds of annelids also developed in which the animal did not attach itself to the sea
floor but continued to crawl and secreted a small conical tent under which it could escape
from predators and probably represented the prototype for the Mollusc group, with a
primitive representative being Neopilina. Today there are at least 60 000 different
species of mollusc. Anatomically these animals usually possess a foot which may be
used for locomotion, a shell, a mantle composed of thin sheets of body tissue that covers
the internal organs, and an internal cavity that coats the central part of the body, in which
most species have gills which extract oxygen from water.
The Molluscs diversified
The shell is secreted by the upper surface of the mantle, with limpets producing shell at
equal rates along the edge of the mantle, in other animals the front end of the mantle
secretes at a faster level than the rear end and produces a flat spiral. The maximum
secretion may be to one side and develops twisted or turreted-shaped shells, or in the case
of cowries the secretion is concentrated along the sides of the mantle creating a shell
resembles a clenched fist. Molluscs may have either single shells (limpets), two shells or
bivalves (mussels) or a number of shell plates (chitons). In some molluscs the shell has
become reduced and totally internal (cuttlefish) whereas in others it is total absent
(octopuses).
Molluscs: Feeding mechanisms
Molluscs have a variety of different feeding mechanisms. The bivalve molluscs can
filter-feed fine particles form the water. Some of the single-shelled molluscs (limpets)
possess a ribbon-shaped tongue or radula, covered with rasping teeth, which enables the
animal to scrape algae from the rock. Whelks have a radula on a stalk that can extend
beyond the shell and be used to bore into the shells of other molluscs. Through these
holes that they have bored they poke the tip of the radula and suck out the flesh of the
victim. The cone-shells also have a stalked radula which is modified into type of harpoon
with which they secure their prey before injecting it with poison. In still more active
carnivores the heavy shell is reduced in size and may even be lost as has occurred in the
sea-slugs which have an upper surface covered with tentacles. One species of sea-slug
actively hunts jelly fish and ingests these animals stinging cells which it then
concentrates in the tentacles and uses them for protection.
Molluscs: Evolving and keeping the shell
An early group of molluscs retained the protection of a shell yet were still able to
maintain a high degree of mobility. This was achieved through the development of a gasfilled floatation tanks. The prototype forms had a flat-coiled shell with an end walled-off
to form a gas chamber. As the animal grew it added buoyancy with the development of
new chambers. Such animals survive today and are known as nautiluses. A tube runs
from the body chamber of the nautilus to the floatation tanks in the shell. The nautilus is
an active carnivore eating animals such as crabs and moves in a form of jet-propulsion
where water is squirted through a siphon. In this animal the original muscular foot is
divided into long grasping tentacles with which it secures its prey. The mouthparts are
modified to form a horny beak with which the nautilus is able to crack shells of other
animals. Variations on the float chamber theme gave rise to the enormously successful
group of animals called the ammonites whose circular shells were up to 2 meter in size.
Molluscs: Secondary loss of the shell
One of these group of molluscs took the same path as the sea slugs and disposed of its
shell entirely (octopuses and squids) whereas relict of the ancestral shell persist as the
cuttlebone found in the cuttlefishe. One species of octopus (Argonauta) secretes a paperthin replica of the nautilus shell, the chambers of which are used to lays its eggs.
Both squids and octopuses have reduced the number of tentacles (10 and 8 respectively),
but squids have become more mobile with the development of undulating lateral fins. The
brains and eyes of these animals is the most advanced of any invertebrate, eyes greater
than 400 mm in size have been recorded for squid. Squids, in particular can reach
immense sizes with one individual 21 m long (found in New Zealand in 1933).
Echinoderms: Penta-symmetrical creatures of the oceans
Another group of animals that had diverged from early stage and also reached immense
sizes are the crinoids or sea lilies which belong to the phylum Echinodermata. These
animals have an architecture plan that is based on a five-fold symmetry and possess large
lime plates that occur just below the skin. Fossil crinoids were up to 20 m long, although
their present day counterparts are considerably reduced in both size and species diversity.
Echinoderms: A hydrostatic structure
The bodies of all members work on a unique hydrostatic principle. The hydrostatic
skeleton is closed fluid-filled system that terminates as a series of blind tubes called tubefeet. Each tube feet ends in a sucker. Changing the local pressure within the tube feet
allows to be extended and contracted. Extensions and contractions of these tube feet
occur as waves down the length of the arms (or ray) and this allows the animal to move
itself and to move particulate matter down the arm. The water from this system circulates
separately from that in the body cavity. It is drawn through a pore into a canal
surrounding the mouth and circulated throughout the body into the myriads of tube feet.
When suspended particles of food touches an arm, the tube feet fasten on to it and pass it
from one to another until it reaches the groove that runs down the upper surface of the
arm to the central mouth. Although stalked, sessile sea-lilies were the most abundant
crinoids in the fossil records, the most common form today is the stalkless feather stars.
Echinoderms diversity: variations on a theme
Five-fold symmetry and hydrostatically operated tube feet also occur in the starfish and
the brittle stars, however their body plan has become inverted and the mouth is on the
undersides. Yet in another group of echinoderms the five-fold symmetry is less
conspicuous and the body plan is elongated with a mouth and anus at the two ends. At
the mouth the tube feet have become modified into tentacles which filter fine food
particles. The five-fold symmetry and hydrostatic mechanisms did not develop further
and the group is generally considered to be an evolutionary cul-de-sac.
Arthropoda: the most successful animal phylum
The third major line in the evolution of invertebrates was the development of the
segmented bodies (Arthropoda) which evolved at a very early stage and are contemporary
with the jellyfish fossil patterns found in Flinders, Australia. This group of animals
shares one important feature with the molluscs, and that is a spherical larvae possessing a
belt of cilia, whereas the echinoderm larvae have a twisted morphology with winding
bands of cilia. This suggests that molluscs and arthropods evolved from flatworms
(Platyhelminthes), with the echinoderms having an independent evolutionary line.
Arthropoda: Segmentation the successful formula
Segmentation may have increased the efficiency for burrowing in mud. A line of
separate limbs that are repeated down the length of the body seems to have been the most
primitive form. Each segment is equipped with its own set of organs - on either side, leglike projections sometimes accompanied by bristles and feathery appendages through
which oxygen could be absorbed, and within the body wall, a pair of tubes opening to
the exterior from which waste is secreted. A gut, a large blood vessel and a nerve cord
run through all segments from the anterior to the posterior end of the organism and coordinates the segmentation. a great variety of these segmented animals have been almost
perfectly fossilized in the Burgees shale of the Rocky Mountains in British Columbia,
Canada.
Early Arthropods: The fossil record
An early segmented animal was the trilobite. These animals had a bony armour
composed of lime and a horny substance called chitin. The armour was not expandable
and therefore shed periodically. Many of these shed exoskeletons have been preserved as
fossils. Where the entire animal is preserved you can observe the jointed legs that are
attached to each segment of the body, the feathery gill next to each leg, two feelers at the
front of the head, the gut running the length of the body, and even muscle fibres along the
back which enabled the animal to roll itself into a ball. Comparatively high resolution
eyes composed of mosaics of separate cells and a crystalline calcite lens. The very thick
lens of some trilobites may have reflected their colonization of deeper water where light
is considerably reduced. However, the optimal properties of the calcite lens operating in
water would not have permitted a fine focus. This shortcoming was compensated by the
evolution of the two-part lens with a waved surface at the junction of the two lens
elements.
The trilobite Asaphiscus wheeleri preserved as a very clear fossil from Cambrianaged shale in Utah
Living descendents of the Trilobites
Although they radiated throughout the oceans, only one descendent of this group survives
today, the horse-shoe crab (Limulus). This animal is larger than its ancestral trilobites,
and segmentation of its armour have fused to form a large domed shield. These animals
generally live at great depths but each spring they migrate towards the coast and during
full moon and high tides they drag themselves onto the beach where they copulate.
Today the similarities between the horse-shoe crabs and the trilobites are only evident in
the larval stage where segmentation of the armour plates are clearly discernable in the
horse-shoe crab larvae.
Crustaceans: Arthropod success in the sea
Another group of armoured animals also evolved from the original segmented worms the
crustaceans which exist today in the form of some 35 000 species. They may prowl
around rocks and reefs as crabs, shrimps, prawns, lobsters and crayfish, they may become
sessile such as barnacles, or congregate and swim in vast shoals such as krill. The size of
the crustacean and the form of the exoskeleton varies considerably from the paper-thin
exoskeleton of the almost microscopic water flea (Daphnia) to the carapace of giant
Japanese spider crab (Macrocheira kaempferi) which measures 3 m from claw to claw.
In the crustaceans the paired legs have become modified for a variety of purposes. At the
anterior end they have become modified into pincers or claws, those in the middle are
paddles, or walking legs or tweezers. Some have feather branches acting as gills through
which oxygen can be absorbed. All limbs are jointed, tubular and operate by way of
muscles. Like the primitive trilobites for crustaceans to grow they need to dispose of
their calcareous carapace. As time approaches for moulting the animal absorbs as much
calcium carbonate from the carapace into the blood stream, and begins to secrete a new
soft wrinkled skin under the carapace. The outgrown armour splits and the crustacean
swells its body by absorbing water, and wrinkled new skin stretches and hardens into a
new carapace.
Arthropod Exoskeleton: Evolving to occupy land
This exoskeleton may work to advantage for animals to colonize land if a mechanism of
breathing in air as opposed to water can be secured. By developing almost closed air
chambers lined with folds of moist skin crustaceans are able to absorb oxygen from air.
In this way sand shrimps, beach hoppers and wood lice have been able to colonize land
that retains a moist environment. The most spectacular of land dwelling crustacean is the
big robber crab Birgus which exploits coconuts.
Other descendent of the invertebrates have left the sea for a terrestrial life style the first of
which were probably derived from segmented marine worms, but more recently included
the familiar snails and slugs. These changes started about 400 million years ago and gave
rise to the most numerous and diverse of land animals; the insects.
Assignments
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
Discuss the variations in shell structure that have occurred in the phylum Mollusca.
Describe the water vascular system that characterizes animals that occur in the phylum
Echinodermata.
Describe the diversity of segmented marine invertebrates that have evolved.
THE FIRST FORESTS
Plants: Evolving to occupy land
The first land available for colonization was inhospitable due to the considerable amounts
of volcanic action. Consequently as volcanoes erupted on land, life in the oceans
multiplied with a diversity of species with different structures and adaptations, but the
land remained unconquered. Marine algae may have secured an existence on the littoral
zones of the ocean in the same way they do today. Around 420 million years ago the first
waxy layers developed in plants to prevent desiccation, but this did not totally free such
plants from an aquatic environment since they required an aquatic medium for
reproduction. Algae reproduce through both asexual division and sexual methods.
Sexual reproduction involves the production of sex cells which require locomotion in
water for the fusion of the cells to take place.
Plants: Fertilization and dispersal the first issues
Such a problem still exists for primitive plants living today such as the liverworts and
mosses. Such plants practice sexual and asexual reproduction in their alternate
generations. The familiar green moss is the generation which produces the sex cells.
Each large egg cell remains attached to the stem at the top of the moss plant, while the
smaller microscopic sperm cells are released into water and thrash their way to fertilize
the egg cell. The egg cell develops while still attached to its parent plant and produces
the next asexual generation which is composed of a thin stem with, at its tip, a hollow
capsule in which a large number of spores are produced. In a dry atmosphere the capsule
splits releasing airborne spores. If the spores land in a suitable site they develop into new
moss plants.
Mosses: Possibly the earliest land plants?
Moss plants have no structural strength and rely on close packing to achieve only modest
heights. Their tissues are soft and permeable and they can only exist and reproduce under
moist environments. Such plants probably represented the earliest colonization of the
terrestrial environment, although no fossil evidence for this has been discovered.
Fossils of the earliest land plants
The earliest fossilized land plants (400 million years ago) were simple leafless branching
strand filaments found in rocks and cherts of the United Kingdom. Like mosses no root
tissue had differentiated, however, long thick-walled cells enabling water to be conducted
along stems had differentiated and represented a major advance which gave plants
structural strength to grow bigger. Such plants, together with primitive mosses and
liverworts created the first vegetation which permitted animals to colonize from the sea
onto the land.
What were the earliest land animals?
The first land-dwelling animals were segmented and probably represented the ancestors
of the millipedes you encounter today and reached spectacular sizes (up to 2m in length).
The exoskeleton inherited from aquatic ancestors needed only minor modification, but
the external gills were unsuitable and in its place a network of breathing tubes (tracheae)
evolved. Each tube has an exterior opening on the side of the exoskeleton, and the
network of tracheae provides each cell with a supply of oxygen.
Living of Land: Issues of reproduction
Reproduction on land required changes since their aquatic ancestors relied on water to
transport the sperm cell to the egg cell. In millipedes the reproductive cells are located
close to the base of the second pair of legs. The male and female animals meet and
intertwine, the male reaches forward with his seventh leg and collects his sperm and
transfers it to the sexual pouch of the female. Such copulation was laborious but safe, but
was not suitable for the predatory animals that evolved then but still survive today as
centipedes, scorpions and spiders. These three groups of animals have all undergone a
reduction in segmentation and all may indulge in cannibalism. As a consequence of this
scorpions armed with large poison glands and spiders have evolved ritualized courtship
patterns prior to copulation.
Land plants: Making their mark
During this early period of evolution in the segmented animals, plant were also evolving,
with the development of rooting systems which were absent in the mosses. Rooting
systems permitted water sources below the ground to be utilized. Consequently root
development permitted plants to survive in less moist environments. Three groups of
plants, all of which have living descendants evolved root structures (club mosses
Lycopodium, horsetails Sphenophyta and ferns Pterophyta) and possessed within their
stems strong woody vessels for the transport of water absorbed by the roots. Such
adaptations provided the structural rigidity for some of these plants to grow big (up to 30
m) and created the first true forests.
A Forest Environment
The development of forests would have necessitated changes in habitat (from the ground
to arboreal) for some animals. Evolving at this time were the first vertebrate animals
which had four legs, a backbone and moist skins and were also carnivorous on the
invertebrates. Among the invertebrates bristletails and springtails evolved and remain
one of the most numerous of invertebrates with the most familiar being the silverfish.
Silverfishes have clear but even more reduced segmentation consisting of a conspicuous
head supporting compound eyes and antennae, a thorax bearing three pairs of jointed legs
(a result of three segments being fused) and segmented abdomen which has lost its limbs
but possesses three filaments at the extreme end. These animals breath much as
millipedes do with a tracheae system, they copulate like scorpions do with the female
walking over packets of sperm and taking them up into the genital pouch.
Insects: The greatest conquerors of all?
The characteristics of six legs and a body divided into three parts became numerically the
most successful group of animals: the insects. Although ancestral insects probably
climbed about the vegetation, one important ingredient for their success was the
development of wings and the ability to fly. How wings evolved is unknown but it may
have reflected attempts on insects to increase surface area and become more efficient at
warming up their bodies so that they can become active (thermoregulation). Winged
insects appeared some three hundred million years ago with animals resembling
dragonflies. In the absence of early competition, early dragonflies radiated with some
species developing enormous sizes (eg wingspan of 700 mm). Dragonflies have two
pairs of wings with a simple up and down movement, and consequently cannot be folded
back. Today's dragonflies have large compound eyes and catch smaller insects in flight,
but are able to hunt only during the day. Consequently today's carnivorous dragonflies
must have been preceded by herbivorous animals or carnivorous forms that prey on nonflying insects. Modern dragonflies probably evolved from primitive omnivorous or
herbivorous insect forms such as cockroaches, grasshoppers, locusts or crickets.
Land Plants: Still working on the reproduction issue
The development of flight in insects was to have a major consequence on the evolution of
plants. Early plants including tree forms existed in two alternating forms, a sexual and an
asexual generation. Becoming tall would have no effect on the transport of spores and
may even enhance their wind-dispersal, however, the distribution of sex cells which,
hitherto, was achieved by the male cells swimming through a droplet of water and
reaching a female cell. This demanded that the sexual generation was small and grew
close to the ground, a situation that is found today in ferns, club mosses and horsetails.
The spores of such plants develop into a filmy plant called a thallus which produces sex
cells on the undersurface where there is permanent moisture. After fertilization of the
female egg cells the thallus develops into the tall spore-bearing plants.
Cycads: Getting to grips with the reproduction on land
A thallus life cycle stage induces considerable vulnerability, since it is small and
possesses little or no protection against herbivory or desiccation. A less vulnerable
sexual stage appeared about 350 million years ago with the evolution of plants like the
cycads which exist today. Cycads superficially resemble ferns, with some species having
spores of the archaic form which are distributed by wind. In other species some spores
become large and remain attached to the parent plant where they develop into a conicalshaped structure containing egg cells (that is functionally equivalent to a thallus). When
a wind-blown spore, now called a pollen lands on these egg bearing cones, no filmy
thallus develops, but a pollen tube which burrows its way into the female cone occurs.
The large sperm cell is transported down to the bottom of the pollen tube, where it enters
a small drop of fluid secreted by the surrounding tissues of the cone, there it swims to the
egg cell and fuses with it and thereby completing the fertilization process.
Conifers: A successful formula
Similar morphological changes resulted in the evolution of the conifer group (pines,
larches, cedars and firs). These plants, unlike cycads produce pollen and egg-bearing
cones on the same plant individual, however, fertilization and the development of the
seed takes longer, but the seeds are equipped with a rich supply of food and a hard,
water-proof coat that permits the seed to remain dormant until conditions are right for
germination and the establishment of the seedlings. Conifers are successful, even today,
with one-third of global forests being composed of them. Both the biggest and most
long-lived individual organism in the world are conifers (the redwoods and the bristlecones respectively).
Earliest plant defences against herbivores
Conifers are also able to repel insect damage with a gummy substance called resin.
Insects are often caught in the resin which has proved to be a good fossilizing medium
called amber. The first amber containing flying insects appeared 100 million years ago
and includes representatives of all major insect groups known today. Each group has
developed its own characteristic way of flying. Dragonflies have two pairs of wings
which flap up and down synchronously, bees and wasps have linked the fore and hind
wings together with hooks, butterflies have overlapped the wings, hawkmoths have
reduced the hind wings considerably in size and latched them onto long narrow forewings with a curved bristle, beetles have the front pair modified into thick covers which
protect the rear flying wings, and flies use only the front pair of wings for flight with the
hind wings reduced to tiny knobs.
Arachnids: Insects Nemesis
Although insects were the first animals to invade the air, they nevertheless fell prey to
their arachnid adversaries, the spiders, who evolved the ability to spin webs between
branches and thereby trap and consume flying insects.
Plants and Insects find “mutual benefit”
Plants also responded to the flying skills of insects by using such mobility for the
distribution of the male reproductive cells (pollen). Unlike spores in the lower plants,
pollen needs to reach the female cell for the development of more adult plants. Winddispersal of pollen which is typical in the pines (Gymnosperms), requires vast quantities
of pollen for even moderate pollination success. Alternatively if insects could be used to
carry pollen to the female cells by using a small incentive (e.g. food), much less pollen
would be required to achieve similar levels of pollination success. Such incentives for
insect pollination evolved with the earliest of the flowering plants; the magnolias which
appeared about one hundred million years ago. In these plants the egg cells are clustered
in the centre, each protected by a green coat with a receptive spike on the top called a
stigma with which it receives pollen and is necessary for fertilization. Grouped around
the egg cells with their stigmas are stamens which produce the pollen. In order to bring
these organs to the notice of insects, the whole structure is surrounded by brightly
coloured modified leaves called petals.
Beetle pollination
Beetles had already learnt to feed on the pollen of cycads, and were one of the first to
transfer their attentions to the early flowers like those of the magnolias and waterlilies.
As they moved from one to another flower, beetles collected meals of pollen and paid for
them by becoming covered in excess pollen which they involuntarily delivered to the
next flower they visited. One danger of having both eggs and pollen in the same
structure is that the plant may pollinate itself, however, this is overcome by egg and
pollen cells being mature at different times.
Plants learn to manipulate
Other flowers developed alternative bribes to pollen this being nectar, a completely
specialized adaptation to recruit even more potential pollination agents which included
bees, flies, butterflies and moths. Even brighter signals were used to draw attention to the
nectar being offered and attractive scented chemicals evolved as additional means of
soliciting the services of insects for pollen transportation. The services of flies were
enlisted with the evolution of flowers that mimicked the scent of rotting flesh, the usual
food of such animals. Some stepelia plants have taken this deception further by
producing brown, wrinkled petals covered with hairs which resemble the decaying skin
of a dead animal. To complete the illusion, the plant generates heat to mimic the warmth
generated by decomposition of flesh. Flies not only visit and transport the pollen of the
stepelia plants, but they even lay their eggs in the flower as if it were carrion.
The most bizarre pollination systems?
Possibly the most bizarre imitations are those occurring in orchids which attract insects
through sexual impersonation. One orchid species produces a flower that closely
resembles the form of a female wasp including eyes, antennae and wings and an odour
(pheromone) that is emitted by the female wasp during the mating period. Male wasps
are deceived into copulating with the flower and so doing get covered with pollen before
carrying on to the next bogus female wasp which will receive and deposit more pollen.
Total dependence: Yuccas and Moths
Sometimes plant and insect become totally independent on each other. Yucca plants
which produce rosettes of cream flowers attract a small moth with a specially curved
proboscis that enables it to gather pollen from the yucca stamens. It moulds the pollen
into a ball and the carries it off to another yucca flower. First it goes to the bottom of the
flower, pierces the base of the ovary with its ovipositor and lays several eggs on some of
the ovules that lie within. Then it climbs back up to the stigma rising from the ovary and
rams the pollen ball into the top. The plant has now been fertilized and in due course,
ovules in the base of the chamber develop into seeds. Those that carry the moth's eggs
will grow particularly large and be eaten by the developing caterpillars. Those ovaries
without caterpillars will not be eaten and permit the yucca to propagate itself.
Assignments
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
Discuss the form of adaptations required by the first invertebrate animals which made the
transition from life in the sea to life on land.
Describe how the first plants and animals evolved and became dependant on each other.
Describe the diversity of flying insect life that has evolved.
THE SWARMING HORDES
Insects: Almost three-quarters of the animal diversity
It is estimated that there are three-times as many insects as all other species of animal put
together. Too date more than 700 000 species have been described, probably only a
fraction of those still waiting to be discovered and described. Insects have invaded all
aspects of terrestrial life. There is no known species of plant that is not attacked by an
insect species. Insects may still remove up to three-quarters of crops grown by people in
Africa.
A Tripartite body plan
This success, diversity and variation is all achieved with a tripartite body plan consisting
of a head bearing a mouth, mouthparts (modified jointed appendages) and most of the
sense organs; a thorax filled with muscles which operate three pairs of legs, and usually
one or two pairs of wings and an abdomen which contains the organs for digestion and
reproduction. All three sections are enclosed within an external skeleton made
principally of chitin, a substance that is chemically similar to cellulose but has both
flexibility and permeability.
Chitin: A secrete ingredient for success?
Insects may cover this chitin with sclerotin to make it hard so as to create armour (e.g.
beetles) and produce mouthparts sharp and tough enough to gnaw wood and cut metals.
It is the responsiveness of this chitinous exoskeleton to evolutionary change that has
permitted insects to diversify. Leg morphology is easily modified to propel an animal for
more than two-hundred times its own length, or to create broad oars to row across the
water or thin hair tipped stilts to stride across the surface of water. Many limbs may
carry special tools moulded from chitin such as pouches to hold pollen, combs to clean
compound eyes, spikes to act as grappling irons and notches to create sounds.
Issues with an Exoskeleton
This exoskeleton still restricts growth and needs to be shed periodically, and a new shell
created to replace it. Primitive insect forms like bristle tails and springtails do not change
their shape significantly with successive changes of the exoskeleton, but this does permit
them to increase their size. The early winged-insect forms (cockroaches, cicadas,
crickets and dragonflies) similarly moult without significant changes to the body shape
with the exception of acquiring wings in the final moult (although damsel flies take two
moults to perfect their wing structure. Even when insects adopt significantly different
environments for their early and later lives, their body structure is recognizably similar.
A “Larval Stage” leads to success
The more advanced insects, undergo structural changes that make it impossible to link the
larvae with the adult forms without observing the changes for oneself. In this way
maggots change to flies, grubs to beetles, caterpillars to butterflies. Since the earlier form
is not required to breed, it has no sex organs and does not need to attract a mate, it needs
no wings to fly, since it has probably been placed in environment that is near optimal for
its development. Such larvae consume great quantities of food and therefore need
efficient jaws and digestive systems. Since these larvae have no exoskeleton, the
locomotion is generally slow and they have little protection against predators. This is of
little consequence to grubs and maggots which live inside the tissues of plants and
animals, but caterpillars which feed in the open frequently use camouflage techniques to
resemble a twig, a bit of leaf or a bird dropping. Other defences may exist including
squirting formic acid, having an unpleasant taste, or covering the body with unpalatable
or even poisonous hairs. Some animals possessing chemical defences advertise this with
a conspicuous coloration which warns potential predators of this fact. Other species with
no such defences mimic the colours of those species possessing chemical defences and
thereby avoid predation. The larval stage of some insects may last a considerable length
of time, with grubs of beetles boring through wood for up to seven years before
developing into adult forms.
Larva: Clothed in silk
Only the larvae of insects possess silk glands which have been used to construct
communal tents, to extrude life-lines guiding them over plants and getting them from one
twig to another. These silk glands are also used to construct a cocoon in which further
development takes place (e.g. moths).
Metamorphosis
Caterpillar larvae undergo one final development before becoming adults
(metamorphosis). The larvae sheds its skin and develops a hard shell around itself and is
now called a pupa. The pupa has spiracles for breathing, and its tip may twitch
sporadically. When the larva first developed from the egg cells it was segregated into
two groups. Some of these cells divided after a few hours but remained generalized in
form, whereas other cells continued to build the caterpillar body. After the larva hatch
these cells enlarge with no further cell division. Within the pupa the original giant cells
of the caterpillar are used to feed cell division of those other group of cells which are reorganizing the new body of the butterfly.
An insect’s first flying lessons
The butterfly exits from its pupa head-first and immediately pumps blood into the
network of veins, and the limp wings begin to take their shape. Now the blood is
withdrawn from the veins of the wing and the veins harden to create rigid struts, at which
point the wings are ready for their maiden flight. All further growth has ceased, and they
use food collected when they were larvae and stored as body tissue. Some species like
Mayflies do not even have mouthparts. In this adult stage their primary function is to
find a mate. However, unlike the larvae, butterflies have large compound eyes, that are
sensitive to most wavelengths. The colours and patterns on their wings are created by
tiny scales which have pigments and microscopic structures that split light, reflecting
back a narrower range of wavelengths. These colourful wing patterns may be useful for
species recognition and mating.
Insects: Finding your soul mate
Other insects use sound to summon prospective mates (e.g. cicadas, crickets and
grasshoppers). Sound in Grasshoppers is produced by sawing the notched edge of their
hindlimb against the strengthened vein of the wing. Cicadas have an abdomen which
contains two chambers, the inner wall of each chamber is stiff and when it moves in or
out it makes a click. In the abdomen behind there is a large muscle which can pull the
wall back 600 times a second and the noise created is amplified in the abdomen using a
hollow vibrating plate and two hollow rectangular resonators. Sound is received from
eardrums on either side of the thorax in cicadas, but grasshoppers use a membrane
situated between two deep slits along their first pair of thighs. With each species having
a unique sound, they can recognize and attract appropriate mates of the same species.
Moths use a third sense, smell to attract mates. Females produce chemical compounds
called pheromones which male moths are able to detect with their large, feathery
antennae.
An Insect’s approach to rearing your young
Using sight, sound and smell adult insects attract their mates and copulation can take
place. The female then lays her fertilized eggs in an environment suitable for her larvae
to exploit. Butterflies seek suitable plants for the young caterpillars, beetles lay eggs in
pellets of buried dung, flies deposit eggs in carrion, wasps catch and paralyse spiders and
lay their eggs on them so that the young larvae can feed on the spiders. Ichneumon
wasps use a beetle grub to lay her eggs, with the hatched larvae eating the grub alive.
Insects: Limitations for size
The only apparent limitation to insect forms appears to be size, the largest moth is 300
mm in wingspan, the heaviest beetle is 100 g in mass. This reflects insects reliance on
tracheae and spiracles, without an effective pumping system to force the air down. Some
insects do use contractions of the abdomen to improve circulation and have tracheae that
swells into thin-walled balloons which can be depressed and expanded.
Insect’s approach to size matters
Insects have, however, transcended even these limits in size, by creating highly social
community living, an example of which is the termite hill. The termites that inhabit these
colonies in effect all belong to the same family and were derived from the same parents.
The body plan of these animals is so modified that they are incapable of an independent
life, the workers are blind and sterile, the soldiers are armed with jaws so large that they
cannot forage and have to be fed by workers. At the centre of the colony is the queen
who is encaserated within earthwalls and has an abdomen that is distended to 120 mm
and produces eggs at a rate of 30 000 per day. She is fed by workers and her eggs
collected for incubation elsewhere in the termitaria. The only other sexually active male
is the wasp-sized king who stays by the queen and is also fed by the workers.
Chemical Communication
An effective communication co-ordinates these individuals and is generally induced by
chemicals, although soldier termites sound an alarm by beating their large hard heads on
the passage walls. Other chemical hormones (also called pheromones) in effect circulate
instructions and dictate both actions and the development of the colony. All members of
the colony exchange food and saliva with each other by way of the workers who also
gather the excrement in order to reprocess it for food to obtain the maximum nutrition
from it. The queen produces a pheromone, which is collected by workers and circulated
through the colony. Although the queen termite produces both sexes, the queen's
pheromones inhibit development maintaining them as sterile, wingless and blind
(=workers). How soldier termites are produced is unknown (either specialized eggs or
preferential treatment of larvae). Soldier termites have their own unique pheromone
which is circulated through the colony and reaches the queen who probably regulates
their numbers.
Establishing a new colony
At certain times, however, the queen does not suppress larval development and sexually
mature winged termites of both sexes are produced and leave the colony by way of splits
in the termitaria and take-off ramps. With the commencement of the first rains the flying
termites pour out. Following dispersal and pairing the wings fall off and the male and
female termites excavate a new nest. These become the royal pair for a new termite
colony. Within the small royal cell they copulate and produce the first larvae which have
to be feed by the parents until they are able to forage independently and continue with the
construction of the new nest and founding of a new colony.
The termite towers
Termites construct fortresses that may contain several tons of mud and contain several
million inhabitants. Ventilation and temperature control are therefore critical for survival
of these communities. Around the margins of these termitaria are tall, thin-walled
chimneys. As the sun warms the walls of these chimneys air becomes hotter than the air
inside of these nests, the air in the chimneys therefore rises and with it draws air from the
termitaria. Since the chimney walls are thin and porous, oxygen from the outside diffuses
in. This air rises to the top of the nest and re-oxygenates the colony. In very hot weather
the workers descend in tunnels that go deep into the ground water, and carries back a
crop full of water that wets the wall and lowers the temperature through evaporation.
Wasp and Bee nests
Wasps and bees also have a colonial lifestyle comparable to termites. Wasps show
transitions in degrees of colonialism. Some hunting wasps are entirely solitary, with a
female wasp constructing a nest of mud in which she lays her eggs and stores a provision
of parasitized wasps. In other species the female wasps remain by the nest and brings
daily food to the larvae. In other wasps the females construct nests next to one another,
some of the nests are abandoned and wasps may join other wasps in constructing theirs.
Eventually one female wasp assumes dominance and lays eggs in the amalgamated nests
with other wasps building more cells to house larvae and collect food.
Dance of the bees
The evolution of community living is also elaborate in bees. A single queen bee is also a
specialist egg-layer, that is supported by worker bees. The community is also bound by a
system of chemical messages (pheromones) but they also use a dance behavioural pattern
to communicate to each other. When a worker bee returns from a new nectar laden
flower a dance behavioural sequence is initiated. If the source is nearby, the bee
performs a simple round dance, alternatively circling in clockwise and counter clockwise
directions. The other bees are excited by the dancing scout and follow it outside, and
they find the food by orientating to chemical signals present on the scouts body. If the
food source is more than 80 m from the hive, the scout expresses this in its dance with a
distance and direction of the source. A waggle dance traces two semi-circles with a
straight run between them. The food's distance is described by sounds and wagging
movements executed during the straight run. The further away the food lies the longer
the sounds last and the more slowly the dancing bee waggles its abdomen. The angle of
the straight run describes the direction of the food source in relation to the sun. A run
straight up the hive wall denotes a location directly towards the sun. When food exists at
an angle to the left or right of the sun, the bee runs at the same angle to the left or right of
the vertical. Even on cloudy days these dances are effective, because bees detect the
sun's location by the analysis of polarized light. The interpretation of these behavioural
patterns have been debated, since inexperienced workers do not seem to be as efficient at
foraging for pollen whereas experienced workers are almost always successful. The
returning scout bee is usually covered with pollen and some researchers feel that the bees
respond to olfactory signals rather than the interpretation of behavioural patterns.
Insect and plant cohabit
The most complex and highly evolved forms of colonialism in the insect world are those
created where the organisms (wasps, bees and ants) live within plants, stimulating the
tissue of their host to provide them with custom-built homes, by growing special galls,
hollow stems or thorns with swollen bases. The leaf-cutting ants of South America build
vast underground nests and have expeditions via long tunnels. They may remove entire
trees (leaves, roots and stems) converting the material to pulp in their chambers which
forms a compost for cultivating edible fungi.
Imperialism- Insect style
Most ants, unlike termites and leaf-cutting ants are carnivorous. Such ants may prey on
termites, devouring the workers and larvae. Yet other ant species make slaves of other
ant species, by raiding a nest and collecting the pupae and rearing them to be slaves. Yet
other carnivorous ants do not make nests, but march in great masses. Such an army of
ants may forage on animals caught in its wake for several weeks. When the larvae
produce pheromones they are circulated within the army and keep it on the move, when
the larvae pupate, no pheromones are produced and the army clusters around roots of a
tree. Individuals clinging to each other create a living nest of tunnels and chambers. The
queen starts producing eggs which hatch into larva, while soldier ants emerge from their
pupae. The next generation of larvae produce pheromones which stimulate the army to
move-off.
Assignments
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
Describe the forms of social life that occur in insects.
Describe the signals used by insects to attract a mate for sexual reproduction.
Many insects have long larval stages and have larvae that differ significantly in
morphology from the adult forms. Discuss this statement giving suitable examples.
THE CONQUEST OF THE WATER AND THE BIRTH OF THE
VERTEBRATES
Although animals without back-bones (invertebrates) are more abundant numerically and
more diverse in species variety, they have never been able to reach the sizes that animals
possessing a backbone can (vertebrates). All animals with a back-bone and some with a
stiffened cartilage rod called a notochord belong to the phylum Chordata. One of the
most primitive members of this group are the Tunicates or sea-squirts. Although the
sessile adult phase bears a superficial resemblance to a sea-anemone (Coelenterata), the
rest of the vertebrate fauna was derived from such a simple organism. Evidences for this
ancestry is in the tunicate's larval stage which resembles a tadpole and has the following
features which are shared with all other vertebrates:
1. Perforations in the wall of the pharynx, or pouches that suggest ancestral perforations.
2. A nerve cord dorsal to the gut that is tubular and reflects its embryonic development
from a tough piece of ectoderm that became roofed over.
3. A stiff rod called a notochord that supports the nerve cord from below.
The larva is short and the animal attaches itself to a rock and loses its tail and becomes a
sedentary filter-feeder.
Free-living chordates
The next most advanced animal in the evolutionary tree is the lancelet or amphioxus,
which is more fish-like in appearance but also has a stiff rod or notochord. This animal is
50 mm in length has a well-developed segmented muscular system that allows it to bury
itself quickly in the sand. This animal has no clearly defined head region, only a lightsensitive spot at the anterior, no heart only a few pulsating arteries, no fins or limbs but
only a slight dilation at the hind end. The strong muscles rhythmically contract against
the notochord and the animal is propelled forward in a series of waves. These lancelets
and the larval tunicates therefore resembled each other and considerable argument arose
as to which form was the most direct ancestor for the rest of the vertebrates. The
embryonic development of many animals often reflects their phylogeny or ancestry.
Consequently, larval termites resemble bristletails and larval horseshoe crabs resemble
the segmented trilobites. It was therefore argued that the lancelet was the ancestor to the
tunicates,
Fossil evidence for the first chordates
However, fossil evidence in the Burgess shales (550 million years ago) included a finned
or backboned swimming animal similar to the living lancelet called a Pikaia and was the
predecessor to a group of fish-like animals that were jawless (apart form modified
parasitic forms) and consequently could only feed on micro-organisms and small animals.
These animals belong to the class Agnatha.
A jawless predator
Another larva provides evidence for the next step in the vertebrate evolution. This is the
Lamprey, class agnatha (= without jaw) which have larvae that are also jawless, blind and
without fins except for a fringe around the tail and very similar to lancelets. These larvae
were once thought to be adult creatures called ammocoetes. The adult lamprey is very
fish-like except being jawless. It possesses the beginnings of a backbone in the form of
cartilaginous elements. They also have a clearly defined head, with two small eyes, a
single nostril leading to a blind sac, and on either side of the neck a row of gill slits. The
mouth is a circular disk and possess a tongue with sharp spines. It is with this disk that
the lamprey clamps itself on to fish which it parasitizes.
Ostracoderms – an extinct group with heavy armour
Within the agnathan group were other small fish-like animals called ostracoderms and
possessed heavy armour-plating which may have originated from deposition of salts
derived from their food. This marks the first presence of bone, the material that was
destined to influence much of the evolution of the vertebrates. These early bony plates
may have provided protection against the large (2 m) sea scorpions that co-existed at the
same time. Heavy deposition of salts in the head region have permitted remarkable
fossilisation of these animals where the structure of the brain and nerve and blood vessels
can be identified. In addition a balancing mechanism composed of two arching tubes at
right angles to the vertical plane has been recognized. The liquid within these tubes,
moved over the sensitive inside surfaces enabled these animals to be aware of their
posture in the water. These animals dominated the freshwater streams 500 million years
ago and the largest representatives reached 600 mm in length. The single median fins
down the midline of their back provided stability in locomotion, but only the group
Cephalaspidomorpha had paired lateral appendages that may have had a similar function
to the lateral fins of true fishes. All these animals had gills located in pouches
Protofish and internal bony skeletons
An important development in one group of protofish was the development of bony rods
stiffening the pillars of flesh between the gill slits. The first pair of which hinged forward
and were supported with muscle tissue, and produced the first jaws. The evolution of
jaws permitted fish and their descendants to utilize larger and harder food, and thus
enabled them to become adapted to many new and diversified ways of living. This
advance was of sufficient importance so that fish and tetrapods (four-legged animals) are
together called gnathostomes (= jaw + mouth). Some of the bony scales in the skin
which covered these animals enlarged and became the first teeth. Lateral flaps of skin
evolved into the first true fins and their swimming skills improved. These animals were
called Placoderms and may have pioneered the gas bladder for vertical movement in
water and eventually evolved into lungs. The most impressive of the placoderms was the
Arthrodira which reached 9 m and possessed large jaws equipped with serrated teeth.
Developing some backbone
One of these animals (Acanthodii) were acquiring an internal bony skeleton and included
the beginnings of a vertebral column running longitudinally through the body and
encompassing the primitive notochord. These were the probable ancestor to the bony fish
we know today and possessed a streamlined body, large lateral eyes and wide mouths
with numerous teeth. Their heads are bony and their small scales are thick and hard, but
unlike the placoderms they did not have armour. The numerous lateral fins of these
animals are unique in that each has a thin membrane supported at its leading edge by a
long stout spine.
Re-inventing the cartilage skeleton
At this time a pronounced split appeared in the fish dynasty, with one line of animals
losing all their bone and developing cartilage, a softer more elastic and lighter material.
The descendants of this are the fish belonging to the class Chondrichthyes and
represented by sharks (orders Galeomorpha and Squalomorpha), rays (order Batoidea)
and chimeras (order Chimaerida). Although this lightened them they would still need to
continue to swim or they would sink. Swimming is still accompanied by a powerful
thrash of the tail and pectoral fins which prevent them from diving nose down. Since the
pectoral fin is stiff these have less mobility than the pectoral fins of the bony fish. Some
of these fish rested by sinking to the sea-floor, and one group has adopted such a position
on a semi-permanent basis (rays and skates). As a consequence they have become
greatly flattened with pectoral fins expanded into undulating lateral triangles which they
use for locomotion and the muscle in the tail is almost completely lost (although it may
bear a poisonous spine at the end). Rays and skates are not as fast swimming as sharks,
but this is of less importance since they feed on molluscs and crustaceans.
Sharks and Mantas
Sharks have mouths on their undersides and water passes through the mouth and over the
gills and out through the slits. With bottom-dwelling mantas and skates this would cause
mud to get into the gills, so instead, they have two openings or spiracles on the upper
surface of the head that take in water and lead it straight to the gills. It is then expelled
on the underside through the gills. One kind of ray, the manta has reverted from bottomdwelling to surface dwelling, using the large lateral extensions to remain afloat.
Swimbladders: refinement
The other group of fish which retain bone in its skeleton, also had to overcome weight
problems in the water. Early fish with heavy bone-based scales, colonized shallow
lagoons and swamps which had warm, poorly oxygenated water. The bichir
(Polypterus)(order Polypteriformes), a heavy scaled fish occurring in Africa indicates
how these early fish overcame such problems. These animals rise regularly to the surface
and take a gulp of air which goes into a pouch leading off the top part of the gut. A
concentration of capillaries in the walls of the pouch absorb the gaseous oxygen. These
air-filled pouches which were the first lungs also provided buoyancy and the ability to
float without using the tail and eventually evolved into swimbladders. With the ability to
absorb gas from the blood systems there was no need to collect air from the surface and
the connecting tube to the throat became no more than a solid thread. The diffusion of
gases into and the expelling of air out of the swimbladder would permit a precise means
of vertical control in the water. The pectoral fins would provide refinement to this
control. However, swimming skills were improved still further with increased tapering of
the twin-bladed symmetrical tail that is driven by banks of muscles on either side of the
backbone. Streamlining was enhanced with reduction of heavy scales into smaller tightly
fitting ones that overlap like tiles of a roof and are covered by slippery mucous, and
pectoral and pelvic fins being able to fold back into depressions in the lateral sides of the
fish. The respiration using gills was further refined with the development of a movable,
bony operculum which by inducing negative pressure forces water over the gills and
improves respiration.
The diversity of morphological forms is testimony to the success of the group. One
group, the flying fish (order Atheriniformes) leap out of the water and glide hundreds of
metres in the air using the elongated pectoral fins. This may be an anti-predator tactic.
Garfish (order Lepisosteiformes) have pectoral fins that have become filmy skulls
rotating slowly bach and forth which permits them to hover in water. Dragonfish (order
Pegasiformes) have lateral fins modified into defensive mechanisms with each ray barbed
with poison.
The swimbladder has released fish from weight problems, and therefore, some like the
box-fish (family Ostraciontidae) and sea-horse (order Gasterosteiformes) have regained
armour.
Down the flanks and around the head of fish runs a series of pores, connected by a canal
running just below the surface. This is called a lateral line and enables the fish to detect
differences of pressure in water. As a fish swims, it creates a pressure wave ahead of it,
when this wave meets another surface the fish can detect pressure changes created by this
surface. It is this ability that permits them to detect other fish and to polarize themselves
into swimming in shoals. Vulnerability to predators is thought to be reduced by shoaling.
Fish also have an acute sense of smell and detect minute changes in the chemical
composition of water. This sense of smell may guide fish to food. Fish also detect sound
with the addition of a third canal (in a horizontal plane and below the sac) which
supplements the two semicircular canals that are found on either side of the skull of the
lamprey. All three canals and the sac have very sensitive linings and contain small
calcium particles which move and vibrate. Sound waves, which travel better in water,
penetrate the semicircular canals without the need for passages which are required by
terrestrial animals.
The eyespot of the lamprey is primitive compared with the bony fishes. The eye of the
bony fish and higher vertebrates is a closed chamber with a transparent window and a
lens in front and a photosensitive lining at the back (retina). The photosensitive lining
contains two kinds of cells, rods for distinguishing light and dark and cones which are
sensitive to colour. Sharks and rays lack cones and are unable to perceive colour; this
may reflect the lack of highly coloured examples within the group. Bony fish have both
types of cells in their retina, and are also characterized by vivid colours and striking
patterns. The Butterfly fish (Family Chaetodontidae) showing particularly diverse
colours and patterns which permits species recognition. Colour is also an important asset
in male fish during spawning. Such displays serve to chase other male fishes away, and
to attract female fish. Pigment granules diffuse within the skin as the fish become excited
and fights other rivals or to stimulate a female fish to lay her eggs.
Eyes of fish have become adapted in various ways to vision below and above water. The
archer fish (Toxotes jaculator) squirts fluid at an insect above the water and knocks them
into the water where they can be eaten. This required compensation since light bends as
it passes from water to air due to differences in density. Anableps has a horizontal
division across its pupils which effectively gives it four eyes, the two lower halves for
underwater use and the two upper halves for above water. Since fish can occur at great
depths (below 750 m) where there is no light, they may posses modified cells producing
luminescent chemicals which are activated rhythmically and may represent some form of
communication to the rest of the shoal. The whiskery angler fish Antennarius scaber
(order Lophiiformes) has a modified dorsal fin spine with an elongated thread at the end
of which are cells producing luminescence. This is used to entice other fish to explore the
light and be consumed.
Water that is covered with floating mats of vegetation is also turbid, and in such an
environment some fish have generated electricity from modified muscles in their flanks.
Electrical signals are transmitted almost continuously creating flow patterns of current in
the immediate vicinity. Any object encountered disrupts these flow patterns and the fish
perceives these changes through receptor pores located over the body. The electric eel of
South America Electrophorus electricus, although not a true eel, has additional body
tissues that produces a massive shock of waves with which it kills or stuns prey items.
From the jawless armour-laden prototype fish have evolved some 30 000 different forms
to occupy seas, lakes and rivers of the world.
Assignments
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
Discuss the development and modifications of the eye that have occured in the fish. Also
discuss why sharks and skates are generally dull-coloured, whereas many bony fish have
bright colours.
Describe the evolutionary transitions from the earliest chordate (e.g. amphioxus) to the
most advanced bony fish.
Describe the morphological differences that exist between the cartilaginous and bony
fish.
THE INVASION OF THE LAND
The first fish may have crawled onto land during Devonian times (350 million years ago)
and probably did so, in response to drying swamps. This required that two problems
must be overcome, how to move without the support of water and how to obtain oxygen
from air rather than water. The mud skipper (eg Periophthalmus sobrinus which occurs
in our mangrove swamps) suggest adaptations that ancestral fish may have developed in
their quest to colonize land. Each pair of front flippers has a fleshy base supported
internally by bones and is able to be used to lever the animal forward. Another animal
which showed the beginnings of limbs is the coelacanth Latimeria chalumnae (order
Crossopterygii), the living fossil that was thought to have been extinct for 70 million
years.
In order to breathe in air the mudskipper retains water in its mouth which swills over the
limning of it. The African Lungfish Protopterus (Order Dipnoi) can burrow into mud,
curls itself into a ball and secretes mucous which creates a parchment-like case around
the hole it has encased itself in and avoid desiccation during the dry season. The lungfish
has a pouch opening from the gut (similar to the primitive bichir fish) which functions as
a lung and extracts air from the tube it created when burrowing through the mud. By
flexing its throat muscles the fish draws air into its pouch which is supplied by numerous
blood vessels which absorb gaseous oxygen. With the termination of the dry season the
fish returns to an aquatic existence and breathes with its gills, but like the Bichir may take
gulps of air if a lack of oxygen develops.
Although all of these animals have been regarded as possible ancestors of the first
tetrapods which colonized the land, their skull morphology is unlike the first fossil
tetrapods which were amphibians. Neither Coelacanth nor Lungfish have a passage
linking the nostrils with the roof of the mouth, a characteristic feature of all land
vertebrates. However, another lobe-fin fish called Eusthenopteron, which only exists
today as fossils, possessed such a passage and well developed lobes. Careful
examination of the fins of these fossils revealed that the base of the lobe was supported
by one stout bone close to the body, two bones joined to it and at the terminal end a group
of small bones; an arrangement found in the limbs of land vertebrates. A link between
lobe-fin fish and amphibians has been found in the fossilized Ichthyostega found in
Greenland in 1938. The swamps through which such animals waded was thick with
horsetails and club moss trees which became fossilized as coal and also contained the
first fossils of the terrestrial vertebrate (tetrapods) which belong to the class Amphibia.
These animals had evolved only 50 million years after the first bony fish and reached
greatest expansion some 100 million years later in the Upper Carboniferous period.
Some of these early forms grew to four metres in size and possessed jaws spiked with
cone-like teeth. Today relatively few amphibians have survived, but they are
nevertheless distributed in tropical and temperate areas of the world and in a variety of
habitats. The modern amphibians differ considerably from their large ancestors. The
living amphibian that most resemble early forms are the salamanders and the newts
which collectively are called Caudata ("tailed ones"). The largest member of this group
comes from Japan and has a body length of 1,7 m (Megalobatrachus).
In general amphibians are only partly successful in their colonization of land, since their
limbs are short and they need to flex their body laterally in order to take reasonable
strides. Amphibian skin is permeable and in a dry atmosphere would quickly dehydrate,
they even do not have the mechanisms to drink water. A moist skin is also required to
supplement respiration, since the lungs are comparatively simple and not totally adequate
for its needs. These limitations restrict amphibians to moist environments. For
reproduction amphibians are also almost entirely dependent on water since, like fish, their
eggs have no water-proof covering and their larvae (tadpoles) are quite fish-like. These
larvae initially have no legs swim using a long tail and breathes using external feathery
gills.
The two life phases of some species of Caudata (entirely aquatic and semi-terrestrial)
have been used to exploit a greater variety of habitats. A Mexican salamander
(Ambystoma mexicanum) regularly changes from an aquatic form to a land form. If
there is a particularly wet season and/or the lake does not shrink greatly the larval stages
are maintained and the larvae may become as big, or bigger than the land-living forms. A
lack of iodine in the water may have prevented metamorphosis. Another species of
Caudata has reverted permanently to an aquatic life. It always breeds in a larval
condition and its external gills develop into branching bushes on either side of the neck.
Using a thyroid extract it can be induced to lose its external gills, develop lungs, and turn
into an animal that resembles a burrowing salamander that lives in Florida. However,
another species called the mud-puppy Necturus maculosus has reverted irrevocably to
water-living and has external feathery gills and very reduced lungs. The Greater Siren
(Siren lacertina) is more elongated, has lost its back legs altogether and also breathes
using gills. The Three-toed Amphiuma (Amphiuma means tridactylum), from southern
USA is extremely elongated with tiny legs that have no function and is known locally as
a Congo eel. This tendency to retain larvae characteristics in adult forms is called
paedomorphosis.
The abandonment of lungs and limbs, the cornerstone adaptations that permitted
colonization of the land is not entirely restricted to aquatic amphibians, but even occurs
in animals that live almost entirely on land. Such animals breathe through their skin and
the moist membranes lining their mouths. The elongated body forms permit maximum
surface area, but they nevertheless remain only a few centimetres in size and are
restricted to very moist environments.
Another group of Amphibians called the caecilians are also limbless, but are adapted to a
burrowing existence and almost resemble earthworms. Their anatomy is so different
from the salamanders that they are classified in the order Gymnophiona. They have
several primitive features such as the retention of small scales in the skin and a very short
tail. The solidly build skull, the elongated body comprising as many as 270 vertebrae, no
internal girdles for supporting limbs and blindness (compensated by having extendable,
sensory tentacles) are all adaptations to a burrowing existence. However, they are
carnivorous and have mouths with a large gape.
There are about 300 species in the order Caudata and 160 in Gymnophiona, but the most
numerous group of amphibians belongs to the order Anura (tail-less ones) with about
2600 species. The Anurans include frogs which are generally characterized by smooth,
moist skins and inhabit moister environments, and toads which have a drier, warty skin
and often occur in drier environments. Unlike the members of Gymnophiona, this group
has shortened the body and have even fused vertebrae together and have developed their
hind legs enormously to become accomplished jumpers. The Goliath Frog (Gigantorana)
can achieve 3m and the tree-living frog Rhacophorus reinwardti can achieve fifteen
metres by gliding. To do this they increased the size of toes and with it the web of skin
that unites them to form a parachute on each leg.
Jumping represents a major way of escaping predators. Since amphibians are generally
soft bodied, they are sought after as food items by larger predators, however, many rely
on having a cryptic coloration of green camouflaged with blotches of brown and grey.
The common European Toad (Bufo bufo) inflates its body and stands on its toes to
appear as large as possible and thereby discourage any potential predator. More active
defence occurs in the fire-bellied toad whose mucous which keeps the skin moist is also
extremely bitter tasting. The poison arrow frogs (Family Dendrobatidae) include some
species whose mucous is lethal to mammals and local people used its poison to tip their
arrows. Such defences are of little value if their attacker dies after they themselves have
been eaten, and therefore are often accompanied by bright warning colours and patterns
(aposematic coloration).
Amphibians are all carnivorous, and indeed some are quite formidable such as the horned
toads (Family Leptodactylidae) which can easily prey on a nestling bird or a small
vertebrate such as a mouse. Although these frogs have a large mouth serviced with sharp
teeth, their purpose is for defence and to grip the prey item, but does not help with
breaking the prey item into smaller parts for digestion. Most amphibians are smaller and
restrict their prey items to invertebrates. For this purpose an extendible tongue attached
to the front of the mouth has evolved. The tongue is sticky at its tip and when it is
flipped out it adheres to the small prey item which is bodily brought back into the mouth.
The tongue helps with swallowing since it produces mucous which lubricates the food
before passing it into the gut.
Amphibian eyes are fundamentally similar to their fish ancestors, however, they do
require a membrane that can be drawn across the eyeballs to keep the surface clean.
However, the mechanisms that fish use to perceive sound using resonances generated in
their swimbladder will not work in air, and consequently ear drums have evolved to
detect sound waves. With their increasing ability to detect sound waves frogs have also
developed the ability to produce sound using the huge swelling of their throats to amplify
the sound produced by air blown from their lungs over simple vocal cords. Such sounds
are unique to individual frog species and are used during courtship (a prelude to mating)
and to recognize frogs of the same species.
Mating for most amphibians still takes place in water, with the males grasping the
females and fertilization taking place externally, with the sperm cells swimming to the
egg cells. Large numbers of eggs are produced to offset high mortalities of eggs and
tadpoles. Other frog species have a different strategy whereby comparatively few eggs
are laid, but considerable parental investment protects them from predators. Some
tropical pond-dwelling frogs (e.g. Dendrobates pumilio and Osteopilus brunneus) find
safety for their tadpoles by depositing eggs in centres of plants such as bromeliads which
create small reservoirs of water in forests where the rainfall is high. These sites are safe
from aquatic predators. In Dendrobates pumilio males and females divide parenting
duties; males guard the eggs until they hatch (10 to 12 days) thereafter the females
assumes care for the young. The females begin by transporting each newborn tadpole to
a bromeliad, at the base of which a small pool of water has collected. Although protected
from desiccation and predation, the tadpole has no food supply and is entirely dependant
on its mother for nutrition until it metamorphoses into a froglet which takes six to eight
weeks. In Brazil, another small frog builds its own ponds at the margins of forest pools,
constructing a crater ringed with low mud (100 mm in height). The eggs are laid and the
tadpoles stay in their exclusive water residence until the rain raises the level of the main
pool and floods the crater created by the parent frogs.
When the first amphibians appeared, the terrestrial environment would have been a much
safer site for the development of their offspring than an aquatic environment which has
many predators (especially fish). As a consequence anurans evolved mechanisms to
exploit the terrestrial environment for the breeding of its young. The midwife toad
Alytes obstetricans (Family Discoglossidae) of Europe lives in holes close to water and
mates on land. After fertilization the long strands of eggs are twisted around the hind leg
of the male toad. The male carries them around until the tadpoles are ready to hatch and
then takes them to water. The South American Centrolene frogs defend calling sites
which are leaves overhanging streams. Such sites are where the eggs are laid, and parent
frogs attend the eggs until they hatch and the tadpoles fall into the water below. In Africa
some species of frog (e.g. Chiromantis) breed on branches of trees above ponds. The
female excretes a liquid which is beaten into a ball of froth by the male frog. The eggs
are then laid and the outside surfaces of the froth harden into a crust which retains
moisture. The female frogs may bring up additional moisture to the nest. The eggs hatch
and tadpoles develop within the hardened froth. The tadpoles are released when the
lower part of the froth ball liquifies and they fall into the water below. Frogs producing
foam nests occurs in five anuran families (Rhacophoridae, Hyperoliidae,
Myobatrachidae, Hylidae and Leptodactylidae). In some tropical American frog species
(Eleutherodactylus) a considerable yolk is provided in each egg which makes it possible
for all stages of larval development to take place within the eggs and fully developed
froglets emerge directly from them, this is term direct development and occurs in nine
families of frogs.
Many frogs invest considerable parental effort. The toad Pipa carvoelhi have a normal
anuran copulation in water, however, only a few eggs are fertilized and the male frog
using his webbed hind feet gathers the eggs and spreads them onto the female's back.
This process is repeated until about a hundred eggs are gathered. The skin below begins
to swell and embed the eggs and a membrane develops over the top of the eggs and
covers them. After 14 days the female's back is rippling with the movements of hatched
tadpoles. After 24 days the young break holes and are released from their mother. The
frog species Gastrotheca have brood patched on their backs where fertilized eggs develop
into tadpoles. When the tadpoles are ready to be released the female finds a shallow
pond to sit in and deposit her young. In Gastrotheca ovifera more yolk is provided with
the eggs, and the tadpoles remain until they are froglets before leaving the pouch. Egg
brooding is usually done by the female frogs, although in the Australian hip-pocket frog
Assa darlingtoni it is the male who broods them.
The stage at hatching for egg-brooding frogs, also called marsupial frogs, is determined
by the amount of yolk in each egg, which in turn reflects the number of eggs produced
per clutch. Species in which the eggs hatch as small tadpoles produce 100 or more ova,
each ca. 2 mm in diameter. Where froglets emerge directly, only about six ova are
produced, each ca. 10 mm in diameter.
A West African frog Nectophrynoides has taken parental effort even further. These frogs
have internal fertilization with the fertilized eggs retained in the oviduct. Tadpoles
develop, complete with mouths and external gills and they feed within the oviduct on tiny
white flakes excreted from its walls. After nine months development which is coordinated with the arrival of the first rains, the female gives birth, by bracing her body
against the ground with her forelegs and then inflating her lungs to full capacity which in
turn swells the abdomen and squeezes the young out by pneumatic pressure. The tiny
frog Rhinoderma found in southern Chile deposits her eggs on moist ground, the males
sit in groups around the eggs and guards them. When developing eggs move within the
gelatinous coats, the males take the eggs into their large vocal sacs where they continue
developing until they are fully-formed froglets. Phyllobates subpunctatus, one of the
South American poison dart frogs (Family Dendrobatidae), also lays the eggs on moist
ground in close proximity to a guarding male frog. When the tadpoles hatch they wriggle
themselves onto the male's back, where his copious quantities of mucous keeps them
attached and prevents them from drying out. These tadpoles have no gills, but obtain
oxygen by absorbing it through the skin of their body and from the surface areas of their
greatly enlarged tails.
However, the most bizarre form of parental care is the Australian frogs Rheobatrachus
silus and Rheobatrachus vitellinus. In these species the female frogs swallows the eggs
after fertilization and broods them in her stomach for six weeks. Such a breeding system
presents interesting problems, such as how the eggs and hatched tadpoles escape being
digested within the mother's stomach? The nurturing females appear to cease feeding
during the breeding period. The production of hydrochloric acid and pepsin are halted in
the stomach by a hormone-like substance prostaglandin E2 which is secreted by the egg
capsules and then by the tadpoles. With this shutting down of normal stomach activities,
the stomach's digestive functions are transformed into that of a protective gestational sac.
The eggs, which range from 21 to 26, are relatively large, ca. 5 mm and rich in yolk.
Consequently, the tadpoles do not need an external source of nutrition but feed
exclusively on yolk throughout their six-week development period. During birth the
female's oesophagus dilates in a manner analogous to the vaginal canal of mammals, and
the young froglets are propelled from her mouth. Within a few days after expulsion of
the young, the stomach begins to function again as a digestive organ, and the frog
resumes feeding. Unfortunately neither of these two species has been found recently, and
it is sadly concluded that these interesting frogs are now extinct.
From these patterns it is clear that size and complexity of parental investment reflects
clutch size. Another trend is that development time appears to reflect climatic conditions.
Frogs within tropical regions develop relatively rapidly, sometimes spending only two or
three weeks in the tadpole stage, whereas those living in cool temperate climates develop
much more slowly. One such species is the spotted frog, Rana pretiosa, which lives in
the cold streams that cascade down the Rocky Mountains. Because the cold water in
which the frogs live slows their metabolism, more than one year is needed to produce
fully yolked eggs, and the females lay eggs only once every two or three years. Tadpoles
also metamorphose more slowly in cooler areas. For example bull frogs in northern USA
(Rana catesbeiana) typically spend two years in the tadpole stage and another species
Ascaphus truei needs three years to reach adulthood.
In arid habitats, development is limited not by temperature but by moisture. One
example are the rainfrogs Breviceps (Family Microhylidae) which lives in arid regions of
Africa. These animals only emerge above ground during heavy downpours. Although
much of the biology of this elusive group of frogs is unknown, it appears that they form
pairs during the breeding season. Adults emerge from their underground burrows and
absorb rainwater through their skins, thus replenishing their body fluids. In particular the
bladder is filled with water. The male is far smaller than the female and is unable to
clasp the female in order to copulate with her. Instead the male glues himself to the
female's back. With the male riding on her back, the female burrows into the ground and
proceeds to lay eggs that are then fertilized by the attached male partner. Periodically the
female wets the eggs from her extended bladder, keeping them moist until the froglets
hatch. This breeding process takes place on only one or two nights per year, when there
is a sufficiently heavy downpour. Once fertilization occurs, growth proceeds rapidly.
The Spadefoot toads (Scaphiopus) in southwestern deserts of the USA, have tadpoles that
develop into frogs in less than two weeks. Such rapid development is necessary in a
habitat where the water will only last for a few weeks.
The zenith of amphibian's adaptations to minimize their dependence on water under arid
conditions is the water-holding frog, Cyclorana which inhabits the central desert regions
of Australia. During the brief and infrequent periods of rain these frogs feed on the flush
of insects, they mate and lay their eggs in tepid shallow pools of water, the eggs hatch
and tadpoles rapidly develop into froglets. As the rain soaks away the frogs and froglets
absorb as much water as possible and bury themselves deep into the sand where they
secrete a membrane around themselves to prevent moisture loss. They remain in this
condition until the first significant rains, which could be in several years time.
Assignments
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
Discuss the adaptations required to make the transition from an aquatic to a terrestrial life
using the amphibian group as an example. What limitations to a terrestrial life do
amphibians exhibit?
Why have some amphibians after evolving limbs then lose them to become limbless?
Support your answer with both terrestrial and aquatic examples.
The anurans have evolved a variety of reproductive strategies to reduce predation of eggs
and tadpoles and to exploit arid regions. Discuss such adaptions with special reference to
the amount of parental investments existing between different anuran species.
A WATER-TIGHT SKIN AND THE SHELLED EGG
The reptiles evolved from an early ancestral group of amphibians (Subclass
Labyrinthodontia) which have been extinct for 175 million years. Terrestrial
Labyrinthodontia had strong limbs, robust bodies. The first animal with a dry skin was
probably Seymouria which lived in the Permian (230 million years ago) is thought to be
the link between the amphibians and the reptiles and was probably the first animal to
have a hard-shelled egg that entirely freed its reproduction from water or extremely moist
habitats. All amphibians require a moist environment to survive and reproduce, but the
reptiles can occupy a dry environment due to their water-tight skin and the shelled eggs.
A modern example of how reptiles manage to survive in hot dry conditions can be found
in the marine iguanas (Amblyrhyncus cristatus) which are able to survive on barren larva
fields on the tropical arid islands of the Galapagos Islands. These animals bask in the sun
which helps raise their body temperatures without the risk of desiccation. Physiological
processes of animal's body, like other chemical reactions, are affected by temperature.
Up to about 40oC the higher the temperature the quicker the physiological processes and
the more energy they produce and the more active the animal can be. Neither reptiles nor
amphibians generate their heat internally like we do, but they draw it directly from the
environment usually in the form of solar radiation. The daily activity cycle of these
marine iguanas maintains the body at the most efficient temperatures. At dawn, when
ambient temperatures are lowest, they climb to ridges and expose themselves broadside
to the rising sun. As temperatures rise, the risk of overheating increases, the iguanas
respond by lifting their bodies off the ground and positioning themselves so that air
currents can pass below them. They can also pack themselves into the few shady places
that exist (such as rock crevices). The sea surrounding the islands is influenced by the
cold Humboldt Current, and is only entered to feed on green alga at the hottest time of
day (noon). During foraging their bodies would cool-off rapidly and they will need to
conserve as much heat as possible. These animals therefore constrict the arteries near the
surface of their bodies so that blood circulates only in the centre of the body.
Nevertheless body temperatures will drop up to 10oC before they have to return to land.
On land they stretch-out their bodies and absorb warmth from the black larva surfaces.
With the setting sun they again congregate on the ridges with their bodies broadside to
the last of the solar radiation for the day. These behavioral sequences maintain the body
temperatures close to 37oC, although it varies considerable more than in endothermic
animals (eg our bodies). Animals like iguanas are ectothermic since their body
temperatures tend to fluctuate. Endothermy has advantages since it permits greater
independence of the prevalent temperatures (eg can maintain activities at night and in
cold regions), but is energetically expensive. About 80% of daily calories is invested in
maintaining body temperatures constant in endothermic animals. In contrast an
ectothermic animal uses only 10% of the energy that an equivalent endothermic animal
would use. As a consequence they survive in desert conditions were endothermic
animals would have more difficulty surviving.
The ability to breed under dry conditions is achieved by a gland located in the lower part
of the oviduct and secretes a parchment-like shell which prevents desiccation of the shell.
However, the shell still needs to be supplied with sufficient yolk to support the
development of the embryo and the shell needs to be porous to enable oxygen to diffuse
through. Clearly fertilization of eggs needs to be internal (male reptiles therefore evolved
a penis) and to be completed before the shell is deposited. The Tuatara Sphenodon
punctatus (Order Rhynchocephalia) an ancient lizard that occurs on New Zealand has no
penis and males and females press their genital openings close together in order to
achieve internal fertilization in a way similar to amphibians. These lizards have another
amphibian feature that is an ability to be active down to 7oC, a much lower temperature
than for any other reptile. Fossilized bones of these creature have be dated to 200 million
years ago and may represent one of the most basic four-legged (tetrapod), tough skinned,
egg-laying ectotherm that was a predecessor to the great dinosaurs that conquered all
parts of the earth (except the polar region). The diversity of dinosaurs also included forms
that returned to the sea (ichthyosaurs and plesiosaurs).
The amphibians and earliest reptiles that evolved from them are often referred to as
cotylosaurs, and the stem reptiles themselves are called captorhinomorphs. Less than 100
million years after their first appearance, the captorhinomorphs had already divided into
three major divisions (Subclasses) based on the skull structure.
One lineage referred to as the Anapsids has turtles and tortoises (Order Chelonia) as
living representatives. The anapsids are characterized by a solid skull roof with no
temporal openings in the skull (viz. area behind the orbits of the eyes).
A second lineage referred to as the Diapsids produced the most diverse and spectacular
radiation of animals. Diapsids skulls primitively possess upper and lower temporal
openings behind the orbit of each eye. Living representatives of this group include
snakes and lizards (Order Squamata) and the Tuatara (Order Spheodontida). Extinct
forms within this group included the marine reptiles (ichyosaurs and plesiosaurs) which
are sometimes referred to as Euryapsids. However, the largest group within this lineage
are the Archosaurs (ruling reptiles) most of which are now extinct except crocodiles and
alligators (Order Crocodylia). Extinct Archosaurs included the famous dinosaurs
represented by two orders; Saurischia (lizard-hipped dinosaurs with a triradiate pelvis)
and Ornithischia (bird-hipped dinosaurs with a tetraradiate pelvis), the flying pterosaurs
(Order Pterosauria) and thecodonts the ancestral stock o f all archosaurs and birds.
Thecodonts were relatively small and often bipedal reptiles that had a resemblance to the
first crocodiles.
The third lineage refers to the Synapsids, which possess skulls with a single (lower)
temporal opening behind the orbit of each eye. These were the first group of reptiles to
colonize land during the Permian period and are referred to as the mammal-like reptiles.
Within the synapsids two orders have been identified. The primitive Order Pelycosauria
was characterized by animals which developed elongated spines from the vertebrae and
are commonly referred to as sailbacks. The most spectacular example was Dimetrodon
with vertebrae projecting more than a metre above the back at their highest point. These
vertebral spines supported a web of skin and probably served as a temperature-regulating
device that added a great area of skin surface for warming up and cooling off. The other
group of synapsids are classified in the Order Therapsida. The therapsids developed into
animals that resembled dog-faced tanks, for their limbs extended beneath their bodies,
rather than to the sides, they may have had fur, and exhibited specializations of bone and
teeth structure. These mammal-like reptiles suffered at least six distinct mass extinctions
during the last eight million years of the Permian. The survivors of each extinction
appeared to be more warm-blooded, to have more specialized jaws and teeth and to
possess a more efficient respiratory system. Although this line ultimately lead to the
evolution of the mammals, they came to dominate only fairly recently during the Tertiary
period (starting some 65 million years ago).
The Triassic period produced new forms of reptiles (archeosaurs), the ichthyosaurs,
crocodiles and the flying pterosaurs and the first of the dinosaur line, which were small
active animals about the size of a pheasant, many of which were bipedal and had
probably evolved high metabolic rates. Some may even have been covered with down
and later feathers, an evolutionary line that ultimately evolved into birds. These
dinosaurs remained in the shadow of the dominant group which were the mammal-like
reptiles. Towards the end of the Triassic at about 220 million years ago, a mass
extinction of the mammal-like reptiles may have facilitated the radiation of the other
reptile group (archeosaurs) during the next million years (Jurassic). The oldest true
dinosaur Eoraptor has been dated at 230 million years ago. This animal was a primitive,
small (ca. 1 metre), carnivorous dinosaur. Like the more recent and better known
dinosaur Tyrannosaurus rex, Eoraptor belonged to the saurischian group of dinosaurs
(lizard-hipped). Eoraptor is considered primitive because it has an exceptionally simple
jaw, and probably evolved shortly after saurischians and ornithischians diverged. Only
10 million years after Eoraptor the entire dinosuar group had already diverged, whereas
the other reptile groups, such as the crocodiles and mammal like reptiles were declining
rapidly.
The richest deposits of dinosaur fossils have been found in the midwestern states of North
America. Although recent excavations to Mongolia suggest that this region will provide
the greatest number of new fossil dinosaur species. Some of these dinosaurs were no
bigger than a chicken called Compsognathus, whereas others represent the largest land
animals that have existed on the earth such as Apatosaurus which measured 25 m long
and weighed at least 30 tonnes. A fossil dinosaur, Seismosaurus, unearthed in 1986
appeared to have been 43 metres long and weighed about 100 tonnes. Another dinosaur
called Ultrasaurus, may have been heavier still with an estimated mass of 150 tonnes.
The simple peg-like teeth of these animals meant that food, particularly plant material
such as the tough leaves of the cycads that existed at that time, had to be broken down in
the stomach. Mammals have specialized teeth that break-down and grind food to a
considerable extent before entering the stomach for further processing. Consequently
herbivorous dinosaurs probably needed large guts and even used stones (gastroliths) to
process their food and this may have been the reason for them becoming so large.
Carnivorous dinosaurs, like Tyrannosaurus, would also need to be large to prey on these
mega-herbivores. Many Apatosaurus fossil bones have teeth marks which correspond to
the fit of carnivorous dinosaur's jaws such as Allosaurus. Some scientists have reinterpreted such findings and have suggested that these apparently carnivores were more
likely to have fed on the large carcasses of the mega-herbivores.
The large size may also reflect temperature control. The bigger the body the more heat it
retains and the more constant the temperature will remain for the animal. Evidence for
warm-bloodedness is that the chest cavities are large enough to hold huge hearts, like
birds do today. The dinosaurs were known to migrate, and both their northerly and
southerly limits to these migration routes would have not been possible for a cold
blooded animal. The bone histology of dinosaurs (particularly the more advanced
thecodonts) suggest that they may have regulated their temperatures the way birds and
mammals do today. Specialized structures such as the parallel rows of plates on
Stegosaurus have been interpreted as additional temperature-control mechanisms. These
plates, although made of bone, are spongy and probably carried many blood vessels
which could either dissipate excess heat or absorb heat from the environment.
Anatomical analyses of many dinosaurs suggested that they were active, fast-moving
animals, and therefore probably possessing endothermic metabolisms. Finally the ratio
of predator-prey ratios of fossilized dinosaurs do not correspond to the expected ratios
assuming them to be ectothermic but does more closely resemble those of endothermic
mammals. It is recognized that endothermy may take several forms and that some
dinosaurs may have fell short of fully fledged endothermy. It has even been speculated
that Tyrannosaurus rex underwent three vastly different growth stages and may have been
equipped with a variable metabolism. A 2 metre juvenile would have been very active,
capable of scampering around like some groundbirds do today. By contrast, mid-sized
individuals, averaging 3.5 to 4.5 metres were probably less agile, and may have traveled
in packs. A fully grown 12 metre adult weighing 8 tons would not have been agile, and
may have reverted to a solitary life-style scavenging on carcasses. Further, all, but a few
highly specialized endotherms have some kind of heat insulation in the form of hair, fat
or feathers. Without it, the demands on energy are so extravagant, that it is difficult for
such an animal to survive. However, the only fossil impressions of a dinosaur skin
discovered suggests that their hides were not furry or leathery, but scaly and covered with
bony bumps. It has even been suggested that the large herbivorous dinosaurs (sauropods)
would have required hundreds of kilos of vegetation a day to sustain their enormous bulk
and that they had a unique endothermic metabolism fueled by the heat given off by nonstop digestion.
The dinosaurs had several extinction phases, with the gigantic dinosaurs, being replaced
by smaller, low browsing, beaked dinosaurs at the end of the Jurassic and early
Cretaceous. Again, another extinction occurred and marked the late Cretaceous period.
These dinosaur extinctions may have been related to the radiation of angiosperm plants
(viz plants possessing flowers) which attracted animals to disseminate their pollen and
seeds. A new generation of low browsing dinosaurs may have promoted the spread of
these plants. Overgrazing by dinosaurs may have threatened many low-growing plants
with extinction, except for the angiosperms which possessed reproductive superiority.
The late Cretaceous period witnessed the Hadrosaurs or duckbill dinosaurs (Anatosaurus,
Lambeosaurus, Corythosaurus and Parasaurolophus) occupying swamps and forests and
large herds of Triceratops and their relatives on the grass plains together with
Tyrannosaurus rex.
The discovery of fossilized egg-filled dinosaur nests belonging to the Hadrosaur
Maiasaura gives new light on the life-styles of dinosaurs. Grouped nests were found in a
single layer of sediment, implying that they were all built in the same year. These nests
were spaced at an average of 7 metres apart:- about the size of an adult Maiasaura. Some
bird species lay their eggs close enough together for maximum mutual protection, yet far
enough apart so that they can move easily past their neighbours. Tiny eggshell fragments
within the nests suggested that baby dinosuars remained in the nests to be cared for and
fed by their mother. Had the Maiasaur simply hatched and wandered off to fend for
themselves, the shells would be broken in a few large pieces rather than smashed into
fragments. It is now accepted that these hadrosaurs nurtured and protected their young,
probably feeding them by mouth like young birds until they were strong enough to leave
the nests.
The amazing aspect of these mesozoic reptiles were their exploitation of not only the
terestrial surface but their conquest of the air by pterosaurs and their recolonization of the
aquatic environment by Ichthyosaurus and Plesiosaurus. The ichthyosaurs were
completely adapted to a marine life, like mammal such as dolphins are today. Fossil
evidence suggested that egg-laying on land had been abandoned, and that the young were
born alive and at sea. The body sahpe was completely reconverted to that of a fish; the
neck telescoped to give a fusiform body shape, the limbs shortened into small steering
devices. LOcomotion was performed, fish-like, by undulations of the trunk and tail; a
fishlike fin was developed on the back (but like that of dolphins, it lacked the skeletal
support found in dorsal fins of fishes), but the tail became a powerful swimming organ, in
appearance like that of a shark. In this last regard, however, there is a notable structural
difference; for whereas in a shark the end of the backbone tilts into the upper lobe of the
tail fin, that of the ichthyosuar turns sharply down at the back, with the fin expanding
above it. Most ichthyosaurs were presumably fish-eaters, but some feed on ammonites.
The plesiosaurs were less extreme in their adaptations and probably were able to wadddle
up on to a beach for egg-laying rather like marine turtles do today. They possessed a long
neck or long snout or both; the body was short, broad, and relatively flat. Reversion to a
truely fishlike means of locomotion was impossible, for the trunk was inflexible and the
tail short; instead the limbs were developed into powerful oarlike structures, with which
the creatures "rowed" its way through the sea.
The pterosaurs were probably the first flying vertebrates, and evolved from an early line
of thecodonts. Although pterosaurs were not ancestral to birds they did share some traits
that indicate similarities in anatomy and physiology such as hollow bones. In addition,
both bird and pterosaur skulls have relatively larger cerebellar and optic lobe capacities
than the skulls of modern reptiles. Many of the earlier pterosaurs were small animals not
even as large as crows. However, pterosaurs of the late Jurassic and Cretaceous periods
grew to be the largest ever flying animals. Pteranodon had a 7 metre wingspan and a
weight of ca. 17 kg, and the discovery of fossilized fish within their fossilized ribs,
indicated that they must have flown great distances over water. What is difficult to
explain is how they kept from crumpling their wings if they splashed into the water after
prey, and even more difficult to understand how such large animals regained altitude.
One suggestion is that they scooped up fish pelican-fashion and soared on ocean breezes.
Even so, the lack of a stabilizing tail and the position of the wings behind the centre of
gravity made them aerodynamically unstable. The rudderlike head may have provided
some lateral stability, but other pterosaurs such as the largest Quetzalcoatlus (which had a
wingspan of 16 metres and a weight of 65 kg) were even more unbalanced and lacked
such stabilizing devices. Flight in Quetzalcoatlus has been compared to shooting an
arrow backwards, even so this large beast must have had some means of contolling its
flight since it evidently feed on the carcasses of other dinosaurs.
The pterosaur wing was supported from an enormously extended fourth digit (finger) on
the front limb. From this the wing was extended, in somewhat batlike fashion, a great
wing membrane. Manipulation of a wing of this sort would appear to have been an
awkward matter, and flight was originally considered to be mostly achieved by soaring
rather than flapping. Further since there are no intermediate fingers extending into the
wing memebrane, it was originally thought to have been very fragile. The hind legs of
pterosaurs, in stark contrast to most birds, were feeble structures, and it is difficult to see
how these creatures could have stood up, let alone get a running take-off as birds do
today.
Some recent findings have required some radical changes to our thinking on the
pterosaurs. Some Soviet scientists have reported that one of the smaller pterosaurs
(Sordes pilosus) had fur like mammals; implying that they were endothermic. Recent
analysis of pterosaur Sandactylus (5 metre wingspan) the skin of the pterosaur wing was
quite thick, with epidermal, dermal and muscle fibre layers, and therefore not just a
membrane. Within the upper dermal layer were blood vessels. This antomy and
arrangement of blood vessels is similar to that of a bats wing which uses its blood vessels
to cool itself while flying. If the pterosaur needed to cool down, the flying must have
involved energy expenditure, and therefore be active (flapping) rather than gliding flight.
The lack of stiffeness in the pterosaur wing is difficult to interpret if they flapped their
wings. It is, however, hypothesized that Sandactylus kept its wings at a constant tension
by moving its hind legs, which were also attached to the wing. The implications of these
findings is that pterosaurs had more control over their flight than scientists had previously
thought, and that their flight was not limited to passive gliding. These pterosaurs were
obviously fascinating animals which dominated the skies for 100 million years,
unfortunately they left no descendants for us to study.
Although the fossils of dinosaurs during the entire mesozoic era suggest a high diversity
of organisms adapted to a variety of habitats, the reason for their final wholesale
extinction some 65 million years ago is not completely resolved. However, this
extinction does correlate with a thin band of iridium-enriched clay that marks the
boundary between Cretaceous and Tertiary periods (nicknamed the K-T boundary).
Because iridium is rare on earth, but common in meteorites, it was proposed that the earth
was hit by an asteroid 10 km in size. More recently proof of such a meteorite has been
found in the Gulf of Mexico (off the continental shelf of the Yucatan Peninsula). This
impact site has formed the Chicxulub crater. To have formed this crater the meteorite
would have needed to be at least 10 km in diameter. The impact of such a meteorite
would have caused massive impact earthquakes, perhaps hundreds of times greater than
the largest measured earthquake. Massive tsunami waves (tidal waves) would have
radiated out.
When such a meteorite struck the earth, dust would have blanketed the globe, darkness
would have occurred for one to three months and land temperatures would have
plummeted. Since the meteorite very likely hit the sea, the water vapour could have
created a greenhouse effect, making the short-term climate exceptionally hot, although in
the long-term the temperature declined. Hot nitric acid would have rained out of the
atmosphere and threatened many organisms with death, particularly those possessing
shells. Recent evidence of large amounts of soot in the K-T sediments suggest that largescale fires accompanied such a catastrophe (as much as 90% of the world's forests may
have burned). Such events would have had a profound effect on the ecosystems of the
world.
One theory suggests that mammals, which were on the brink of a great radiation during
the Cenozoic, may have been predators of dinosaur eggs, or in some other way
outcompeted the dinosaurs for resources. At this time mammals were only represented
by shrew-like creatures, a few centimetres in size. Numerous, but tiny cone-shaped teeth
from these mammals were found together with the gigantic fossilized bones of the great
dinosaurs.
In the fossil records of the Montana Badlands there is a black marker of coal and some
excellently preserved fossilized tree stumps. Below this marker was the last of the cycad
and tree fern forest, but the tree stumps represent the coniferous redwoods (Sequoia).
These later plants prefer a much cooler climate than the cycads and tree ferns. Although
a large body can retain heat more efficiently, if it becomes cooled, it becomes
increasingly more difficult to gain heat. In contrast very small animals can find microhabitats that reduce exposure to unfavourable conditions and can more quickly warm
their bodies up during favourable conditions. Aquatic animals also have a greater buffer
against temperature since water maintains heat more efficiently than land. Consequently
the three main types of reptiles that endured the late Cretaceous extinction were lizards,
tortoises and turtles and crocodiles, all either small-sized or aquatic animals.
Crocodiles (Order Crocodilia) are the largest living reptiles and possibly the most
advanced, having a nearly complete four-chambered heart. The nostrils are at the end of
the snout and the eyes protrude from the head so that these animals can float near the
surface of water with only these parts exposed above the water. It was possibly these
features that allowed them to survive the sudden global cooling that almost definitely
occurred at the end of the Cretaceous period. Under hot conditions crocodiles open their
mouths and air passes over the soft skin on the inside of the mouth and cools the animal
down. The crocodile eyes are unusual in that the photo- pigments receptive to light are
different in the upper and lower hemispheres of each retina. The upper retinal
hemisphere which looks down into the water has a photopigment similar to that of
freshwater animals (porphyropsin), whereas the ventral retinal hemisphere has the
pigment of terrestrial animals (rhodopsin). The skin is thick and covered with horny
epidermal scales and dorsal bony plates (osteoderms) which may extend to the ventral
surface and are like those in turtle shells.
The social lives of crocodiles is complex. Male Nile crocodiles establish and defend
breeding territories adjacent to the water and courtship occurs in the water. As the
females approach; the males roar with such intensity that their flanks vibrate throwing up
clouds of spray from the water, and their jaws clap furiously. Mating lasts for a few
minutes with the male clasping the female. Their jaws and tails become intertwined
during copulation. The female excavates a hole in the bank close to the waterline and
lays about forty eggs in several batches. She ensures that the eggs are buried so that
temperature remains relatively constant to within 3oC. Saltwater crocodiles build
mounds of vegetation as a nest and sprays urine to cool it if it becomes to hot. The
alligators occurring in the New World piles up rotting vegetation into a nest which is
regularly turned over in order to provide the eggs with appropriate temperature and
moisture conditions. Just before hatching the female Nile Crocodile waits and when she
hears the pipping calls of the hatching babies she will scrap the earth away and will pick
her young up and put them into a pouch at the bottom of her mouth and will transfer them
to the water. The male will escort these baby crocodiles to a nursery area where they will
remain for the next few months with the parents closely guarding them. The Crocodiles
and its allies invest considerable parental care in the rearing of its young after their
hatching. Many dinosaurs were also thought to invest in considerable parental care, since
they built fairly elaborate nests out of mud which would have retained the young
dinosaurs until they were large enough to climb over the perimeter of the nest edge.
Tortoises, terrapins and turtles belong to the Order Chelonia and have an ancestry that is
even older than the crocodiles. The strengthened bony plates occurring in crocodiles
(ossicles) have in tortoises become modified to form a continuous dorsal carapace and a
ventral plastron. This represents the most effective armour developed by any vertebrate
group, and this pattern has changed very little since it first evolved. The turtles reverted
to an aquatic life style where the heavy shell was less of an impediment to locomotion.
However, the shelled egg, an essential adaptation to terrestrial life, did become an
impediment since the membrane beneath the shell by which the embryo breathes through
the shell pores functions by gaseous exchange and cannot work in water. Consequently
turtles come on to beaches to lay their eggs in a terrestrial environment. However, when
the young turtles hatch they have a perilous journey from where they hatched (above
spring high tide) to the sea, and many succumb to predation.
The third group of reptilian survivors are the lizards (Order Squamata) and are very much
more numerous (3000 species) and have many more modifications arising from their
ancestral stock than either of the other surviving reptile groups. Snakes are essentially
highly specialized lizards that have elongated bodies through increasing the number of
vertebrae and have lost their limbs and even have a reduction of the left lung.
Lizards belong to the suborder Sauria and includes geckos (Family Gekkonidae), iguanas
(Iguanidae), chameleons (Chamaeleonidae), skinks (Scincidae), worm lizards
(Amphisbaeridae) and monitor lizards (Varanidae). They have all enhanced their watertight integument with the development of scales, which have become highly modified.
The Australian shingleback skink (Trachysaurus) has stout polished scales, the Gila
monster (Neloderma) has round pink and black ones (and has additional protection by
being venomous) and the horned lizards occurring in arid areas have enlarged them into
spiny appendages which are scored with fine grooves which allow dew to condense on
them and be collected in the mouth. Spines in the chameleons have also become horned
with one to four occurring in the head region. The scales on the underside of the toes of
geckoes have become highly modified with numerous microscopic hairs (lamellae) which
enable them to climb smooth surfaces (including glass) with relative ease by each hair
engaging on the smallest irregularity of the surface.
Many lizard families have members with reduced limbs that may even be lost altogether
and parallels the amphibian groups Gymnophiona and Caudata. Skinks show a
progression of limb reduction. The snake lizards of South Africa (Family Pygopodidae),
even within their single genus, have some species with a complete complement of
functional legs each with five toes; another species possesses very small limbs, with only
two fully developed toes on each foot and a third species has hind legs with a single toe
and no externally visible front limbs.
A hundred million years ago limb reduction occurred among ancient lizards and resulted
in the evolution of snakes (Suborder Serpentes) of which about 2300 species live today.
They differ from lizards in the following respects: (1) the right and left halves of the
lower jaw are not firmly united, instead they are connected by an elastic ligament; (2)
there is no pectoral girdle; (3) a urinary bladder is absent; (4) the braincase is closed
anteriorly; (5) the eyelids are fused over the eyes but a transparent window exists which
allows the snake to see; and (6) no external ear openings exist.
These adaptations and loss of structures suggest that the snake's ancestors had previously
adopted a burrowing existence, and their surface dwelling is secondary. The loss of legs
for locomotion on the surface has been overcome with the development of flank muscles
that flex in alternate bands so that their body is drawn up in a series of S-shaped curves.
As the contractions travel in waves down the body the flanks are pressed against
obstacles on the ground such as stones and the snake is able to push itself forward. When
snakes hunt they are able to creep up on their prey without oscillating its body. The
scales on the underside are shaped like narrow rectangles running across the width of the
body and overlapping one another with their free edges to the rear. The snake is able to
hitch these scales up and forward in groups by contracting its belly muscles. The back
edges catch the ground and as the contractions pass downwards in waves, the snake
advances smoothly and silently with no lateral movement.
Snakes are predators with prey being seized with their mouths. In boas and pythons they
swiftly coil themselves around the body of the prey and suffocate it. With the backward
pointing teeth the snake engages onto the prey and the snake draws it into the mouth by
using the loosely connected lower jaw. Other snakes deliver venom via specially
modified teeth to kill the prey before ingesting it. In back-fanged snakes a poison gland
lies above the teeth and the venom trickles down a groove in the tooth. The snake
therefore has to drive its fangs deep into the prey before it is able to deliver its venom.
Other snakes have their fangs placed in the front of the upper jaw and have an enclosed
canal through which the venom is delivered. Cobras (Naja) and mambas (Dendroaspis)
have short immobile fangs which inject the venom, whereas vipers have long fangs which
are kept hinged back and are rotated forward when it attacks it prey. Still other snakes
spit poison into the eyes of it prey.
Possibly the most advanced snakes are the pit vipers (Family Cotalidae) which include
the rattle snakes (Crotalus) of the southwestern regions of the United States. These
animal invest heavily in parental care and like some amphibians retain their eggs inside
their body. The shell is reduced to a thin membrane so that the embryos, as they lie
inside the oviduct, not only feed on their yolk but draw sustenance from their mother's
blood diffusing from the walls of the oviduct pressed against them. Such a system for
nourishing of its young is functionally analogous to the placenta used by mammals. The
mother snake will also safeguard her young after they have hatched, warning intruders
with sound of the vibrating rattle at the end of the tail. Each time a rattle snake sheds its
skin a special, hollow scale remains and accumulates at the end of their tail. Up to
twenty scales may accumulate.
Rattle snakes are nocturnal hunters and use a pit located between the nostril and the eye
to detect infra-red radiation. The detection of heat given by a small mammal is also
directional, and therefore it is able to attack its prey even in pitch darkness. Being
ectothermic, food requirements are, however, small and therefore less time is spent
foraging than the equivalent sized endothermic mammal. This ensures their success even
in the most inhospitably dry regions of the world.
Assignments
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
Discuss the origins, morphology and lifestyle of animals belonging to the suborder
Serpentes.
Discuss the adaptive radiation of reptiles living in the Mesozoic period.
Review the evidence we have that dinosaurs were warm-blooded animals.
Describe the adaptions that pterosaurs required in order to fly. In what ways is the
pterosaur wing similar and different to the wings of birds and bats.
What adaptations allowed reptiles to better colonize the terrestrial environments than
their amphibian counterparts?
LORDS OF THE AIR
Many characteristics of birds show close resemblance to those of reptiles and in
particular the early bipedal reptiles before they evolved into the great dinosaurs.
In the early Triassic (225-200 million years ago) small pseudosuchians such as
Saltoposuchus showed the essential characteristics of birds including bipedalism. There
are no fossils detailing the change from the ectothermic bipedal reptiles into endothermic
flying birds except for five fossil specimens of the upper Jurassic (about 150 million
years ago) found in the lithographic slates of Solnhofen, Bavaria. These Archaeopteryx
lithographica probably achieved some degree of gliding, and are certainly the earliest
known animal to possess feathers. Anatomically these animals are much less specialized
than the modern birds but does represent the earliest animal classified as a member of
Aves and is in its own subclass Archaeornithes. All other birds were extinct or living
belong to the subclass Neornithes. In 1860 the first fossilized feather was found, and a
year later the first Archaeopteryx was found. The whole body axis was elongated, the
dorsal vertebrae were not fixed and only five were fused to form the sacrum. There was a
long tail, with feathers arranged in parallel rows along its sides. The fore-limbs ended in
three clawed digits, with separate metacarpals and carpals. This limb was used as a wing
since feathers were attached to the ulna and hand, but the wing was small and the shape
rounded. The pelvic girdle and hind-limb resembled that of the archosaurs. In the skull
there were sharp teeth in both jaws, and the eyes and brain were considerably smaller
than modern birds. The bones were not hollowed and since the sternum bone (keel) was
not well developed, it could not have had muscles that could achieved flapping flight. It
has been suggested that it used its feathers which probably originally evolved as some
form of insulation, as a kind of net to trap insects while running fast across land.
Alternatively it was suggested that it was arboreal and the feathers which were originally
derived from reptilian scales, enabled Archaeopteryx to glide short distances much as
gliding lizards do today (e.g. Draco volans). Thus the two theories that flight evolved
'from ground up' and 'from trees down' have been proposed. The descendants of
Archaeopteryx and other ancient birds underwent a dramatic adaptive radiation during
the Cretaceous period when both aquatic and terrestrial habitats were invaded.
Hesperornis was a loon-like diver that possessed teeth, and had already lost its power of
flight since the wings had become functionless and is the only other bird species known
to have teeth.
That Archaeopteryx almost definitely used its claws on the front wings to climb is clearly
paralleled by the Hoatzin (Opisthocomus) a heavily built bird occurring in South America
and belonging to the cuckoo family. Its young possess conspicuous claws on the digits of
the wings with which it is able to climb away from possible predators. These claws are
usually considered to be a secondary development, however, their resemblance to the
claws of the Archaeopteryx is remarkable. When the Hoatzin chicks grow up they lose
these claws, but the adult birds are nevertheless poor fliers.
The debate as to whether Archaeopteryx could or could not fly still continues. It has
been argued that Archaeopteryx was too heavy and that its muscles were to light to power
it, and that they used their feathers for gliding or cooling themselves. Some researchers
have argued that Archaeopteryx had the muscles of a cold-blooded reptile. These are
twice as powerful per unit weight as those of warm-blooded animals, and may have
allowed Archaeopteryx to fly short distances which makes more ecological sense than a
warm-blooded Archaeopteryx
possessing wings and feathers but not the ability to fly.
Now with the discovery of a fossil bird in northeastern China which has provided the first
evidence that fairly modern tree-perching birds had evolved by 135 million years ago,
only 15 million years after Archaeopteryx.
This sparrow-sized bird, which is as yet unnamed, has an opposable first digit and slender
claws on its legs. This would have allowed it to firmly grasp a tree banch and to "perch"
(the flat forward pointing claws of Archaeopteryx mark it as a ground-dwelling animal).
This small bird had a well developed keel on its sternum which was the anchor site for
strong flight muscles and also possessed a pygostyle (fused cluster of tail vertebrae to
which long tail feathers are attached). This gave the bird a centre of gravity in the centre
of the wings, whereas the long-feathered tail of Archaeopteryx puts the centre gravity
well back of the wing and just above its feet which is a better position for an animal that
runs. This Chinese bird did, however, retain some primitive traits. These include small
remnants of claws and fingers, stomach ribs and the bird may have had teeth. All of
these are present in Archaeopteryx and carnivorous dinosaurs but not in modern birds.
This Chineese fossil does present problems such as how an animal like Archaeopteryx
could have evolved into this bird like animal within 10 - 15 million years?
It is now almost certain that Archaeopteryx was not a direct ancestor to the modern birds,
but would have been an offshoot. The fossil of a 4 metre long coelurosaur called
Deinonychus showed an anatomy almost identical to Archaeopteryx except that it lacked
wings and feathers and was around 50 million years older than Archaeopteryx. Other
bird-like dinosaurs include Avimimus, a 1,5 metre bipedal fossil found in Mongolia.
This animal had a short deep head, toothless beak, long neck and tail and possibly
feathers, which would make it the most ancient of feathered animals. Yet another fossil
discovered in North America called Protoavis, may have been a bird or a dinosaur, but
certainly pre-dates Archaeopteryx. However, the fossil that has attracted the most
amount of interest in relation to the links between birds and dinosaurs is Mononychus, a
turkey-sized predator equiped with sharp teeth and a long tail and looked very similar to
other theropods. Mononychus does share some anatomical features with birds that are
not found in any of the other bird-like dinosaur fossil including Archaeopteryx. For
example Archaeopteryx has a fibula (the thin bone in the lower hind limb) that touches
the ankle, in birds and Mononychus this does not happen. All birds have a keeled
sternum for attachment of wing muscles. Mononychus also has a keeled sternum and
some of its wristbones are fused together which is also an adaptation for flight. This
evidence suggest that Mononychus evolved from a flying animal, just as ostriches are
descended from flying birds. If this is the case Mononychus probably had feathers and
the real ancestor of birds goes back still further in the fossil record.
Although we have a poor fossil record describing the evolution of birds there is little
doubt that they evolved directly from a small coelurosaurian dinosaur. However, the
conquest of the air by birds was not only achieved with the adaptation of the feathers and
powerful wing muscles, but also necessitated considerable weight reductions. The bones
of birds are extremely thin and hollow inside, with structural strength being created by
cross struts. The heavy extension of the spine that supported Archaeopteryx's tail has
been replaced with stout quilled feathers. The heavy jaw with teeth has been replaced
with a beak composed of lightweight protein called keratin.
The basic bird plan of structure originating in the Jurassic has been modified to produce
over 8600 living species. The factors that promoted such species radiation are unclear
since there is a poverty of fossil records and it is not possible to trace individual lines,
which you can do for other vertebrate groups. It is clear that the process of change has
been radical and accomplished in an extremely short evolutionary period.
In particular the bill structure appears to be easily and quickly moulded by evolutionary
processes. From an ancestral finch-like bird with a short straight beak, the Hawaiian
Honey-creepers (Family Drepanididae) have evolved bill structures that are adapted to
feeding on insects, nectar, fruit and seeds in a period of a few thousand years. Darwin
noted similar variation in the bills of the finches of the Galapagos islands. Elsewhere in
the bird world the evolution of bill structure has occurred for a much longer time and we
therefore see bills adapted to seed-eating (sparrows; Ploceidae), fruit-eating (hornbills
and toucans; Bucerotidae and Ramphastidae respectively) insect-eating (nightjars;
Caprimulgidae), tearing (eagles and hawks; Accipitridae), probing (stilts;
Recurvirostridae), filtering (flamingoes; Phoenicopteridae) and capturing of fish
(cormorants; Phalacrocoracidae). The feet of birds also show adaptations to scratching
for food (pheasants; Phasianidae), wading (heron; Ardeidae), grasping (eagles), perching
(warblers; Muscicapidae) and swimming (ducks; Anatidae).
Feathers are also highly evolved in the differentiation of different feathers (primary and
secondary wing, tail, inner and outer contour feathers, down and filoplume) as well as
adaptations to meet different habitats due to the unequalled insulation properties of
feathers which permit the Emperor Penguin (Aptenodytes forsteri) to be the only animal
that can endure winter on the Antarctic ice cap. Most birds have an oil gland near the
base of the tail. The bird takes this oil with its beak and coats individual feathers to
waterproof them and maintain their insulation. Other birds, including herons, parrots
(Psittacidae) and toucans lack this gland and condition feathers with a fine talc like dust,
powder-down, that is produced by the continuous fraying of the tips of special feathers.
Cormorants and darters, spend a great deal of their time diving in water, their feathers are
not waterproofed, permitting them to get completely wet. This is of advantage since it
reduces buoyancy and they can dive deeper and more easily in pursuit of their fish prey.
After foraging they stretch their wings to dry.
Feathers are unique to birds, but were derived from scales and arise to form papillae. A
papilla consists of a projection of vascularized dermal tissue that grows out of an
epidermal pit, called the feather follicle. A typical feather consists of a stiff axial rod, or
shaft. The proximal portion of the shaft, the quill is hollow whereas the distal end is
solid. The shaft bears two rows of branches, or barbs, which in turn support two rows of
smaller, numerous barbules. The feathery vane is composed of a double series of barbs
and barbules. The barbules on the side of the barb towards the tip of the feather bear
hooklets or barbicels, that form bridges with ridges on the adjacent proximal barbules.
The vane is thus lightweight and pliable, but also extremely strong and resilient. At least
once a year each feather is shed and a new feather develops from the same papilla. Birds
usually shed, or moult, their old feathers during late summer. There may be partial or
complete moult in spring when the bird assumes a more colourful breeding plumage.
The acquisition of breeding plumage may also result from wear or the breaking-off of
feather tips, thus exposing different colours beneath.
Feather coloration is due to two basic pigments known as melanins which are pigment
granules of brown, black or yellow and the carotenoids which are either red or yellow.
Green, blue and iridescent markings on sunbirds and other species are due to the peculiar
surface and (Nectarinidae) internal structure of their feathers. Absence of pigments result
in partial or complete albinism.
The first juvenile plumage of birds is usually replaced before the first winter. This winter
plumage usually resembles that of an adult female irrespective of whether the juvenile is
male or female. Only in the second year does differentiation of plumage between males
and females occur. Mature male and female plumages frequently differ in colour (sexual
chromatic dimorphism), especially during the breeding season, when the male may be
particularly brightly coloured (eg Red Bishop birds Euplectes orix). Such colour changes
are used during courtship with male birds advertising themselves. Breeding plumage
may facilitate mate recognization within a species, and is particularly important when
many related species coexist in the same area. In particular striking combinations of
colour are used in finches (Fringillidae) and parakeets/parrots. Worldwide ducks
assemble in multispecies flocks, but during breeding each drake (male) species will
acquire a unique colour and pattern combination particularly in the head regions which
will distinguish that species from other duck species in his quest to find a mate. Colour
may also be used to effectively camouflage birds. The most striking being the ptarmigan
(Lagopus mutus; Tetraonidae), this grouse is white during winter when snow is on the
ground, but mottled brown during the rest of the year.
Feathers have become enlarged and specialized and are used with or without changes of
plumage colour to attract mates. The Pennant-wing Nightjar (Macrodipteryx vexillaria)
acquires long pennants from the primary feathers. In the Crested Grebe (Podiceps
cristatus; Podicipedidae) both sexes develop elongated chestnut-brown feathers on their
cheeks, a deep brown ruff beneath the beak and a pair of horn-like tufts of glossy black
feathers on the head. Sexual difference has been taken to the most extreme for any
animal with the male pheasants, peacocks, grouse, manakins, and birds of paradise all of
which grow feathers to a great size. The Great Argus pheasant (Argusianus argus) has
wing feathers that are over a metre long and are lined with huge eye spots. The Peacock
(Pavo cristatus), which is basically a pheasant, has tail feathers up to 1.8 m long, with a
conspicuous pattern that resembles large eyespots.
The most spectacular bird plumages occur in the Birds of Paradise (Paradisaeidae) from
the island of New Guinea. The King of Saxony (Pteridophora alberti)has two long quills
from his forehead each bearing a line of enamelled blue pennants; the Superb Bird
(Lophorina superba) has an immense emerald shield which it can expand until it is as
broad as the bird is tall; the Twelve-wired Bird of Paradise (Seleucidis melanoleuca) has
a shimmering green bib and a huge inflatable yellow waistcoat with bare quills, the wires
of its name, curling down behind it. The most celebrated birds of paradise are those
possessing plumes arising from beneath their wing coverts. There are several species,
each with a plume of a different colour (yellow, red or white). These birds display
communally, with dance displays being held in a prominent position on a branch which
has had twigs and leaves stripped off it. In this way a dull coloured female is attract and
she flits across to the branch where one of the male birds jumps aggressively onto her
back. Copulation is quick, and the female returns to the nest that she has already
prepared for her now fertilized eggs. The male birds which had been burdened with the
plumes for several months now losses them.
Although bright colours are important for courtship in some birds. other birds have used
behavioural patterns to attract their mates. The Satin Bower Bird (Ptilonorhynchus
violaceus; Ptilonorhynchidae) bird Australia constructs an avenue of twigs on which he
attaches a variety of objects which are either yellow-green, or preferably a shade of blue
that closely matches his plumage colour. The nature of the objects collected is
unimportant and may include berries, feather from other birds and even pieces of plastic.
These birds are even known to steal desirable objects from a neighbouring nest and
certainly mash blue berries with his beak and uses the blue-purple pulp to paint the walls
of his bower. With this bower he tries to lure the female bower bird for courtship and
copulation.
Copulation in birds is generally clumsy, since the male birds with few exceptions have no
penis. The mating birds cling and may twist about until the two vents are brought
together and sperm is transferred to the females. Unlike other tetrapods birds only lay
eggs, a characteristic inherited from the archosaurian ancestors. It is possible that
vivipary would have been too great a load for a female to carry in flight throughout the
weeks necessary for their development and therefore the eggs within the females are laid
soon after fertilization.
Birds now have to pay the penalty for being endothermic, for reptiles can bury their eggs
and abandon them. Bird's eggs like the adults themselves, need to be kept at a constant
temperature which is usually several degrees above ambient temperatures. Birds
therefore incubate their eggs. Some birds just before egg-laying moult a group of
feathers on their undersides and expose a bare patch of skin which becomes distended
with minute blood vessels. The eggs are kept against this patch and kept at the same
temperature as the parent bird. But not all birds produce this patch by moulting. Ducks
and Geese mechanically pluck out their own feathers. The blue-footed Booby (Sula
nebouxii; Sulidae), not only uses its feet for display but also uses them as insulators.
The other disadvantage of egg laying is the need to build a nest, or in some way to safe
guard the eggs. This puts both eggs and parents at risk. Vertical cliffs being almost
inaccessible represent one safe site, providing the eggs do not roll off. This is minimized
by producing eggs that are pointed at the one end which permits them to roll in a circular
direction. Other birds, particularly those belong to the plover group (Order
Charadriiformes) lay their eggs on open fields and gravel plains, but are usually highly
cryptic and not easily found. More commonly birds construct nests to provide some form
of protection. Woodpeckers (Picidae) excavate or enlarge holes in trees, kingfishers
(Alcedinidae) use holes in river banks. The Tailor bird of India, (Orthotomus sutorius), a
warbler, sews together the growing leaves of a tree by piercing holes in their margin and
tying them together with strands of plant fibre. The weaver bird weaves plant material
together to form an almost basket-like structure which is attached to a thin twig and
hangs upside down. Other species of weaver birds collaborate and build elaborate
community nests. The oven bird of Argentina (Furnarius rufus; Furnariidae) builds its
nest out of mud and against fence posts and bare branches. Hornbills, also nests in holes
in trees and incaserates using mud the incubating female and feeds both the female and
the young hatchlings through a small hole in the mud wall. Cave swiftlets (Collocalia
inexpectata; Apodidae) in southeast Asia construct artificial nests from glutinous spittle
which is attached to the walls of the cave.
Several bird species, including the famous cuckoo (e.g. Cuculus carnosus; Cuculidae)
have escaped the labour of incubation and chick rearing by depositing their eggs in the
nest of other birds and allowing foster parents to rear its young. Adaptations for such
parasitism include close mimicry of eggs between the cuckoo and its host, and the more
rapid development of the cuckoo chicks so that they hatch first and can dispose the
legitimate eggs of its foster parents. However, all hatchling bird species do have a small
egg-tooth at the tip of their beak which they use to break the egg. The egg has provided a
small air sac at the end of the egg to provide the first air for the chick. Hatchlings can be
divided into two categories. Chicks that can run away almost immediately from the nest
and are fully covered with down feathers and can feed on their own but still have parental
supervision are said to be precocial. This type of hatchling is most common to birds that
do not build nests, but lay their eggs in the open such as the plover group. Chicks that at
birth are naked and helpless and need to be fed by the parents are said to be altricial and
restricted to bird species that construct nests.
The anatomy of birds is intimately connected to their ability to fly and this is apparent in
the bird shape which offers minimum resistance to the air. Several adaptations result in a
low centre of gravity, which tends to prevent the body from turning over during flight.
The wings are attached high up on the trunk, as are the light organs such as lungs,
whereas heavy flight muscles and muscular digestive organs are positioned ventrally.
The pattern, speed and endurance of flight are reflected in the shape of wings. Highly
aerial birds; which includes swifts (Apodidae), swallows (Hemiprocnidae), terns
(Laridae) and albatrosses (Diomedeidae); have long pointed wings which enable them to
soar in the air for long periods using the minimum amount of energy. Other bird species
have short rounded wings that enable them to take off quickly and fly rapidly for short
distances (eg sparrows). Vultures (Accipitridae) which fly in circles at low speeds using
thermal air currents have broad rectangular wings that permit slow flight. Humming
birds (Trochilidae) are even able to achieve hovering flight, by tilting their bodies so that
they are almost upright and they can beat their wings as fast as 80 times per second.
Flight has, however, permitted birds to be both the fastest moving animals and the
animals that travel the most distance. The Carrier Pigeon (Columba livia; Columbidae)
attains a maximum racing speed of 96 km/h, ducks can reach 145 km/h and the swift
(Apus apus) 170 km/h in level flight. The swift may travel up to 900 km each day to
collect aerial insects which is its only source of food, and this species even copulates in
flight. The Peregrine Falcon (Falco peregrinus) during a dive can achieve speeds of 267290 km/h, and has swept its wings back to reduce drag even further.
No other creatures can fly as fast or as far as birds. Many species of bird make long
journeys. The White Stork (Ciconia ciconia; Ciconiidae) travels every autumn down to
Africa and returns to Europe in the spring navigating with such accuracy that the same
pair, year after year will occupy the same nest on the same roof top. However, the Arctic
Tern (Sterna paradisea), holds the record for long-distance migration. The extremes of its
Arctic nesting and Antarctic wintering ranges are 16 700 km apart. Since the routes
taken are circuitous, these birds may fly 40 300 km each year. During the autumn, many
birds gather in flocks and fly southward, returning the following spring. A lesser and
opposite movement occurs in the Southern Hemisphere, where the seasons are reversed.
Some other birds perform altitudinal migrations into mountainous regions for the summer
and return to the lowlands to winter. In Africa young Starred Robins (Pogonocichla
stellata; Turdidae) moves from the high interior forests to the warmer river valleys with
the onset of autumn and winter.
Most species used established routes for migration and travel more or less on schedule,
arriving and leaving regularly. Migration, breeding, and moult are phases in the annual
cycle of birds that are regulated by the endocrine system. Migration is a semiannual
event, dependent especially on the pituitary gland in the brain. Usually prior to migration
fat reserves, not present at other times, are accumulated rapidly for extra fuel during the
long flights. Also, many strictly diurnal birds become nocturnal during migration.
Seasonal differences (photoperiodism) influence migratory behaviour of some northern
species. Generally birds migrate close to the earth's surface, although some bird species
may migrate at more than 1 km altitude. Most birds migrate at between 50 and 80 km/h
and stop and feed as they proceed along the migration front. Although some birds use
obvious landmarks such as coasts, rivers and mountain ranges other birds will migrate
without the aid of directional features. Evidence suggest that migration in daytime is
guided by the position of the sun and at night by the patterns of stars. This would
necessitate that migrations need to be done on clear nights. On cloudy nights birds tend
to get lost and if they are released in a planetarium where the constellations have been
rotated so that they no longer match the position of the stars in the heavens, the birds will
orientate with the visible, artificial constellations. Still other bird species appear to be
able to use the earth's magnetic field as a guide.
Despite the large amount of adaptation required for flight, there are nevertheless a large
number of birds that have abandoned flight. The older bird fossils dating some thirty
million years after Archaeopteryx including gull-like forms (Ichthyornis) which were
skilled flyers with a keeled chest bone and no bony tail. In essence they were modern
birds. At the same time, however, lived huge swimming birds Hesperornis, which were
nearly as big as a man and had already ceased to be able to fly. Fossils of those other
non-flying birds, the penguins, also appeared around this time. Fossils of another large
flightless bird Diatryma stalked the plains of Wyoming, while a similar bird Phororhacos.
This bird was about 2m tall, carnivorous, and equipped with a huge bill. It is possible
that this group was successful in the absence of other large carnivores representing either
reptile or mammal classes. Large carnivores in the former class were already extinct in
the former class and were yet to evolve in the latter class. Diatryma may have been the
early ancestor to Gruiformes group of birds (Rails and Cranes) which even today have
representatives (eg flightless rails of Gough Island) that showed a marked tenancy to lose
flight when they colonize islands that have few or predators. The cormorants of the
Galapagos Islands have such small wings that they cannot fly any longer. On the
Madagscarene Islands, the dodo (Raphus cucullatus), was a very large pigeon that
adopted a terrestrial habit and was exterminated by the human introduction of dogs to the
island in the seventeenth century. The Elephant bird Aepyornis was about 3m tall and
possessed the largest known eggs for any bird species (148 times the size of a hens egg
by volume). Moas (Diornis) were another giant flightless bird over 3m tall and occurred
on New Zealand.
Currently four orders of birds species fall into the general category of wingless and
flightless terrestrial birds. These include ostriches (Struthioniformes), rheas (Reiformes),
cassowaries and emus (Casuariiformes) and kiwis (Dinornithiformes)
Assignments
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
Discuss the general adaptations birds have evolved for flight. Your answer should include
sections on anatomical modification, physilogical adaptations, feathers and wings.
Describe the modifications that have occured in the beak and feet of birds. Discuss how
such a diversity of forms evolved, relating these forms to environmental factors and or
food items that they forage.
Birds have been described as the living relatives of dinosaurs, briefly discuss the validity
of such a statement.
EGGS, POUCHES AND PLACENTAS
The duck-billed platypus Ornithorhynchus anatinus (bird-billed) from Australia is animal
belonging to the most primitive order of mammals (Monotremata). This animal is the size
of a rabbit, possesses thick fur, webbed and clawed feet, a cloaca combining both
excretory and reproductive functions and a large pliable flat beak like a duck's. It lives in
the rivers of eastern Australia, swimming using its webbed flat feet and steering with its
hind-limbs. When it dives, it closes its ears and tiny eyes with little muscular flaps of
skin and hunts for aquatic invertebrates using its bill, which is rich in nerve endings and
very sensitive. It is also a powerful burrower, excavating tunnels up to 18 metres in
length through the river banks. These animals roll back the webbing of their fore feet into
their palms and this frees the claws for burrowing. Within these tunnels the female
constructs an underground nest of grass and reeds and lays two eggs that are nearly
spherical, the size of marbles, and soft shelled and therefore similar to a reptile's egg.
Since platypuses have fur, they are warm blooded and possess rudimentary mammary
glands; they definitely belong to the class Mammalia and is one of only two primitive
living mammal families which lays eggs. The female platypus develops on her belly
special glands, that are similar in structure to sweat glands but are enlarged and produce a
thick rich milk which oozes into the fur. The young platypus suck the fur. This is the
beginning of the true mammary gland found in all higher mammals. The other important
mammalian feature of endothermy is also incompletely developed and the platypus
allows its body temperatures to fluctuate more greatly than other mammals (viz can drop
to 300C).
The only other animal that can parallel this mixture of primitive features is the spiny anteater called Echidna which taxonomists renamed Tachyglossus (swift-tongued). These
animals are spiny with a long tube-like snout that has no teeth but does possess a long
tongue which flicks out to catch insects. Its front legs are equipped with long digging
claws. At the beginning of each mating season the female develops a small pouch into
which she later transfers her single egg. The mammary glands discharge directly into the
pouch and the milk is sucked of the hairs.
The Echidna and Platypus are of great antiquity, but we have no hard evidence to indicate
which fossil reptiles were their ancestors. Our knowledge of many of the candidates is
based to a considerable degree on its teeth, one of the most durable parts of any animal's
anatomy. Fossilized teeth provide information about an animal's diet and habits. They
are also highly characteristic of a species and similarities between teeth are strong
evidence of genealogical relationships. Both Platypus and Echidna became highly
specialised for underwater foraging and ant eating respectively and consequently lost
their teeth (although young Platypuses still produce three tiny ones soon after birth which
are lost in a very short time). We therefore have virtually nothing to help us link these
creatures to any group of fossil reptiles. This is further complicated since the features that
characterize mammals are hair, warm bloodedness and milk producing glands which
cannot be easily deduced from fossils.
We know that dinosaurs, such as Stegosaurus undoubtedly developed very effective
methods of absorbing heat quickly from the sun and thereby maintained higher than
ambient body temperatures. Mammals, however, evolved from an earlier group of
reptiles (the Synapsids, often referred to as mammal-like reptiles). One of the earliest
group of synapsids, the pelycosaurs, also had similar adaptations to the Stegosaurus
dinosaurs. Dimetrodon, grew long spines from its backbone which supported a sail of
skin which must have served as a solar panel in a similar way to the Stegosaur's plates.
Although the pelycosaurs persisted for a considerable time their sail like crests
disappeared in later forms. In seems extremely unlikely that even if there was a warming
of the climate, the forces of evolution would allow an animal to lose such a valuable
method of heat control unless it was able to replace it with an adaptation that is more
efficient. It has been hypothesized that the pelycosaurs and their successors, the
therapsids, were to some degree endothermic.
One of the therapsid lines were the theriodonts which were small carnivorous animals
(less than 1m) and were almost certainly the evolutionary line that lead to the mammals.
There is some doubt as to whether theriodonts should be classified as reptiles or as very
primitive mammals. An example of which is Cynognathus, an animal approximately a
metre long, possessing a large dog-like skull with highly specialized and differentiated
teeth (remember reptiles are generally characterized by simple, undifferentiated peg-like
teeth). These teeth suggested that Cynognathus teeth were for chewing and cutting food
rather than swallowing it whole. There is also a well developed secondary palate, which
separates the nasal passage from the mouth, which permits continued eating while the
mouth is filled with food. All these features suggest that the animal was very active and
probably requiring an endothermic metabolism. To maintain such a metabolism would
require some form of body insulation, possibly even fur.
These fossils indicate that some theriodonts were far advanced towards the mammals in
certain characters, but still remained comparatively primitive in other respects. The
mixture of conservative and advanced features makes it difficult to identify the final line
that evolved towards the mammals.
The environmental conditions that stimulated such changes may have produced similar
adaptations in more than one group of animals. It is likely that mammalian traits were
acquired by several separate reptilian groups. It was originally hypothesized that the line
of reptiles from which the platypus and echidna stemmed was not necessarily the same as
that which was to give rise to other mammals. In other words mammals had a
polyphyletic origin (derived from more than one ancestor) rather than a monophyletic
origin (derived from a single ancestor). Recent evidence based on the skull morphology
of Probainognathus is argued for monophyletic origin for the mammals. Much of this
debate depends on whether the advanced theriodonts were reptiles or represented the first
mammals. What is certain that monotremes diverged from the main mammalian line
during the Triassic, whereas the other major division in the mammals, namely differential
of placental and marsupial forms only occurred during the late Cretaceous period.
Whatever the exact shape of the genealogical tree, at least one group of the reptiles
completed the transition to a mammalian status some 200 million years ago. A fossil
from the upper Triassic of a small animal (Megazostrodon) discovered in 1966 in
southern Africa, is possibly the earliest true mammal. This creature was only about 100
mm long and resembled in body form a modern day shrew. Details of its jaw and skull
link it firmly with true mammals and its teeth were specialised for eating insects. There
is little doubt that it must have been both warm-blooded and fury. What we cannot
determine is whether it laid eggs like a platypus or gave birth to live young and suckled
them by means of a breast.
Even with the advantage of warm bloodedness the first small mammals were quite over
shadowed in both numbers and size by the dinosaurs until 65 million years ago. Equipped
with warm bloodedness, mammal were able to be active at night when the great reptiles
became torpid and therefore survived in the shadow of the dinosaurs.
The earliest mammals were probably like the opossums that today live in the Americas
particularly those belonging to the genus Didelphis. The Virginia opossum Didelphis
marsupialis of North America is a large rat shaped creature, with small eyes and a long
naked tail which it can wrap round a branch with sufficient strength to support its own
weight. It has a large mouth that opens wide and is equipped with a great number of
small sharp teeth. It is a tough adaptable creature that has spread through the Americas,
from Argentina in the south to Canada in the north. One of the most extraordinary
aspects of this animal is its manner of reproduction. The female has a capacious pouch on
her underside in which she rears her young. The young are extremely small and without
fur and have attach themselves to the mother's teats. The method by which they get there
is one of the most fascinating. The opossums copulate and fertilization of the female's
eggs occurs internally. The young embryos, however, have only enough yolk to maintain
themselves for the first few days of their life. At twelve days and eighteen hours the
animals are expelled into the outside world. This represents the shortest gestation period
known for any mammal species. These young are born so premature that they are no
larger than bees, and so unformed that they are not called infants but are rather referred to
as neonates. As the neonates emerge from their mothers cloaca, they haul themselves
through the fur of her belly to the opening the pouch, a distance of some 80 mm. Only
about half of the neonates reach the pouch and each animals attaches itself to one of
thirteen nipples and starts to take milk. If more than thirteen complete the journey, only
those that attach themselves to a teat will survive. Nine or ten weeks later, the young
clamber out of the pouch. They are now fully formed, the size of mice, and cling to their
mother's fur. In about three months they leave their mother for an independent life of
their own. Mammals that bread in this way (by means of a pouch) are all placed in the
order Marsupialia.
There are seventy six species of opossum (Family Didelphidae) in America, with the
smallest (Marmosa murina) being mouse sized and not possessing a pouch (the young
simply cling to the teats between their mother's hind legs. The largest is the water
opossum or Yapok (Chironectes minimus) and is almost the size of a small otter, and
possesses webbed feet for swimming. Its young are saved from drowning in the pouch
when their mother goes, a sphincter (ring-shaped muscle) which closes the entrance the
entrance of the pouch. The young inside are able to endure several minutes of
submergence and breathe air within the pouch that has a higher concentration of carbon
dioxide than most mammals could survive.
The earliest mammalian fossils that have been positively identified as being marsupial
were found in the Americas and this may be where the group originated, however, the
greatest assemblage of marsupials occuring today is in Australia. The earliest marsupials
(Alphodon and Eodelphis from Cretaceous North America) closely resemble the living
Didelphis opossums that occur in the Americas.
From didelphid ancestors certain South American marsupials specialized into aggressive
carnivores during Tertiary times. These were the borhyaenids, of which the Miocene
genus Borhyaena was typical and resembled a large wolf. The skull was very dog-like,
with the canines enlarged as piercing and stabbing teeth, and some of the molars
modified into shearing blades. The body was long, limbs exceptionally strong and the
feet were equipped with exceptionally sharp claws. Others such as Thylacosmilus was as
large as a tiger; possessed a short skull and tremendously elongated bladelike upper
canine tooth, whereas in the lower jaw there was a deep flange of bone to protect this
tooth when the mouth was closed. These carnivorous marsupials became extinct with the
influx of placental carnivores from North America.
In order to explain how the marsupials got from South America where they originally
radiated to Australia we have to return to the period when the dinosaurs were still at the
height of their dominance. At that time, the continents of the world were grouped
together in a single large land mass. Consequently fossils of closely related dinosaurs
have been found in all of today's continents. The early mammal like reptiles would have
similarly widespread distributions. About 135 million years ago the large single land
mass (Pangaea) split into two a northern supercontinent called Laurasia comprising
today's Europe, Asia and North America; and in the south, another super-continent called
Gondwana made up of South America, Africa, Antarctica and Australia.
The primary evidence for this grouping and the subsequent splitting and drifting is
geological. It comes from studies of the way in which today's continents fit together, the
continuities of the rocks between their opposite edges, the orientation of magnetic
crystals in rocks which shows the position that they held when they were first formed, the
dating of the mid ocean ridges and their islands and drillings taken from the ocean floors.
The distribution of many animals and plants adds corroborative evidence. Giant
flightless birds provide a particularly clear case since they appeared very early in the
history of the birds. One group which included the ferocious Diatryma, evolved in the
northern super-continent are all extinct. The other group called ratites evolved in the
southern supercontinent, and are represented by the Rhea (Rhea americana) in South
America, the Ostrich (Struthio camelus) in Africa, the Emu (Dromaius novaehollandiae)
and Cassowary (Casuarius spp.) in Australia and the Kiwi (Apteryx) in New Zealand.
These birds are so similar that it seems very probable that they are descended from a
single flightless ancestor which had distribution right across the Gondwana
supercontinent. When the land masses separated the different groups of flightless birds
continued to evolve independently of each other into their present-day forms.
Other evidence for the splitting up of the super-continents comes from fleas, which are
parasitic and travel with the animals they live on but readily develop into new species and
move on to new hosts. Some families of highly characteristic fleas are found only in
Australia and South America, with the most probable explanation being that they
originated on group of animals that had a wide distribution across Gondwana. Botanical
evidence is found with the southern beech, a forest forming tree that flourishes only in the
temperate lands of the southern hemisphere. This distribution can also be explained by
the break-up of Gondwana. During this break-up Africa separated and drifted northwards
and Australia and Antarctica remained joined to one another and were linked either by
way of a land bridge or a chain of islands, to the southern tip of South America. At this
point, it seems, the pouched animals (marsupials) were developing from the early an
mammal stock. If these developments took place in South America, as some evidence
suggests, then the early marsupials could have spread across into the Australian Antarctic
block by way of these land-bridges or by island hopping. Fossil evidence supporting this
theory comes from two very closely related marsupial animals; Polydolops and
Antarctodolops being found in South America and Antarctica respectively.
Meanwhile, primitive mammals were also evolving in the northern super-continent.
They were to develop a different way of nourishing their young. Instead of transferring
them at a very early stage into an external pouch, they retained them within the body of
the female and supported them by means of a device called the placenta. We will
examine this technique of reproduction later.
The South American marsupials flourished greatly while they had the continent to
themselves since the southern supercontinent was fragmenting and drifting apart and
South America was moving slowly northwards. In due course, it connected with North
America by way of a land bridge in the neighbourhood of Panama. Down this corridor
came the placental mammals to dispute the possession of South America with the
marsupial residents. In the course of this rivalry, most species of marsupials disappeared,
leaving only the tough, opportunistic opossums. One of these has even invaded North
America, the land from where the placental invaders had come from. That marsupial
invader is the Virginia opossum.
None of the marsupials that lived in the central part of the southern super-continent which
became Antarctica survived. By that time Antarctica had drifted over the South Pole
where it was so cold that it developed an immense ice cap and life on the land became
insupportable. The eastern section of the super-continent, which became Australia had
drifted in a north- east direction into the emptiness of the Pacific basin and has since
remained totally separate from any other continent. The marsupials that occurred on this
section of the super-continent have continued to evolve without any invasion from
placental animals until man introduced them. During this time, the marsupials radiated
into a great number of different forms in order to take advantage of the wide range of
environments and opportunities available to them. Fossil remains of some spectacular
species that once existed have been discovered in the limestone caves of Naracoorte, 250
kms south of Adelaide. Among them are the remains of a huge marsupial the size of a
cow, with a head like a small giraffe that browsed on the branches of trees. One
specimen Thylacoleo was originally thought to be a carnivore due to the back teeth that
were elongated into formidable shearing blades, and called a marsupial lion. More
recently the front legs have shown that this animal was well suited for a tree climbing
existence and used its elongated back teeth to cut down hard fruits.
Australian marsupials still survive within a dozen main families and are represented by
nearly two hundred species. Many of these creatures parallel the placental forms that
evolved in the northern hemisphere. For example there are carnivorous marsupials that
will tackle reptiles and nestling birds and are called marsupial cats (Dasyurus) and until
very recently there was also a marsupial wolf called a Thylacine. Since this animal took
to preying on newly introduced sheep it was hunted and eventually exterminated by local
farmers.
Sometimes the resemblance between placental and marsupial forms is so close that you
need to examine the animals closely in order to distinguish them. The sugar gliders
Petaurus spp. are small leaf and blossom eating marsupials that live in eucalyptus trees.
They have a parachute of skin connecting its fore and hind legs which enable them to
glide from branch to branch and resemble almost exactly the North American flying
squirrel (Petaurista alborufus). The similarity is based on similar lifestyles requiring
similar forms. For example in order to have lifestyle that relies on gliding you will need
to have structures that function as parachutes. A burrowing lifestyle also demands
particular structures that are similar for marsupial and placental animals alike. Placental
moles (e.g. Cape Golden Mole Chrysochloris asiatica) and marsupial moles (Notoryctes)
both have short silky fur, reduced eyes, powerful digging forelegs and a stumpy tail. The
distinguishing feature is that the female marsupial mole possesses a pouch, which unlike
other marsupials opens from the rear and therefore does not fill with earth when she
burrows.
Not all marsupials have such close placental equivalents. The koala (Phascolarctos
cinereus) is a medium sized tree-living creature that feeds on leaves and is comparatively
slow moving. Its ecological equivalent are monkeys which are far more athletic, active
and intelligent. The numbat (Myrmecobius fasciatus) is an ant eating marsupial
possessing a long sticky tongue used to collect its food items; a feature common to all ant
eaters. Further adaptations for ant-eating are not nearly so extreme for the numbat as
those of other ant-eaters, e.g. the giant ant eater (Myrmecophaga tridactyla) of South
America which has evolved a long curving tube-like snout and lost all its teeth. The
numbat jaw is are not nearly so elongated and it still possesses all its teeth.
Other marsupial forms are more unique in their adaptations for example the boodie
(Bettongia lesueur) a shy, strictly nocturnal rat kangaroo, possessing small pointed canine
teeth to help fed on other small animals. It makes its nest in a burrow, industriously
collecting material for it in a most ingenious way. It picks up a few straws in its mouth,
stacks them in a bundle on the ground and then pushes them back over its long tail with
its hind legs. The tail then curls up tightly so that the straw is effectively baled and the
boodie move away by hopping. Boodies locomote using only their back legs which have
very long feet. An animal like the boodie may have been the ancestor to the spectacular
radiation of bipedalism that resulted in the kangaroos and wallabies
The development of the kangaroos may be related to Australia's continuing drift
northwards and the consequent drying and warming of its climate. This would have
caused a reduction in forest cover and replacement by grasslands. Living in an open
grassland would require that the herbivores feeding on the grass an ability to escape
predators. In kangaroos the hind legs have become enormously powerful and the long
muscular tail is held out stiffly behind to acts as a counterbalance which gives the
animals a potential to reach speeds of 60 kph and to clear fences nearly 3 metres high.
The second difficulty that grass eaters must overcome is the wear and tear on their teeth.
Grass is tough, due to the silicates that occur in them, and breaking it down into a pulp in
the mouth is very abrasive on the teeth. Grazers elsewhere have molars with open roots
so that wear can be compensated by continuous growth throughout the animal's life. In
kangaroos the roots of the teeth are closed, and they have evolved a different system of
tooth replacement. There are four pairs of cheek teeth on either side of the jaws. Only
the front ones engage. As they are worn down to the roots, they fall out and those from
the rear migrate forward to take their place. By the time the animal is fifteen or twenty
years old, its last molars are in use.
There are some forty different species in the kangaroo family. The smaller ones are
usually called wallabies. The largest is the red kangaroo Macropus rufus which is as tall
as a man and the largest living marsupial. Kangaroos reproduce in much the same way as
the opossums. The egg which is still enclosed in the vestiges of a shell a few microns
thick and has only a small quantity of yolk within it, and descends from the ovary into the
uterus. There, lying free, it is fertilised and begins its development. If this is the first time
that the female has mated, the fertilized egg does not stay there long. In the case of the
red kangaroo it is only thirty three days before the neonate emerges. Usually only one is
born at a time. It is a blind, hairless an only a few centimetres long; its hind legs are mere
buds, but its forelegs are better developed and with these it hauls its way through the
thick fur on its mother's abdomen. The neonate's journey to the pouch takes about three
minutes. Once there, it fastens on to one of four teats and starts to feed. Almost
immediately, the mother's sexual cycle starts again. Another egg descends into the uterus
and she becomes sexually receptive and she mates and the egg is fertilised. But then an
extraordinary thing happens, the egg's development is temporarily halted. Meanwhile, the
neonate in the pouch is growing prodigiously. After 190 days, the baby is sufficiently
large and independent to make its first foray out of the pouch. From then on it spends
increasing time in the outside world and eventually, after 235 days, it leaves the pouch
for the last time.
If there is a drought at this time, as happens often in central Australia the fertilised egg in
the uterus still remains dormant. But if there has been rain and there is good pasture, then
the egg resumes its development. Thirty three days later, another bean sized neonate will
emerge from the mother's cloaca. The female will then immediately mate again. But the
first-born does not give up its milk supply so easily. It returns regularly to feed from its
own teat. The female kangaroo in effect has three young dependents on her, each at a
different stage of development. One active young at foot which grazes but comes back to
suckle, a second, the tiny neonate, sucking at her teat in the pouch; and a third the
fertilised egg waiting further development.
It is a commonly held notion that the marsupials are backward creatures, scarcely much
of an improvement on those primitive egg layers, the platypus and echidna. That is a
long way from the truth. The marsupial method of reproduction must certainly have
appeared very early in mammal history, but the kangaroos have refined it marvellously.
No other creature anywhere can compare with the female kangaroo who, for much of her
adult life, supports a family of three in varying stages of development.
The mammalian body is a very complicated machine that takes a long time to develop.
Even as an embryo it is warm blooded and burns up fuel very quickly. Both these
characters demand that the developing young should be supplied with considerable
quantities of food. All mammals have found methods of providing far more than could
ever be packed within the confines of a shelled egg. We do not know whether the early
mammals in the northern supercontinent ever passed through a marsupial stage before
developing the placenta. It could be that they sprang from a branch of the mammal like
reptiles that never acquired pouches. The placental and marsupial forms probably arose
independantly from a common ancestor, and they evolved side by side. Certainly the
fossil record of the placentals is as ancient as that of the marsupials, and they arose
sometime during the Cretaceous period. During the early stages of their evolutionary
histories they were probably well matched, so that marsupial adaptations were about as
efficient in evolutionary terms as placental adaptations. However, during the Cenozoic,
the placental animals came to dominate in all areas of the world except the large island of
Australia, which until the advent of many had never witnessed placental mammals. In
Australia the marsupial animals achieved the sophisticated levels of efficiency occurring
in the Red Kangaroo.
In the northern continents the placental method of mammalian reproduction evolved with
many ensuing benefits. The placenta allows the young to remain within the uterus for a
very long time. It is a flat disc that becomes attached to the wall of the uterus and is
connected by the umbilical cord to the foetus. The junction with the uterine wall is
highly convoluted so that the surface area between the placenta and the maternal tissues
is very great. It is here than that the interchange between the mother and foetus takes
place. Blood itself does not pass from mother to young, but oxygen from her lungs and
nutrients derived from her food both dissolved in her blood, diffuse across the junction
and so enters the blood of the foetus. There is also traffic in the other direction. The
waste products produced by the foetus are absorbed by the mother's blood and then
excreted through her kidneys.
All of this makes for great biochemical complications. But there are further ones. The
mammalian sexual cycle involves the regular production of a new egg. This causes no
problem to the marsupial, for in every species, the neonate emerges before the next egg is
due to be produced. In the placental animal the foetus, however, stays in the uterus for a
very much longer period. So the placental foetus secretes a hormone which suspends the
mother's sexual cycle for as long as the placenta is in place so that no more eggs are
produced to compete with the foetus in the uterus.
There is also another problem. The foetus' tissues are not the same genetically, as the
mother's. They contain genetic material from the father. So when it becomes connected
to the mother's body, it risks immunological rejection in the same way as a transplant
does. Just how the placenta prevents rejection is not completely understood.
So by these means, the babies of placental mammals can remain in the uterus until, if
necessary, they are so well developed that they can be fully mobile as soon as they are
born. The placental breeding technique spares the young the hazardous journey outside
their mother's body at a very early stage that a marsupial neonate has to undertake, and
allows their mother to supply their every want during the long period they remain within
her. So whales and seals can carry their unborn young even as they swim for months
through freezing seas. No marsupial with air breathing neonates in a pouch could ever
succeed in doing such a thing. It is possible that the placental technique of reproduction
was to prove one of the crucial factors in the mammals' ultimate success in colonising the
whole of the earth.
Assignments
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
Compare the placental and marsupial modes of reproduction.
Briefly describe how mammals evolved from the synapsid reptiles.
Describe the process of continental drift and how it has influenced the global
distributions of marsupial and placental mammals.
Briefly discuss the adaptive radiation that has occured in the Australian marsupial
mammals and compare such adaptative radiation with that found in placental animals.
THEME AND VARIATION
In the forests of Borneo lives a small, furry, long tailed creature resembling a squirrel but
called a Tree Shrew, (Tupaia glis). Unlike a squirrel this creature does not eat vegetable
matter, but hunts small invertebrates. When first discovered its phylogenetic
relationships with other animals was much debated and it was called a tree shrew based
on its dental similarity (small, pointed unspecialized teeth) to insectivores. Some
scientists suggested that the structure of its genitals indicated a relationship with
marsupials, whereas others analysing the structure of its skull noted an exceptionally
large brain, and proposed that it was a very distant ancestor of monkeys and apes. The
debate is not over yet. Currently the balance of opinion has swung away from viewing
the Tree Shrew as an ancestral monkey and favours classifying it within its own
mammalian order (Scandentia) but recognizing that its closely allied to other primitive
mammals such as shrews which are classifed within the order Insectivora.
The fact that characteristics of so many different kinds of mammals can be seen in the
Tree Shrew, suggests than it might well resemble the ancient creature from which all
placental mammals are descended. Certainly, judging from fossil skeletons such as
Megazostrodon, the first mammals to exist in the dinosaur dominated forests must have
looked very like it: small, long tailed and pointed nosed, and, by inference, furry, warm
blooded, active and insect eating. The reign of the reptiles had been a long one. They
had come to dominate about 250 million years ago. They had browsed the forests,
munched the lush vegetation of the swamps, and carnivorous forms had evolved which
preyed on the plant eaters. Still other species lived by scavenging carrion. The
plesiosaurs and ichthyosaurs were forms that returned to the seas and preyed upon fish;
while pterosaurs took to the skies. Then, 65 million years ago, they all disappeared. In
the void created by the demise of the dinosaurs a radiation of the placental mammals
began.
Tree shrews and other primitive insect eating mammals (with representatives in
mammalian orders Insectivora and Macroscelida) have survived and are scattered
worldwide. In Malaysia, alongside the Tree Shrew, lives an unkempt irritable creature
with a long nose bristling with whiskers and smelling of rotten garlic and is known as a
moon rat (Echinosorex gymnurus). In Africa there is the otter shrew (Potamogale velox),
the biggest of all and a powerful swimmer; and a whole group the size of rats which hop,
have slender elegant legs and mobile thin trunks and are called elephant shrews (Order
Macroscelida). In the Caribbean there is another insectivore called the Solenodon
(Solendon paradoxus). However, the most spectacular radiation of insectivores has
occurred in Madagascar and are called tenrecs. Some of these animal are striped and
hairy with stiffened quills (Hemicentetes semispinosus), whereas others have all their
hairs stiffened into spines on their backs (Echinops telfairi) and resemble European
hedgehogs (Erinaceus europaeus), and yet others have become large and lost their tails
(Tenrec ecaudatus).
Europe also has a number of insectivores including hedgehogs (Erinaceus europaeus),
shrews (e.g. Sorex araneus) and moles (e.g. Chrysochloris asiatica). The spines of a
hedgehog are no more than modified hairs. In many parts of the world shrews are
abundant animals and although small, are very ferocious, attacking any small creature
they encounter including one another. To sustain themselves, they have to eat great
quantities of earthworms and insects every day. Among the shrew is one of the smallest
mammals, the pygmy shrew (Suncus etruscus) which weighs only 1.5 to 2.5 g). Shrews
communicate with one another by shrill high pitched squeaks. They also produce noises
of a frequency that is far above the range of our ears, their eyesight is very poor and there
is some indication that they use these ultra sounds as a simple form of echo location.
Several species of shrew have taken to water in their search for prey items. In Europe,
there are two near relatives called the desmans - one lives in Russia (Desmana moschata)
and the other in the Pyrenees (Galemys pyrenaicus) which use long mobile noses as
snorkels, turning them up so that they project above the water as their owners swim about
busily searching for food. One insectivore group searched for its prey entirely
underground, the mole. Judging from the structure of its paddle shaped forelegs and
powerful shoulders, it is possible that the mole's ancestors were once water living shrews
and the mole has simply adapted the same sort of actions for moving along its tunnels.
Fur, underground, might be a mechanical handicap, but many moles live in temperate
areas and they need fur for insulation. So it has become very short and without any
particular grain so that it points in all directions and the animal can move forwards or
backwards along its tight tunnels with ease. Eyes are of little use underground, would
easily clog with mud, so they are much reduced in size. Moles locate their prey using
their nose which is an organ of both smell and touch, since it is covered with many
sensory bristles. At the rear, in has a short stumpy tail also covered with bristles which
make it aware of what is happening behind it. The star nosed mole of America
(Condylura cristata) has an additional device, an elegant rosette of fleshy feelers around
its nose which in can expand or retract. It may be simply a tactile organ or it may be a
means of detecting changes in the chemical content of the air.
Mole tunnels are not simply passageways but traps. Earthworms, beetles, insect larvae,
in the soil may suddenly fall into a mole's tunnel where the mole harvests the food item.
Incessantly active, it patrols its extensive network at least once every three or four hours
and consumes vast numbers of invertebrates each day. On the rare occasions when so
many worms collect in the tunnels that even a mole's appetite is sated, it gathers up the
surplus, gives each of them a quick bite to immobilise them, and then stores them away in
an underground larder. Some of these stores have been found with thousands of
paralysed invertebrates in them.
A few insectivores specialised in eating one particular kind of invertebrate, ants and
termites. In order to do this a long, sticky tongue is required. Many unrelated creatures,
specializing on this diet, have independently evolved such an organ. The numbat, the
marsupial ant-eater from Australia, the monotreme echidna and even ant eating birds,
woodpeckers and wrynecks, have developed one that fits inside a special compartment of
the skull and in some extends round the eye sockets.
But the most extreme version of such a tongue is that evolved by the placental mammals
including the pangolins (Order Pholidota) the aardvarks (Order Tubulidentata) and South
American ant-eaters (Edentata). In Africa and Asia, there are seven different species of
pangolin including the local species called the Cape Pangolin Manis temminckii a
medium-sized creatures 850 mm long with short legs and long stout prehensile tails. The
Giant Pangolin (Manis gigantea) is 1,5 metre in length and has a tongue that can extend
400 mm beyond its mouth. The sheath that houses it extends right down the front of the
animal's chest and is actually connected with its pelvis. The pangolin has lost all of its
teeth and its lower jaw is reduced to two slivers of bone. The ants and termites collected
by the mucus on the tongue are swallowed and then mashed by the muscular movements
of the stomach which is horny and sometimes contains pebbles to assist in the grinding
process.
Without teeth and without any turn of speed, the pangolin has to be well protected. It has
an armour of horny scales that overlap like shingles on a roof. At the slightest danger the
animal tucks its head into its stomach and wraps itself into a ball with its muscular tail
clasped tightly around it.
South America has evolved its own particular group of insect eaters (Order Edentata) and
their ancestors were among those placental mammals that, migrated down from north
America through Panama and mingled with the marsupials. However the land bridge did
not, in this first instance, last long. After a few million years, it became submerged
beneath the sea and once more the continent was cut off and its animals evolved in
isolation. Eventually, contact was re established and there was a second invasion from
the north as a consequence of which many of the recently evolved South American
placentals disappeared, although not all. One of the less specialised of the survivors are
the armadillos. Like the pangolins, they are protected by armour which consists of a
broad shield over the shoulder and another over the pelvis, with a varying number of half
rings over the middle of the back to give a little flexibility. Armadillos eat insects, other
invertebrates, carrion, and any small creatures, like lizards, that they manage to catch.
Their standard method of seeking food is to dig. They all have an excellent sense of
smell and when they detect something edible in the ground, they start excavating with
manic speed. When you watch them digging, it seems impossibly that they are able to
breathe while excavating, and in fact they are able to hold their breath for up to six
minutes while digging.
There are twenty living species of armadillo, a mere fraction of what formerly existed.
An extinct gigantic armadillo called the glyptodont (Glyptodon) that had a single piece
domed shell as big as a small car. One such shell has been found and it appears to have
been used by early man as a tent. In the glyptodonts, not only was the body heavily
armoured, but the top of the head was covered with a thick shield of bony armour, as was
the tail. The ends of the tail were often provided with an enlarged, spiked knob of bone,
which was probably used for defence. The biggest surviving species is the Giant
Armadillo, (Priodontes giganteus) the size of a pig, which lives in the forests of Brazil.
Like all the group, it is very largely insectivorous and consumes great quantities of ants.
In Paraguay the little three banded armadillo (Tolypeutes matacus) trots about on the tips
of its claws and can roll into a neatly fitting impregnable ball. Down in the pampas of
Argentina there are small Hairy Armadillos (Chaetophractus villosus) that are mole like
and seldom come to the surface except at night. All armadillos have teeth. The Giant
Armadillo has about a hundred, which is almost a mammalian record, but they are small,
simple and peg like.
The specialist ant eaters of South America, however, like the pangolin of Africa, have
lost their teeth entirely. There are three species of them, the smallest being the Dwarf
Ant eater (Cyclopes didactylus) which lives entirely in trees and exclusively on termites.
A bigger version, the Tamandua (Tamandua tetradactyla) is cat sized has a prehensile tail
and short coarse fur. It too is a tree dweller but it often comes down to the ground. On
the open plains, where termite hills stand as thick as tombstones in a graveyard, lives the
Giant Ant eater (Mymecophaga tridactyla) which is about 2 metres long. Its forelegs are
bowed, and its claws are so long that it has to tuck them inward and walk on the sides of
its feet. With these claws it easily tear open termite hills. Its toothless jaws form a tube
even longer than its forelegs. When it feeds, its huge thong of a tongue flicks in and out
of its tiny mouth with great rapidity and probes deep into the termite hill.
All ant eaters are slow movers and are without teeth and armour to defend and protect
themselves. The Dwarf Ant eater and Tamandua favour tree living ants and termites and
spend most of their time up in the branches out of the way of most predators. The Giant
Ant-eater is less defenceless than might at first appear. Its huge front claws can do severe
damage even to a large predator such as the jaguar.
The mammals that we have studied so far, almost all feed on invertebrates, particularly
insects, however a large number of insects fly and therefore are able to escape such
predators. Insects first took to the air some 300 million years ago and had the skies to
themselves until the arrival of the flying reptiles like the pterosaurs, some hundred
million years later. Whether the reptiles flew at night is not known although unlikely
bearing in mind the reptilian problem of maintaining body temperature. Birds eventually
succeeded them, but there is no reason to suppose than there were any more night flying
birds in the past than there are today which is very few. Consequently the night skies
offer the best refuge from predation until another variation on the insectivore theme
evolved: the bats.
There were probably many mammalian attempts at flying before the success of the highly
specialized bats. In Malaysia and the Philippines there lives an odd animal called the
colugo or flying lemur (Cynocephalus volans) and has been classified in its own order
Dermoptera. It is about the size of a large rabbit but its entire body, from its neck to the
end of its tail, is covered by a softly furred cloak of skin. When the animal hangs beneath
a branch or presses itself against a tree trunk, its camouflaged patterning on its fur makes
it almost invisible, but when it extends its legs, the cloak becomes a gliding membrane.
The colugo's gliding technique has several parallels. The marsupial sugar glider planes
through the air in just the same way. Two groups of squirrels have also independently
acquired the talent. But the colugo has the biggest and most completely enveloping
membrane and took to the habit early in mammalian history, for it is certainly a very
primitive member of the group and seems to be a direct descendant of an insectivore
ancestor. A few Palaeocene and Eocene fossils from North America are very similar to
the living Colugo and therefore it is considered to be a fairly primitive animal. Having
perfected a gliding life style, the Colugo has remained unchallenged and unchanged.
Colugos cannot be regarded as a link with the bats, for its anatomy is entirely different in
many fundamental aspects, but it is an indication of a stage that some early insectivores
may have passed through on their way to achieving flapping flight that occurs in bats
which are classified in the order Chiroptera.
The first fossil evidence of fully developed bats were dated at fifty million years ago
(Icaronycteris), so the evolution of flight started early on in the radiation of the placental
mammals. Bats are the only mammals that have mastered true, flapping flight. The bat's
flying membrane stretches not just from the wrist, like the colugo, but along the extended
second finger. The other two fingers form struts extending back to the trailing edge.
Only the thumb remains free and small. This retains its nail and the bat uses it in its toilet
and to help it clamber about its roost. A keel has developed on its chest bone which
serves as an attachment for the muscles which flap the wings.
The bats have many of the modifications developed by birds in order to save body
weight. The bones in the tail are thinned to mere straws to support the flying membrane
or have been lost altogether. Though they have not lost their teeth, their heads are short
and often snub nosed and so avoid being nose heavy in the air. They had one problem
that birds did not face. Their mammalian ancestors had perfected the technique of
nourishing their young internally by means of a placenta. Evolutionary developments
can seldom be reversed so bats have not been able to revert to egg laying with the
associated benefit of weight saving that occurs in birds. The female bat must therefore
fly with the heavy load of her developing foetus within her. In consequence, bats usually
have one young born per breeding season. This, in turn, means that if the population is to
be maintained, the females must compensate by having long reproductive lives, and bats
are for their size, surprisingly long-lived creatures, with a life expectancy of up to twenty
years.
Today, most bats fly at night and it is likely that this was always the case since the birds
had already laid claim to the day. To do so, however, the bat had to develop an efficient
navigational system. It is based on ultra sound like those made by the shrews and other
primitive insectivores. The bats use them for sonar, an extremely sophisticated method
of echo location. This is similar in principle to radar, but radar employs radio waves
whereas sonar uses sound waves. These are frequencies that lie a long way above the
range of the human ear. Most of the sounds we hear have frequencies of around several
hundred vibrations a second. Some of us, particularly when we are young, can with
difficulty distinguish sounds with a frequency of 20 000 vibrations a second. A bat flying
by sonar, uses sounds of between 50 000 and 200 000 vibrations a second. It sends out
these sounds in short bursts, like clicks, twenty or thirty times every second and its
hearing is so acute that from the echo each signal makes, the bat is able to judge the
position not only of objects around it but of its prey which is also likely to be flying quite
fast. Most bats wait to receive the echo of one signal before emitting the next. The
closer the bat is to an object, the shorter the time taken for the echo to come back, so the
bat can increase the number of signals it sends the closer it gets to its prey and thus track
it with increasing accuracy as it closes in for the kill.
Hunting success, however can mean momentary loss of it senses for if its mouth is filled
by an insect, a bat cannot squeak in the normal way. Some species avoid this difficulty
by squeaking through their noses and developed a variety of grotesque nasal outgrowths
which serve to concentrate the beam of the squeak and act like miniature megaphones.
The echoes are picked up by the ears and these too are elaborate, extremely sensitive and
capable, in many cases, of being twisted to detect a signal. So the face of many bats is
dominated by sonar equipment - elaborate translucent ears, ribbed with cartilage and
laced with an intricate pattern of scarlet blood vessels; and on the nose, large protrusions
to detect sounds. The combination and patterns of protrusions on the nose and ear
structure is species specific so that each can produce a unique call. Receptors
synchronized to particular sounds filter out signals from other bat species. The system,
described in such terms, sounds simple but when you encounter several million bats
flying simultaneously in pitch darkness represented by eight species as occurs in the
Gomanton Caves in Borneo you realize that echolocation has become a highly
sophisticated sensory apparatus.
A few insects have developed systems to protect themselves from predation from bats. In
America, there are moths that have the ability to tune in to the frequency of the bat's
sonar. As soon as they hear a bat approaching, they drop to the ground. Other species go
into a spiralling dive which the bats find hard to follow. Yet others manage to jam the
signal or send back high frequency sounds that convince the bat that they are inedible or
are objects to be avoided.
Not all bats feed on insects. Some such as the Pallas' long-tongued bat (Glossophaga
soricina) have discovered that nectar is very nutritious, and have refined their flying skills
so that they can hover in front of flowers, just like humming birds, and gather nectar by
probing deep into the blossoms with long thin tongues. Just as a great number of plants
have evolved to exploit the services of insects as pollinators, so too some rely on bats.
Some cacti, for example, only open their blossoms at night. These are large, robust and
light-coloured, for in the darkness colour is valueless. Their scent, however, is heavy and
strong and the petals project well above the armoury of spines on the stems so that the
bats are able to visit without damaging their wing membranes.
The biggest of all bats live only on fruit. They are called flying foxes (e.g. Pteropus
giganteus), not only because of their size and some of them have a wing span of one and
a half metres but because their coats are reddish brown and their faces are fox like.
They have large eyes but only small ears and lack any kind of nose protrusions and they
are not equipped with any form of echolocation apparatus. Whether this major difference
between them and other insectivorous bats indicates that the two groups derive from
separate branches of primitive insectivores is not yet agreed. Unlike insectivorous bats,
fruit bats do not live in caves but in the tops of trees in large communal roosts. In the
evening, they set of in parties to feed. Their silhouette is quite unlike that of birds, for
they lack a projecting tail and their flight is very different from the fluttering of insect
hunting bats. Their huge wings beat steadily as long skeins of them keep a level
purposeful course across the evening sky. They may travel as far as 70 kilometres in
their search for fruit.
Other bats have taken to feeding on meat. Some prey on roosting birds, some take frogs
and small lizards. The Yellow-eared bat (Phyllostomus hastatus) even feeds on other bats.
An American species even manages to fish (Noctilo leporinus). At dusk, it beats up and
down over ponds, lakes, or even the sea. The tail membrane of most bats extends to the
ankles. In the fishing bat, it is attached much higher up at the knee, so that the legs are
quite free. The bat can therefore trail its feet in the water, keeping the membrane out of
the way by folding up its tail. Its toes are large and armed with hook shaped claws. When
they strike a fish, the bat scoops it up into its mouth and kills it with a powerful crunch of
its teeth.
The vampire bat (Desmodus rotundus) has become very specialised indeed. Its front
teeth are modified into two triangular razors. It settles gently on a sleeping mammal, a
cow or even a human being. Its saliva contains an anti coagulant, so that the blood, when
it appears, will continue to ooze for some time before a clot forms. The vampire then
squats beside the wound lapping the blood. They fly by sonar and it is said that the
reason that dogs, whose hearing is also tuned to very high frequencies, are so seldom
attacked by them is that they can hear the vampire bats coming.
The diveristy of bats is amazing with some 950 species. Possible the most unique
adaptation that has occurred is the Yellow-eared Bat (Uroderma bilobatum). Unlike most
bats, which make no nest or shelter of any sort, this bat cuts a row of holes in a bannana
leaf so that the edges drop and forms a tent under which it hangs during the day.
Not only have mammals taken to the air, but they have also returned to an aquatic
environment. The mammals that are the most fish-like are Whales and Dolphins and are
classified in the order Cetacea. Despite their appearance they are warm blooded and milk
producing animals that have a long ancestry, with fossils dating back to the beginning of
the great radiation of the mammals fifty million years ago. The earliest known cetacean
is Pakicetus, the fossils of which are found in river sediments, indicating that these
primitive cetaceans had not ventured into marine environments. The earliest fossil that
resembled a marine whale is Basilosaurus, which occured about 42 million years ago and
had already reached a length of 20 metres, possessed a very long tail and its forelimbs
were modified into paddles. The hind limbs were small, but still included a foot
possessing three toes.
The problems associated with a return to an aquatic existance include locomotion,
respiration and reproduction. Yet such adaptations were undertaken in an extremely
short period, although it is difficult to comprehend how such an immense animals as the
130 ton blue whale (Balaenoptera musculus) really descended from a tiny creature like
the tree shrew. Their ancestors must have entered the sea at a time when the only
mammals in existence were the little insectivores. But their anatomy is now so extreme in
their adaption to swimming that it gives no clue as to how the transition back to the seas
was made. It may be that the two main groups of whales; the carnivorous forms
possessing teeth (suborder Odontoceti) and the filter feeding forms using a baleen
(suborder Mysticeti) have different ancestries, those with teeth having come from
insectivores by way of primitive carnivores and the rest, the baleen whales, being
descended more directly.
The major differences between the whales and the early mammals are all attributable to
adaptations for a swimming life. The forelimbs have become paddles. The rear limbs
have been lost altogether, though there are a few small bones buried deep in the whale's
body to prove that the whale ancestors really did, at one time, have back legs. Fur, that
hallmark of mammals, functions as an insulator due to air being trapped between hairs
and is therefore of little use to a creature that never comes onto dry land. Consequently
whales have lost that too, though there are a few bristles on the snout to demonstrate that
they once had a coat. Insulation, however, is still needed and whales have developed
blubber, a thick layer of fat beneath the skin that prevents their body heat from escaping
even in the coldest sea. The mammals' dependency on air for breathing must be a
considered a real handicap in water, but the whale has minimized that problem by
breathing more efficiently than most land livers. Man only clears about 15% of the air in
his lungs with a normal breath. The whale, in one of its roaring, spouting exhalations,
gets rid of about 90% of its spent air. As a result it only has to take air in at extended
intervals. It also has in its muscles a particularly high concentration of a substance called
myoglobin that enables it to store oxygen. This form of oxygen storage allows the fin
back whale, to reach depths of 500 metres and swim for forty minutes without surfacing
for air.
One group of whales has specialised in feeding on tiny shrimp like crustaceans, krill,
which swim in vast quantities in the sea. Just as teeth are of no value to mammals feeding
on ants, so they are of no use to those animals eating krill. These whales have lost their
teeth and instead have baleen, sheets of horn, feathered at the edges, that hang down like
stiff parallel curtains from the roof of the mouth. The whale takes a large mouthful of
water in the middle of the shoal of krill, half shuts its jaws and then expels the water by
pressing its tongue forward so that the krill remains and can be swallowed. Sometimes it
gathers the krill by slowly cruising where it is thickest. It also can concentrate a
dispersed shoal by diving beneath it and then spiralling up, expelling bubbles as it goes,
so that the krill is driven towards the centre of the spiral. Then the whale with its jaws
pointing upwards, rises vertically in the centre of the spiral it has created and gathers
them in one gulp. On such a diet, the baleen whales have grown to an immense size. The
blue whale (Balaenoptera musculus) the biggest of any animal to inhabit our planet,
grows to over 30 metres long and weighs up to 130 tonnes. There is a positive advantage
to a whale being so large. Maintaining body temperature is easier the bigger you are and
the lower the ratio between your volume and surface area. This phenomenon had
affected the dinosaurs but their dimensions were limited by the mechanical strength of
bone. Above a certain weight, limbs would simply break. The whales are less hampered.
The function of their bones is largely to give rigidity. Support for their bodies comes
from the water. Nor does a life spent gently cruising after krill demand great agility.
The toothed whales fed on different prey. The largest of them, the squid eating sperm
whale (Physeter macrocephalus), only attains half the size of the blue whale. The smaller
ones, dolphins, porpoises and killer whales, hunt both fish and squid and have become
extremely fast swimmers, some reputedly being able to reach speeds of over 40 kph.
Moving at such speeds, navigation becomes critically important. Fish are helped by their
lateral line system, but mammals lost that far back in their ancestry and the toothed
whales have instead a system based on the sounds used by shrews and elaborated by bats,
sonar. Dolphins such as Bottle-nosed (Tursiops truncatus) produce the ultra sound with
larynx and maybe an organ in the font of the head, the melon. The frequencies they use
are around 200 000 vibrations a second, which is comparable to those used by bats. With
this aid, they can not only sense obstacles in their path, but identify from the quality of
the echo, the nature of these objects ahead. This can be demonstrated easily enough, for
dolphins flourish in oceanaria and eagerly cooperate in training. Blindfolded dolphins
demonstrate that they can, without difficulty, pick out particular shapes of floating rings
and will swiftly swim through the water, with blindfolds on their eyes.
Dolphins produce a great variety of other noises quite apart from ultra sounds and there
has been considerable speculation as to whether these sounds constitute a language. So
far, we have identified some twenty different sounds that dolphins make. Some seem to
serve to keep a school together when they are travelling at speed, other appears to be
warning cries. But no one yet has demonstrated that dolphins ever put these sounds
together to form the equivalent of the two word sentence that can justifiably be regarded
as the beginning of true language, a phenomenon already demonstrated for Chimpanzees
(Pan troglodytes).
The great whales also have voices. Humpbacks (Megaptera novaeanglia), one of the
baleen whales, congregate every spring in Hawaii to give birth to their young and to
mate. Some of them also sing. Their song consists of a sequence of yelps, growls, high
pitched squeals and long drawn out rumbles. And the whales declaim these songs hour
after hour in extended stately recitals. They contain unchanging sequences of tones that
have been called themes. Each theme may be repeated over and over again the number
of times varies but the order of the themes in a song is always the same in any one
season. Typically, a complete song lasts for about ten minutes, but some have been
recorded that continue for half an hour and whales may sing, repeating their songs,
virtually continuously for over twenty four hours. Each whale has its own characteristic
song but it composes it from themes which it shares with the rest of the whale community
in Hawaii. The whales stay in Hawaiian waters for several months, calving, mating and
singing. Then, within a few days, the deep blue bays and straits off the Hawaiian islands
are empty. The whales have gone. Humpbacks appear a few weeks later off Alaska. It
is very likely that these are the Hawaiian animals but more studies will have to be made
before we can be certain that they are. Next spring, they reappear in Hawaii and once
more begin to sing. But this time they have new themes in their repertoire and have
dropped many of the old ones.
We still do not know why whales sing although each individual whale can be identified
by its song, which may mean that whales can do the same. Water transmits sound better
than air so it may well be that sections of these songs, particularly those low vibrating
notes, can be heard several kilometres away informing them of the whereabouts and
activities of the whole whale community.
Ant eaters, bats, moles and whales are all early descendants of the first mammals and
have developed elaborate specializations to eat other small and large animals, But there
are other sources of nutriment to be trapped as well plants. This is the next step in the
radiation of the placental animals, the first of which Some creatures developed that ate
grass and moved from the forest onto the plains to graze. They were followed by the
flesheaters and in the open, the two inter-dependent communities evolved, side by side,
each advance in hunting efficiency producing responses in defence from the hunted. A
second group of creatures established their lives in the tree tops.
Assignments
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
Discuss echolocation in bats and whales.
Describe how placental mammals have colonized land, water and air.
Discuss adaptive radiation in the order Insectivora.
Discuss adaptations to eating ants and termites in the mammalian orders Edentata,
Pholidota, Tubulidentata, Marsupialia and Monotremata.
Discuss adaptive radiation in the orders Insectivora and Chiroptera.
THE HUNTERS AND THE HUNTED
Forests offer an ever renewing, inexhaustible supply of food for evolving animals. The
first vertebrate herbivores probably evolve to utilize and digest such vegetation.
Herbivorous dinosaurs had fed on them, smashing saplings in the forests of ash, elm and
beech in North America, crashing through the palms and lianas of the tropics. With the
extinction of the dinosaurs, only invertebrates such as insects would continue,
unobtrusively, to claim their share, gnawing at the wood, scissoring the leaves into
fragments. A few lizard species would have teared away at leaf fronds, and birds, would
have been acquiring a taste for the newly evolving fruit, and obliging the plants with
distribution of their seeds. About 50 to 60 million years ago there appeared to be no large
herbivores using these plants. Eating plants is no easy business. It demands particular
skills and structures just like any other specialized diet. For one thing, vegetable matter
is not particularly nutritious and great quantities of material needs to be extracted to
obtain enough calories to sustain a large animal. Some dedicated vegetarians have to
spend three quarters of their waking hours foraging. This in turn would expose an animal
to risk by a predator. One way for an animal to minimise such a risk is to grab as much
as possible, as quickly as possible, and to run of with it to somewhere safe a strategy that
the African Giant Rat (Cricetomys gambianus) employs. This rodent emerges cautiously
from its burrow at night and when it is sure that there is no danger, frantically loads its
cheek pouches with anything that looks remotely edible. Seeds, nuts, fruits, roots,
occasionally a snail or a beetle all go in. The pouches are very large and when they are
crammed full it scurries back to its burrow.
Plant eaters have to have particularly good teeth. Not only do they use them for very
long periods but the material they have to deal with is tough. Rats, like other members of
the order Rodentia squirrels, mice, beavers, porcupines cope with that problem by
maintaining open roots to their front gnawing teeth, the incisors, so that they continue to
grow throughout the animal's life compensating for wear. They are kept sharp by a
simple but effective self-stropping process. The main body of the rodent incisor is of
dentine, but its front surface is covered by a thick and often brightly coloured layer of
enamel which is even harder. The cutting edge of the tooth thus becomes shaped like a
chisel. As the top incisors grind over the lower ones the dentine is worn away more
quickly and this exposes the blades of enamel at the front keeping a sharp chisel edge.
Once gnawed, ground and pulped, the food has to be digested. This too presents major
problems. Cellulose, the material from which the cell walls of plants are built, is one of
the most stable of organic substances. Digestive enzymes produced by mammals are
unable to break cellulose down, and this can be achieved by either mechanical means
through extended chewing or by bacteria which are able to dissolve the cellulose through
fermentation. Herbivore digestive systems maintain bacterial cultures to break down this
cellulose. Even with bacterial help, digestion of an entirely vegetarian meal can take a
long time.
In rabbits (order Lagomorpha) and rodents additional digestion is provided by re-eating
soft faecal pellets (coprophagy), so that the material is twice processed and the last
vestiges of nourishment are extracted. Only after this second processing are the faeces
deposited outside the burrow as the familiar dry pellets.
The two members of the order Proboscidea; the African and Indian Elephant (Loxodonta
africana and Elephas maximus) have particularly acute problems for they eat, in addition
leaves, a great deal of fibrous twigs and woody material. Apart from their tusks their
only teeth are molars at the back of the mouth, which form massive grinders. As they
wear down they are replaced every few years by new ones erupting from behind and
migrating forward along the jaw. The molars pulp and crush with enormous power, but
even so, the elephants food is so woody it requires a very long period of digestion to
extract anything of value from it. The elephant's stomach, however, is big enough to
provide it. A meal taken by a human being normally passes through the body in about
twenty-four hours. An elephant's takes about two and a half days to make the same
journey and for most of that time it is kept stewing in the digestive juices and bacterial
broth of the stomach. Much earlier in history some dinosaurs, eating ferns and cycads,
had encountered the same problem and solved it the same way by becoming giants.
Elephant dung, even after all this protracted treatment, still contains a great deal of twigs,
fibres and seeds that have remained virtually un-touched. Some plants that have been
stripped by elephants for millennia have reacted to the treatment by coating their seeds
with rinds thick enough to withstand a prolonged soaking in the digestive juices. The
paradoxical consequence has been that now, unless the rind is weakened by passing
through an elephant digestive system, the seeds are unable to germinate.
The most elaborate apparatus for digesting cellulose is the familiar one used by the
ruminants such as antelope, deer, buffalo as well as domestic sheep and cows (order
Artiodactyla). They clip grass from their pasture with the lower incisors, pressing it
against the tongue or the gums of the upper jaw, which has no teeth in the front. They
then swallow it immediately and it goes down to the rumen, a chamber of the stomach
which contains a particularly rich brew of bacteria. There it is churned back and forth for
several hours, squeezed by a muscular bag, while the bacteria attack the cellulose.
Eventually, the mash is brought up the throat, a mouthful at a time, to be chewed in a
particularly thorough way by the molars. Ruminants can move their jaws not only up and
down but backwards, forwards and sideways. This ruminating can be done, however, at
leisure and in safety, when the animal has left the exposed feeding grounds and is
relaxing in the shade during the heat of the day. Eventually the mouthful is swallowed
for the second time. It goes past the rumen and on to the stomach proper which has
absorptive.
Leaves have one further shortcoming as food. In temperate parts of the world (viz
deciduous forests), many disappear almost entirely for months at a time. The creatures
dependent upon them must, therefore, make special preparations as winter approaches.
Asiatic sheep (Ovis ammon) turn their food into fat and store it as cushions around the
base of their tails. Other species not only feed and fatten themselves as much as they can,
but reduce the demands of the next few months to a minimum by hibernating. The
triggers to initiate hibernation have not all been precisely identified. It is certainly not
simply a drop in the temperature since animals kept in a constantly warm environment
will still hibernate. In some cases it appears to related to shortening of daylight hours. It
may be that the stimulus comes from the fat reserves themselves. When the animal has
accumulated sufficient fat biochemical processes initiate hibernation.
A hibernating dormouse (Glis glis) is spherical, with its head tucked into its stomach, its
soft furry tail wrapped around itself. In this posture the amount of heat that seeps away
from the body is reduced. Its heart beat slows considerably and the breathing becomes so
shallow and infrequent that it is difficult to detect. The muscles stiffen and the whole
body feels cold, since body temperature is reduced to save energy. In this state of
suspended animation, the body's food demands are so low that the fat store can provide
enough to keep essential processes ticking over for months. Extreme cold, however, will
waken the animal to prevent it being frozen alive. When awakened the animal begins to
shiver violently, warming itself by burning fuel in its muscles. It may even, in an
emergency, squander some of its remaining reserves of fat by trotting about until the
worst of the cold is past and it can go back to sleep again. Normally it is only the warmth
of spring that brings the dormouse and other winter sleepers out of their hibernation.
Their appetites are now huge and urgent, for during the winter, they may have lost as
much as half of their body weight.
With such methods as these, a great variety of animals nourish themselves on the
vegetable foods provided by the forests of the world. Up in the topmost branches,
rodents such as the grey squirrel (Sciurus carolinensis) scamper along the twigs,
collecting bark and shoots, acorns and catkins. Some species have even developed furry
membranes between their hind and fore legs so that they can glide between the branches
and thereby improve their foraging efficiency. These are called flying squirrels, and
there are over forty species of them, and they are concentrated almost in the Asiatic
region (e.g. the Red and White Flying Squirrel Petaurista alborufus) with seven species
occurring in Africa (e.g. Pel's Flying Squirrel Anomalurus peli) two species occurring in
North America (e.g. Southern Flying Squirrel Glaucomys volans).
In the upper branches live the monkeys (order Primates). Many species will take a wide
variety of food - insects, eggs, nestlings and fruit; but others will only take the leaves of
particular trees and have complicated stomachs to deal with them. Life in the forest
canopy has lead to a high degree of co-ordination, particularly with respect to the
grasping manipulative hands and a quick intelligences, features that ultimately lead to the
evolution of the human being.
However, one of the first creatures to make an existence high up in the tropical forest
canopy of South America was the sloth, a distant relative of the ant-eaters and a member
of the order Edentata, and it adopted a solution almost exactly opposite to those of the
monkeys. There are two main kinds of sloth, the two-toed (belonging to the genus
Bradypus and the three-toed (genus Choloepus). Of these, the three-toed sloths are
considerably more slothful. It hangs upside down from a branch suspended by hook-like
claws at the ends of its long bony arms. It feeds on only one kind of leaf, Cecropia,
which happily for the sloth grows in quantity and is easily found. No predators attack the
sloth - few indeed can even reach it - and nothing competes with it for Cecropia leaves.
Without fear of predation and plentiful food sources without competition from other
predators allows them to spend up to eighteen hours each day asleep. A green algae
grows on its coarse hair and communities of a parasitic moth live in the depths of this
coat producing caterpillars which graze on the alga-covered hair. Its muscles are such
that it is quite incapable of moving at any speed whatsoever. It is virtually dumb and
hearing poor. Even its sense of smell, though better than ours, is less acute than that of
most mammals.
These animals live a solitary life except when finding a mate to breed with? With its
poor senses, it is no easy matter to find one, however, since the sloth's digestion also
works as slowly as the rest of its bodily processes it defecates and urinates once a week.
To accomplish these processes it descends to the ground and habitually uses the same
spot. This is the one time in its life that it is exposed to predators such as jaguars
(Panthera onca), but also provides opportunities to meet mates and to breed with them. Its
dung and urine have extremely pungent smells, and the sense of smell is the only one of
the sloths faculties that is not seriously blurred. So a sloth midden is the one place in the
forest that another sloth could easily find a mate.
The forest floor is not rich in vegetation. In some areas the shade is so dense that there is
nothing but a deep layer of decomposing leaves with the occasional fungi. Where the
canopy is thinner, there may be small bushes, a few herbs on the ground and some
spindly saplings. In Africa and Asia such plants provide food for small antelope e.g.
duiker (Cephalophus species). These animals are extremely shy and difficult to observe
as the forage for leaf material in the dappled light. These animals are very similar to the
primitive ruminants that were among the first-leaf eating specialists that evolved some
fifty million years ago.
In South American forests, the major herbivores are not hoofed animals but rodents such
as the paca (Cuniculus paca) and agouti (e.g. Dasyprocta leporina). They have body
forms, shy habits and a solitary life style. Browsing on the taller shrubs and saplings
requires greater stature and most tropical forests have some form of large herbivore,
which are secretative, generally uncommon and difficult to observe. In Malaya and
South America, there are nocturnal tapirs Tapirus indicus and Tapirus terrestris), which
belongs to the order Perissodactyla (odd-toed ungulates). In parts of Southeast Asia,
another odd-toed ungulate occurs, the Sumatran Rhinoceros (Didermocerus sumatrensis),
with a slightly hairy hide. In the Central African basin forests occurs the even-toed
ungulate called the Okapi (Okapia johnstoni; order Artiodactyla), and is a short-necked
primitive cousin of the Giraffe (Giraffa camelopardalis). It is an amazing fact that so
large and conspicuously marked a creature as the Okapi was unknown to science until
1901.
All these ground-living forest dwellers, large and small, are solitary since the forest floor
seldom produces sufficient leaves to sustain a large group in one area for any length of
time. Further if several animals are to maintain a relationship they require some kind of
communication. It is not possible to see far into the forest and signalling by sound would
attract the attention of potential predators. These animals also maintain territories which
they mark with dung or secretions of a gland close to the eye and rely on concealment to
protect themselves from predation.
The hunters that seek them such prey are also solitary. Examples are the jaguar preying
on the tapir, and the leopard (Panthera pardus) preying on the duiker. A wandering
Brown Bear (Ursus arctos) will eat most things including a small antelope. The smaller
hunters such as genets (Genetta species), jungle cats (e.g. Felis chaus), civets (e.g.
Viverra species) and weasels (e.g. Mustela) prey on small rodents as well as birds and
reptiles.
Of all the carnivore hunters (order Carnivora), the cats (Family Felidae) are the most
specialized for meat-eating. Their claws are kept sharp by being retracted into sheaths.
When they attack, they hook their victim with them and then deliver a piercing bite to the
neck that severs the spinal cord. The long dagger-like tooth on either side of the mouth,
just behind the front teeth, typical of a meat-eater, is used to slash open its prey. The
jagged teeth further back in the jaw shear bones. They are all the tools of butchery. None
of the dogs or cats can really chew. Most simply bolt their food down in chunks. Flesh is
far easier to digest than leaves and twigs and the hunters stomach is not so elaborate.
The relationships between predator and prey are very different on the open grassy plains.
Grass may look to be a simple almost primitive plant, little more than leaves with roots.
In fact, it is a highly advanced one, bearing tiny, unobtrusive flowers which rely not on
insects to distribute their pollen but on wind. It produces horizontal stems running close
to the surface or just below it. When fire sweeps across the plains, consuming the old dry
leaves, the stems and the root stocks are unharmed and resprouts almost immediately.
Grass leaves grow, not from the tip as do those of bushes and trees, but from the base.
This is of benefit to the grazing animals for it means that even though the leaves have
been cropped, they will continue to grow and new leaves will become available to be
eaten.
The grass itself benefits from the presence of the grazing herds for they trample and eat
the seedlings of woody plants that might take root on the plain and eventually displace
the grasslands. It seems likely therefore that the spread of the grassland and the evolution
of grazing animals proceeded together, and that the grassland maintains the herbivores
and the herbivores maintain the grassland by preventing woody species from colonizing
it.
On an open plain such as an African grassland a single herbivore is an easy target for a
predator unless you are very large such as an Elephant (Loxodonta africanus), Black and
White Rhinoceroses (Diceros rhinoceros and Ceratotherium simum) and Buffalo
(Syncerus caffer). The dense vegetation of a forest makes it easier for a herbivore to
move around without being seen, and a smaller size would tend to be favoured. On the
plains a small size is not an advantage, in fact a large size may reduce the risk of
predation. Great bulk with a tough skin may be deterrents to predation. However, for
smaller animals, the dangers of predation are high.
Some sought safety in burrows, and in grassland which are free of roots of large trees, it
is easy to construct extended tunnel systems without hinderance. One of the most
specialized of burrowers is the naked mole-rat (Heterocephalus glaber; order Rodentia) of
East Africa. It eats the roots of grasses together with bulbs and tubers. Mole-rats live in
families and excavate elaborate underground quarters with special dormitories, nurseries,
larders and lavatories. Life spent entirely underground in the warm, dry earth of the
African plains has changed them dramatically. They have lost use of their eyes and are
now hairless. These naked sausage-shaped animals have huge incisor teeth that project
clear of the head in a bony semicircle in front of the face. They are used for both feeding
and as burrowing tools. Gnawing one's way through earth could clearly be a distasteful
business, but the mole-rat avoids mouthfuls of soil by pressing back its lips behind the
protruding teeth and the mouth is kept tightly shut while the teeth excavate through the
soil.
When they dig, they work in teams. The one at the front gnaws away dislodging the soil
behind it where the second member of the team hurls the soil back between its legs onto
the third member of the team. The soil is passed in this way until the last member of the
line receives it and throws it vigorously out of the entrance of the tunnel. A patch of
ground colonized by mole-rats is riddled with small heaps of earth which demarcate the
entrance to the burrows.
Few, if any, predators are able to make a meal of a mole-rat. It can dig faster than any
predator and it has no need to come to the surface. But those burrowers than eat not grass
blades must emerge from their holes and then become targets for predation. The plains of
North America are colonized by rodents called prairie dogs or Marmots (e.g. Cynomys
ludovicianus). They not only graze above ground but do so during the day when coyotes,
bobcats, ferrets and hawks are about, all predators of the prairie dog. These animals have
developed defences which depend upon a highly organized social system. They live in
huge concentrations called towns which may contain up to a thousand animals. Each
town is divided up into a number of communities called coteries of about thirty
individuals, all of whom know one another well. Many have interconnecting burrows.
The coteries always have some members on sentry duty, sitting upright on the mound of
excavated earth beside the burrow entrance where they can get the best view of what is
going on. If a potential predator is spotted the sentry lets out a series of whistling barks.
Different kinds of predators elicit different calls so that the other prairie dogs know where
the danger comes from. The call is repeated by others nearby and so spreads through the
town, putting every-one on guard. The inhabitants do not immediately take to flight but
take up strategic positions close to their holes. From there, standing on their hind legs,
they stare at the intruder, watching its every move. So as a coyote trots through the town,
the alarm spreads from coterie to coterie and the intruder is met with fixed glares from
the citizens who let it come tantalisingly close before they duck into their burrows.
The social life of the prairie dog is not limited to defence. The adults, sitting outside their
burrows, proclaim their ownership by giving yet another kind of whistle, accompanied by
a small leap into the air. During the breeding season, the coterie members keep very
much to themselves and defend their boundaries against any intruder. The prairie dogs
tend the vegetation within the town with great care. Their grazing is so intense that many
of the plants they favour become eaten out. The animals then move to a different part of
their territory and let the old pasture recover. They also cultivate selectively. Sage,
although one of the commoner plants is not a favoured food item. If a seedling of one
takes root or if there is one growing in a newly colonized patch of territory, they do not
simply ignore it but deliberately cut it down and so allow more room for the plants they
prefer.
On the pampas of Argentina, the role of the prairie dog is taken over by another rodent,
the viscacha (Viscacha maximus). It, too, lives in dense communities but it grazes only
at dusk and at dawn. Like many creatures that are active in the twilight, they have
prominent recognition marks, broad horizontal black and white stripes across the face.
They build cairns over their burrows. If they find any sizeable stone during their
excavations they drag it up to the surface and dump it in the pile on the top.
The viscacha is another descendant of the first mass placental migration from North
America which invaded grasslands and forests of South America. This invasion included
some strange herbivores, most of which are now extinct. Which the separation of South
from North America some of herbivores evolved to great sizes and included an animal
that resembled a camel (Alticamelus) but stood over 3 metres tall. Another called the
Ground Sloth, Megatherium a relation of the sloth, was 7 metres tall and lumbered across
the ground, feeding on bushes and trees.
When the Panama bridge was re-established for a second time, creatures from the north
again invaded South America many of these animals such as the giant camel and the sloth
died out. In Patagonia, at the southernmost tip of the continent, the remains of a ground
sloth were found. The cold temperatures had virtually freeze-dried the large bones and
shaggy coated hide of this animal. Grass stems in the dung left by the animal appeared to
have clean edges as if they had been cut by artificial means. This evidence has given
rise to the hypothesis that the prehistoric Indians kept this animals in caves and feed them
bales of grass.
At the time that the sloths and other members of the Edentates (e.g. Glyptodon were
evolving in the south, on the other side of the Panama strait in North America, another
different group of grass-eaters were developing on the prairies. Their ancestors were
forest-living creatures, not unlike tapirs but far smaller. Their molar teeth were rounded
and suited to forest browsing. On the plains, in order to escape their predators, they
began to run faster. The earliest forms (Hyracotherium) run on four toes on their
frontlimbs and three toes on their hindlimbs. The longer the limbs, the better they serve
as levers and, properly muscled, the faster they can propel their owners. As time passed
these grazers lengthened their legs by rising off the ground onto their toes. The side toes
started to dwindle and the animal, an early horse the size of a dog, was running on a
single elongated middle toe (Mesohippus). The reduction of the side toes continued
(Merychippus). The ankle bones thus became placed halfway up its legs, the side toes
were reduced to internal vestiges called the splint bones, and the nail thickened to form
the protective shock-absorbent hooves (Pliohippus).
These changes in the limbs were accompanied by others changes. The grasses of the
plains were becoming tougher to chew and contained within their leaves tiny sharp
crystals of silica which wore teeth badly. So the proto-horses changed their rounded
molars into bigger and bigger grinders with hard ridges of dentine in them. One of the
problems of the grazing life is that an animal, with its head on the ground for such long
periods, cannot keep a good lookout for predators. The higher the eyes are placed on the
head the better the visibility. This requirement, together with the necessity to provide
room for the enlarged molars, resulted in a considerable elongation of the skull. So the
early horses evolved into the forms we know today (e.g. Equus). They spread across the
plains of America and eventually, at a time when the Bering Strait was dry and connected
North America with Asia, they reached Europe. From there they spread south and
colonised the plains of Africa. Later, they died out in North America and only
reappeared when they were introduced by European man. In Europe and Africa, they
flourished as horses (Equus), donkeys (Equus asinus) and zebras (Equus burchelli).
The zebras share the African plains with other running grazers which, during the same
period, had been evolving along lines of their own. They were the descendants of the
forest dwelling antelopes, like the duikers of today. They had already elongated their
legs for running within the forest though in a slightly different way from that of the
horses, retaining not one toe on the ground but two. Now, out on the plains, their legs
grew even longer and they became the cloven-hoofed grazers - antelope, gazelle and
deer. Today they flourish in such numbers that they constitute some of the most
spectacular assemblages of wildlife to be seen anywhere in the world.
On the edges of the plains in the open bush, where a small amount of vegetation cover
still occurs, antelope such as the dik-dik (Madoqua) live alone or in pairs within
territories that they mark and defend very like their forest-dwelling relations do. Farther
out in the open, where concealment is no longer possible, the antelope seek safety in
numbers, gathering together in large herds. They lift their heads regularly from grazing
to look around, and with so many sharp eyes and sensitive nostrils on the alert, it is more
difficult for a hunter to take the herd by surprise. If an attack does eventually come, then
the fleeing herd makes it difficult for a predator to target onto an individual prey item.
Keeping together in such numbers makes great demands on the pasture and the herds
have migrate regularly over great areas. Wildebeest (Connochaetes taurinus) seem able
to detect a shower of rain falling as far away as 50 kilometres and will move off to find it
and crop the newly sprouting grass. But this nomadic existence complicates the social
arrangements for breeding that in the forest, based on a single pair, had been so simple.
For some - the Impala (Aepyceros melampus), Springbok (Antidorcas marsupialis) and
Kudus (Tragelaphus strepsiceros):- territory remains nonetheless the basis of their
arrangements. Males and females form separate herds. A few dominant bucks leave the
bachelor herd to establish individual territories for themselves. Each marks the boundary
of its land, defends it against other males and tries to attract females into it and mate with
them. This however is a demanding business and most of the bucks who undertake it are
exhausted and badly out of condition after three months or so. Eventually, they are then
forced to yield to stronger, more rested rivals and they go back to join the bachelor herd.
The eland (Taurotragus oryx), the largest of the antelopes, and the plains zebra are among
the few that have finally broken the bond with territoriality altogether. They form herds
in which both sexes are always present and the males settle their problems over females
by battling between themselves wherever the herd happens to be.
In order to catch these grazers, predators need to improve their own running abilities.
Instead of elongating limbs and running on their toes, they have increased their strides by
making their spines extremely flexible. At full stretch, travelling at high speed, their hind
and front legs overlap one another beneath the body. The cheetah (Acinonyx jubatus) has
a thin elongated body and is said to be the fastest runnner on earth, capable of speeds, in
excess of 110 kph. But this method is very energy-consuming and great muscular
strength is needed to keep the spine springing back and forth and the cheetah cannot
maintain such speeds for more than a minute or so. Consequently this method of
locomotion is fine for an attacking animal but would not be suitable for a fleeing animal.
Lions (Panthera leo) are nowhere near as fast as the cheetah. Their top speed is about 80
kph. A wildebeest can do about the same and keep it up for much longer. So lions
generally hunt as a team. They set off in line abreast creeping close to the ground and as
they approach a group of prey - the lions at the ends of the line move a little quicker so
that they encircle the herd. Finally, these break cover, driving the prey towards the lions
in the centre of the line. Such tactics often result in several of the team making kills.
Hyenas (Crocuta crocuta) are even slower runners than lions and in consequence their
hunting methods have to be even more subtle and dependent on teamwork. The females
have separate dens where they rear their pups, but the pack as a whole works together and
holds and defends a territory. They have a rich vocabulary of sound and gestures with
which they communicate among themselves. They growl and whoop, grunt, yelp and
whine as a means of communicating amongst themselves. They also use their tails as a
means of communication. Tails are normally carried pointing down. An erect tail
indicates aggression; pointed forward over the back, social excitement; held between the
legs tight under the belly, fear. By hunting in well-co-ordinated teams, they have become
so successful that in parts of the African plains, they make the majority of kills and the
lions merely use their bigger size to bully their way on to a carcass.
Hyenas usually hunt at night. Sometimes they set off in small groups of two or three and
then a wildebeest is likely to be their intended prey. They test the herds by charging them
and then slowing down to watch the fleeing animals closely, as if trying to detect any
weakness among individuals. In the end, they appear to select one animal and begin to
chase it doggedly, cantering after it, snapping at its heels until it is finally goaded into
turning and facing its persecutors. When it does that, it is doomed. While it faces one
hyena, the others lunge at its belly, sinking their teeth into the unfortunate animal. The
wildebeest is soon crippled, and disembowelled.
Zebra are a more difficult prey. To hunt them, the hyenas unite to form a large team.
Through behavioural gestures they reaffirm bonds between one another. When they are
in groups like this, they will trot straight past herds of wildebeest, paying no attention to
them. At last they sight a small group of zebra, led by a dominant stallion. This usually
raises the alarm with a braying danger call and the herd gallops away the dominant
stallion taking the rear, placing himself between the pursuing hyenas and his mares and
foals. The hyenas follow in a crescent behind. The stallion will swerve and attack the
pack with his powerful kicks and bites and even chase the leading hyena, who may be
forced to drop back and allow others to make the running. But eventually one of the pack
will get past the stallion and begin to snap at a mare or a foal. As the chase relentlessly
continues, one gets a tooth-hold on a leg or the belly or the genitals and the animal is
dragged down. While the rest of the herd canters to safety, the hyenas leap on the fallen
zebra, ripping it to pieces.
Assignments
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
Discuss why herbivore occurring in the forest environment live alone or in pairs, whereas
those herbivores occurring in an open grassland environment live in gregarious groups.
Give examples of herbivores living in both environments.
Discuss why a herbivorous diet imposes certain problems for digestion and how some
animals have overcome such problems. Also explain why some herbivores have become
such large animals.
Discuss the identifying characteristics of each order of placental mammal. Your answer
should include a list of animals representing each order.
Ungulates have increased their running speed by increasing the length of their limbs,
whereas many carnivore hunters have increased their running speed by increasing the
flexibility of their backs. Discuss these adaptions, giving examples of animals that have
evolved them.
Discuss how underground social mammals organize their communities and protect
themselves from predation.
A LIFE IN THE TREES
In order to live in trees, two abilities are extremely useful, a talent for judging distances,
and a capacity for holding on to branches which requires a pair of forward-facing eyes
that can both focus on the same object and hands with grasping fingers. Only members
of the order primates (monkeys, apes and humans) have these characteristics.
There is no doubt that the early insect-eating shrew-like mammals which were the
ancestors of such diverse creatures as bats, whales and ant-eaters, also gave rise to the
primates. Indeed, an animal like the Tree Shrew (Tupaia glis) could have been an
ancestor to the primates. The Tupaia has two characteristics which it shares with the
primates; its eye-sockets are completely encircled by bone and its tongue is underlain by
a cartilaginous sub-tongue; other insectivores do not possess these characteristics. But
the Tree Shrew does not have the other primate hallmarks, namely hands with thumbs
that are opposable to the fingers which is required for a true grasping hand, and eyes that
face forward with overlapping fields of view so that distances can be judged.
Another group of animals with unmistakably monkey-like characteristics are called the
prosimians or 'pre-monkeys'. Typical of them is the Ring-tailed lemur (Lemur catta) of
Madagascar. These animals spend a lot of time on the ground in troops. Scent plays a
very important part in their lives. Their nose is nowhere near as well developed as that of
a Tree Shrew, but it is still very fox-like in proportion and it too has a moist muzzle with
bare skin around the nostrils. These animals also possess three kinds of scent glands.
One pair on the inside of the wrist which opens through spurs; another high up on the
chest, close to the armpits and a third around the genitals. With these, the males and to a
lesser extent the females produce signals. Such signals are often left on particular plants.
Typically a lemur will come upon a sapling, smell it carefully, checking whether it has
been visited before, then put its hands on the ground, hoist its rear as high as it can and
rub its genitals several times on the bark. Often, within a minute or so, another individual
will come and repeat the performance. Males also grasp a sapling with both hands swing
their shoulders so that they twist from side to side. Their wrist spurs rub against the bark,
making deep scratches that are impregnated with their musk.
The male Ring-tail uses scent not only as a signature but as a means of offence. When he
prepares for battle with a rival, he vigorously folds his arms several times and rubs his
wrists against his armpit glands. Then he brings his tail forward between his hind legs
and in front of his chest and draws in several times between his wrist spurs so that it is
loaded with scent. Thus armed, rivals face each other on all fours, lift their haunches
high and thrash their splendid tails over their backs with the fur bristling, so that the smell
is fanned forwards. Troops meeting on the frontier between territories may do battle in
this way for as long as an hour, hopping and skipping, squealing and yawning, and
excitedly marking saplings with their wrist spurs.
The Ring-tail also spends a lot of its time in trees. Here, its behaviour is more monkeylike. The eyes on the front of its head give it a binocular view and their hands with their
mobile fingers and opposable thumbs grasp branches. The fingers ending in short nails
rather than claws are sufficiently dexterous to enable the animal to pluck fruit and leaves
from the tips of branches. Although this lemur is quite big it can leap safely from tree to
tree.
The ability to grip is put to good use by infant lemurs. which cling to their mother's fur
and thereby travels with her wherever she goes and is provided with parental protection at
all times. As a consequence of this intensive parental investment Ring-tails usually have
only one baby at a time.
In Madagascar there are 21 species of lemur and its relatives, with most of them spending
much of time in the trees. The Sifaka (Propithecus verreauxi), a little larger than the
Ring-tail, has become a specialist jumper. Its legs are considerably longer that its arms
and enables it to leap four or five metres from one tree to another. However, when these
animals come to the ground they cannot use all four feet but have to hop using two feet.
Sifakas have scent glands beneath their chins; they mark their territory by rubbing them
on an upright branch and then reinforce the effect by dribbling urine over the bark,
wriggling their hips and slowly drawing themselves up the branch as they do so.
The Indris (Indri indri) is the most arboreal of all the lemurs and hardly ever comes down
to the ground. It is the biggest of all living lemurs with a head and body nearly a metre
long, and its legs are even longer in proportion than those of a Sifaka, the big toes are
widely separated from the rest and about twice the length, so that each foot resembles a
huge calliper with which the animal can grasp thick trunks. Indris also use scent in
marking the trees, though to a much lesser extent than the lemurs. Instead territories are
established using their voices. Every morning and evening, a family fills its patch of
forest with an unearthly wailing chorus.
Although the Ring-tail, Sifaka, Indris and several other Madagascan lemurs are active
during the day, their eyes have a reflecting layer behind the retina which increases the
ability to see in very dim light. This is a characteristic of animals that move at night and
strong evidence that these lemurs were nocturnal until quite recently. Many other lemurs
and their relatives are, however, nocturnal.
The Grey Gentle lemur (Hapalemur griseus), which is about the size of a rabbit, lives in
holes in trees and only comes out at night. The smallest of the group is the mouse-lemur,
with a snub nose and large eyes. The Indris has a closely related nocturnal equivalent the
Wooly Indris (Avahi laniger). Oddest and most specialised is the Aye-aye (Daubentonia
madagascariensis), an animal the size of an otter, with a black shaggy fur, a bushy tail
and large membranous ears. One finger on each hand is enormously elongated and
seemingly withered, so that it has become a bony articulated probe. With this the Ayeaye extracts beetle larvae, its main food, from their holes in rotting wood.
Fifty million years ago, there were lemurs and other prosimians not only in Madagascar,
but in Europe and North America. Around thirty million years ago, Madagascar became
separated from the continent of Africa, where more advanced primates evolved. These
primates never reached Madagascar, and lemurs survive today. Elsewhere, lemurs died
out, being unable to compete with the monkeys. Since monkeys with the single
exception of the South American Douroucouli (Aotus trivirgatus), are diurnal, other
prosimians which are nocturnal have been able to co-exist with the monkeys.
In Africa, the prosimian group is represented by the Bush Babies (Galago and Euoticus
species), the Potto (Perodicticus potto) and the Angwantibo (Arctocebus calabarensis).
In Asia, the prosimians are represented by the Loris (Loris and Nycticebus species) and
the tarsier (e.g. Tarsius syrichta). The Loris have large eyes and sign post their trees with
scent and use it for route-finding in the dark. They use urine to signpost, but because
they live in the tops of trees, they urinate on their hands and feet, rub them together and
then on to the topmost branches in their territory.
The Tarsier, is the size and shape of a tall Bush Baby. It has a long near-naked tail tufted
at the end, greatly elongated leaping legs and long fingered grasping hands and gigantic
glaring eyes (150 times bigger in proportion to the rest of its body, than our own) which
face directly forward. If this animal needs to see something to one side, it has to turn its
whole head. Together with these spectacular eyes, the tarsier has paper-thin ears, like
those of a bat, that can be twisted and crinkled so as to focus on a particular sound. With
these two highly developed sensory organs it hunts at night for insects, small reptiles and
even fledgling birds. It also marks territories with urine although its sense of smell is not
likely to be good. A look at its nose not only confirms this but reveals that the animal is
quite distinct from all other prosimians. For one thing, the eyes are so huge that there is
little room in the font of the skull for the nose itself and the internal nasal passages are
very much reduced in caparison with, say, a Bush Baby's. The nostrils are not commashaped nor are they surrounded by bare moist skin, as are the noses of lemurs and other
prosimians. In this it resembles monkeys and apes and it is tempting therefore to see the
tarsier as representing an ancestral form from which all the higher primates are
descended. Indeed, this was once held to be the case. Today it is argued that this little
creature is so specialised a leaper and nocturnal hunter that it could hardly have given rise
directly to monkeys. Nonetheless, it is seen as a close relative of those early primates
which, fifty million years ago, spread widely through the world displacing most of the
prosimians and ultimately populating both the Old and New worlds with monkeys.
Monkeys differ significantly from all the prosimians, except the tarsier in that their world
is dominated not by smell but by sight. Clearly it is important for creatures of any size
living in trees and, on occasion, jumping between them, to be able to see where they are
going. So daylight suits them better than darkness and all monkeys, except for the South
American Douroucouli, (Aotus trivirgatus) are active at that time. Their eyesight is better
than that of the prosimians. Not only do they see in depth, they have greatly improved
colour perception. With accuracy of vision they can judge the ripeness of distant fruit
and the freshness of leaves. They can detect the presence in the trees of other creatures
which, in a monochrome world, might be invisible. And they can use colour in their
communications between one another; monkeys because their colour-vision is so good,
have themselves become the most highly coloured of all mammals.
In Africa there lives de Brazza's Guenon (Cercopithecus neglectus) which has a white
beard, blue spectacles, orange forehead and black cap, the Mandrill (Mandrillus sphinx)
with a scarlet and blue face, and the vervet monkey, the males of which have startling
blue genitals; in China, the Golden Snub-nosed Monkey (Pygathrix roxellana) with a
metallic golden coat and an aquamarine face; in the Amazon forests, the Red Uakari with
a scarlet naked face (Cacajao calvus). With these colourful displays they advertise and
threaten and proclaim both their species identity their sex.
They also use sound in a similarly extravagant way, for up in the trees they are beyond
the reach of most predators. Howler monkeys (e.g. Alouatta seniculus) in South America
sit morning and evening, and sing in chorus. Their larynx is extraordinarily large and
their throats swell into resonating balloons. The resulting chorus can be heard for several
kilometres.
The monkeys that reached South America and became isolated there when the isthmus of
Panama sank beneath the sea, have developed very much along their own lines. That
they all are derived from one common stock is deduced from the number of anatomical
features they share e.g. all have fat noses with widely spaced nostrils opening to the side
whereas monkeys elsewhere have thin noses with forward or downward pointing nostrils.
One South American group, the marmosets (e.g. Callithrix penicillata) and tamarins (e.g.
Leontocebus leucopus), still use scent a great deal in communication even though they
are active during the day. The males gnaw the bark of a branch and then soak it with
urine. But they also have extremely elaborate adornments - moustaches, ear-tufts and
wig-like crests - which they display during their social encounters; and they threaten one
another with high-pitched twittering calls. Their manner of rearing their young is less
specialized than for the old world apes.
Marmosets are the smallest of all true monkeys and seem to have moved from the basic
monkey life style to that of a squirrel; eating nuts, catching insects and licking sap from
bark gnawed by their special forward-pointing incisors. The pygmy marmoset (Cebuella
pygmaea) has a body length of 100 mm and runs along branches, keeping a foothold on
the bark with claws. Use of claws is a recent reversion, for the embryonic marmoset
begins to develop monkey nails on its fingers and only later do they develop into claws.
Generally the primates have tended to evolve larger sizes, with the marmosets being an
exception. Greater weight, however, places greater demands on the grasping hands and
the South American monkeys have developed a unique way of supplementing them.
Their tails have turned into a fifth grasping limb. It is equipped with special muscles so
that it can curl and twine, and at the end its inner surface has lost its hair and developed a
ridged skin like that on its fingers. So powerful is it that a spider monkey (e.g. Saimiri
sciureus) can hang by its tail while foraging for fruit with both hands.
Old world monkeys have not developed the prehensile tails, however, they do extend
them horizontally when they run along branches, as a balancing aid. The failure of the
African monkeys to use a prehensile tail meant that if they did grow larger, they would
find an arboreal life increasingly awkward and consequently spend more time on the
ground. This is clearly evident by the lack of ground living New World monkeys,
whereas in the Old World there are many. The primate's tail seems of less value for
terrestrial life and there has been a tendancy to reduce and even lose the tail. The
mandrill (Mandrillus sphinx) and drill (Mandrillus leucophaeus), have tails that are
reduced to a tiny stump.
The Macaque monkey (Macaca) is one of the most adaptable of primates capable of
surviving in extreme conditions. There are about six different species and subspecies
distributed from the Atlantic Ocean to the Pacific. One group (Macaca sylvana) lives on
Gibraltar, the only non-human primate living naturally in Europe.
The Rhesus Macaque (Macaca mulatta) is one of the commonest monkeys in India, often
living close to urban areas. In Indonesia the crab-eating Macaque (Macaca fascicularis)
has become a competent swimmer and dives in the mangrove swamps for crabs and other
crustaceans. In Malaysia, the pig-tailed macaque (Macaca nemestrina) has been trained
to harvest coconuts. The Japanese Macaque (Macaca fuscata) is the most northerly living
monkeys and has a shaggy coat to protect it from the cold winters.
Macaques spend most of their time on the ground. Their hands and eyes, inherited from
an arboreal existence, together with adaptive learning abilities have permitted a
successful transition to a terrestrial existence.
The adaptability of the Japanese Macaques is illustrated by their use of hot volcanic
springs to provide relief from the cold winters, by washing dirt off food items such as
sweet potatoes and even separating rice grains from dirt by throwing them into water and
scooping off the floating grain. This ability to resolve problems is usually mastered by
one individual and the behaviour patterns associated with these are spread to all members
of the troop.
This ability and readiness to learn from your companions results in the community
having shared skills and knowledge, shared ways of doing things - in short, a culture.
The word is normally used only in the context of human societies, but the beginnings of
a culture can be seen in the way the Japaneese Macaques communicate amongst
themselves and organize their communities.
However, one of the most significant behavioural patterns that occurred in the evolving
primates was bi-pedalism. Moving on to two legs, would free the upper limbs, and
paticularly the hands to explore objects which eventually lead to the use of tools by apemen. To trace the origins of these animals, we have to go back some thirty million years.
At that time, one group of lower primates were increasing in size. This brought a change
in the way they moved through the trees. Instead of balancing on the top of a branch and
running along it, they began to swing along beneath it. Swinging successfully involves
physical changes. Arms lengthened, a tail that was used for balancing, disappeared; and
the musculature and skeleton of the body changed so that the backbone and abdomen was
supported in vertical rather than a horizontal planes. Those changes produced the
members of the Family Hominidae and include Gibbons (Hylobates); and the Great Apes
which includes the Orang-Utan (Pongo pygmaeus) from Asia, the Gorilla (Gorilla
gorilla), the Chimpanzee (Pan troglodytes), the Bonobo (Pan paniscus) from Africa and
Humans (Homo sapiens).
The great red-haired Orang Utan of Borneo and Sumatra is the heaviest tree-dweller in
existence. A male may stand over one and a half metres tall, have arms with a spread of
two and a half metres and weigh a massive 200 kilograms. The digits on all four limbs
have powerful grips, so that the animal is best described as being four-handed and the
ligaments of the hip joints are so long and loose that an orang utan, particularly when it is
young, can stick its legs out at astonishing angles. Plainly, they are excellently adapted
for the arboreal life.
At the same time, their size does seem to be something of a handicap to them. Branches
break under their weight. Often they are unable to get fruit they relish because it is
hanging far out on a branch that would never support them. Moving from tree to tree can
also cause problems. There is little difficulty if substantial branches from each tree
overlap, but that is not invariably the case. The Orang Utan deals with that problem
either by reaching out until he can clasp a stout branch, or by rocking the tree that he is in
until it bends over far enough for him to climb across.
Ingenious though these techniques may be they can hardly be reckoned easy or swift.
Indeed, sometimes an old male gets so large that he apparently finds the whole process
too exhausting and whenever he wants to travel any distance, he comes down and
lumbers across the forest floor. There is also evidence that the arboreal way of life is
fraught with danger for the Orang Utan. A study of adult skeletons showed, rather
pathetically, that 34 percent had, at one time or another, broken their bones.
The males, as they grow old, develop immense pouches which hang down from the throat
like gigantic double chins - not simply fat, but true pouches that can be inhaled with air.
They extend far down the chest across into the armpits and right over the back to the
shoulder blades. Although they may have been used by ancestral Orang Utans as
resonators to amplify their voice like howler monkeys, the modern Orang Utan does not
sing. His most impressive sound is his `long call', a lengthy sequence of sighs and groans
which continues for two or three minutes. To produce it, he partly inflates his throat
pouch and the call ends with a number of short bubbling sighs as the pouch deflates. But
he makes this call infrequently, and most of his vocalisations consist of grunts, squeaks,
hoots, heavy sighs and a sucking
noise made through pursed lips. It is a varied repertoire but a quiet one that can only be
heard fairly close by. The animal more often than not is alone and during these
monologues he gives the impression of a recluse, mumbling and grumbling to himself in
an absent-minded way. Males take up this solitary life as soon as they leave their
mothers, travelling and eating by themselves and only seeking company when they
briefly come together with a female to mate.
Female Orang Utans are about half the size of their mates but they too are solitary
animals and travel through the forest accompanied only by their young. This preference
for solitude may well be connected with their size. Orang Utans are fruit-eaters, and
being so big have to find considerable quantities of it every day to sustain themselves.
Fruiting trees, however, are uncommon and widely scattered through the forest, at widely
varying intervals. Some only bear fruit once every twenty-five years. Others do so
almost continuously for about a century but only on one branch at a time. Yet others
have no regular pattern and are triggered irregularly by a particular change in the weather
such as the sudden drop in temperature that proceeds a heavy thunder storm. Even when
they do produce fruit, it may only be on the tree for a week or so before it becomes overripe, falls or is exploited. So the Orang Utans have to make long journeys, continually
searching, and may well find it more profitable to keep their discoveries to themselves.
The gibbons, also fruit-eaters, have followed a very different line of development.
lncreasing size may have been the stimulus that made apes start to swing beneath
branches but the ancestral gibbons subsequently exploited the new style of locomotion to
the full by becoming smaller again. In the end they developed into even more
accomplished acrobats than any balancing, branch running monkey. A gibbon in motion
in the tree tops is one of the most glorious sights the tropical forest has to offer. With a
supple grace that is breath-taking, they hurl themselves nine or ten metres across space,
grabbing isolated branches and swinging themselves off again in another dazzling swoop
through the air. The arms that enable them to be acrobats in the air are as long as their
legs and torso combined, and if they do come to the ground, they have to be held above
its head out of the way. Its versatile grasping primate hands have also become
specialised at the cost of some of their manipulative abilities. Swinging at gibbon speed
requires that the hands be used as hooks that can be latched swiftly on to a branch and
then detached almost instantaneously. Thumbs get in the way, so they have moved down
towards the wrist and become much reduced.
Because Gibbons are small, there is usually enough fruit on a tree to satiate several of
them, so it is practical for them to travel together and they live in tightly knit families. A
pair is accompanied by up to four of their offspring of varying ages. Every morning, the
family sings in chorus. The male starts with one or two isolated and tentative hoots,
others join in, the group launches into a ecstatic song and finally the female takes over
with a rising peal that gets faster and faster and higher and higher until it becomes a trill
of tonal purity that no human soprano could ever challenge.
The parallel with the indri of Madagascar is an obvious one. Because of different
ancestral histories, one creature uses its fore limbs as its major propellant, the other its
hind. Otherwise, the tropical rain forest in diffent parts of the world has produced
creatures that are remarkably similar- families of singing, vegetarian gymnasts.
The African apes, in great contrast to their Asian relations, are much more terrestrial in
their habits. Gorillas live in central Africa, one form in the forests of the Congo basin,
another slightly larger one in the cool sodden moss-forests that cover the flanks of
volcanoes on the borders of Rwanda and Zaire. Young gorillas often climb trees, but they
do so rather carefully without the confidence of Orang Utans. This is not surprising since
the gorilla foot cannot grasp in the way that an Orang Utan's can, so the arms have to
provide the main means of hauling up the body. When gorillas descend, they do so feetfirst, lowering themselves with their arms, sometimes sliding down, braking by pressing
the soles of their feet flat on the trunk and showering moss, creepers and bark all around
them.
The big adult males are so huge, weighing up to 275 kilograms, that only the stoutest
trees can support them. They climb rarely and do not have much reason to, for although
the shape of their teeth and the nature of their digestive system suggest that they were
once primarily fruit-eaters, like the Orang Utan, they now subsist very largely on
vegetation that can be reached without climbing, such as nettles, bedstraw creeper and
giant celery. Usually, they also sleep on the ground, making a bed among the flattened
vegetation on which they have fed. They live in family groups of a dozen or so, each
being led by a silver-backed patriarch, who has several adult females attached to him.
They sit quietly grazing, ripping huge handfuls of stems from the ground with slow,
irresistible sweeps of their immense hands, lolling among the dense nettles and celery,
sometimes grooming one another. For the most part they sit in silence. Occasionally
they exchange quiet grunts or gurgles and if an individual wanders away from the main
group it makes a belching sound every now and then so that the rest know where it is.
While the adults doze, the young play and wrestle and occasionally rear up on their hind
legs to beat a quick tattoo on their chests, rehearsing the gesture the adults use in display.
The silver-back leads and protects his group. If he is frightened or angered by intruders
he may roar defiance and even charge. A blow of his fist can smash a man's bones.
Pestered by a younger rival, who may be trying to lure away one of the females of his
group, he will even fight although this is a rare event.
Several groups of Gorillas have been studied for many years and, through the patience
and understanding of the scientists, have come to accept other people, provided they are
properly introduced and behave in a proper fashion. Encountering a gorilla family and
being allowed to sit with them is a moving experience. They are in many ways so like us.
Their sight and sense of hearing and smell are closely similar to our own, so that they
perceive the world in very much the same way as we do. Like us, they live in largely
permanent family groups. Their life expectancy is about the same as ours and they move
from childhood to maturity and from maturity to senility at very similar ages. We even
share the same kind of gestural language and one that you must observe when you are
with them. A stare is rude or, put in a less anthropocentric way, threatening - a challenge
that invites reprisal. Keeping the head low and the eyes down is a way of expressing
submission and friendliness.
The placid disposition of the gorilla is connected with its diet and what it has to do to get
it. It lives entirely on vegetation of which there is an infinite supply growing
immediately to hand. As it is so big and powerful it has no real enemies and there is no
need for it to be particularly nimble in either body or mind.
The other African ape, the Chimpanzee, has a very different diet - and temperament.
Whereas a Gorilla may eat two dozen kinds of leaves and fruit, the Chimpanzee samples
two hundred or so and in addition, termites, ants, honey, birds' eggs, birds and even
small mammals like monkeys. To do this, it has to be both agile and inquisitive. Several
groups of chimpanzees, living in the forests on the eastern shores of Lake Tanganyika,
are being studied by a Japanese scientific team and are now so accustomed to the
presence of human beings that you can sit among them for hours at a time. The size of
their groups varies, but they are very much bigger than those of the Gorilla and may
contain as many as fifty animals. Chimpanzees are adept climbers, sleeping and feeding
in trees, but they habitually travel and rest on the ground, even in thick forest. There they
move on all fours, their hands knuckle-down and their long stiffly-held arms keeping
their shoulders high. Even when the group is settled and at ease on the ground, there is
constant activity.
The sexual bonds between individuals are variable. Some females and some males are
monogamous. Other males will mate with many females, and the females themselves,
when their hind-quarters inflame into pink fleshy cushions and they become sexually
receptive, often court and mate with numerous males. The tie between the young and
their mothers is very close. Immediately after birth, the infant clings to its mother's hair
with its tiny fists, though at first it is not strong enough to stay there for long without
maternal support. It will remain close to its mother, riding on her back like a jockey
when the group travels, until it is about five years old. This close dependence, made
possible by the baby's grasping hands, has a profound effect on Chimpanzee society, for
as a result the young learn a great deal from their mother and she is able to keep a close
eye on them as they grow up, supervising what they do, pulling them back from danger,
showing them from her own example how to behave.
There is a constant interplay between adults in a resting group. New arrivals will greet
one another, by offering the back of their outstretched hand to be sniffed and touched
with the lips. Elderly males, grey and balding with bright eyes and wrinkled faces, often
sit away from the main activity. They may be as much as forty years old and they often
give expression of short-tempered irascibility. They are treated with considerable
respect, the females rushing up to them smacking their lips and effusively hooting. All of
the group, young and old, spend hours grooming one another, carefully sorting through
the coarse black hair, scratching the skin with a fingernail to remove a parasite or a scale.
So anxious are they to perform this service to one another and so pleasurable do they find
it that sometimes a chain of five or six individuals may form, each absorbingly grooming
another. It has become a truly social activity and a gesture of friendship.
One way or another, the group investigates everything around it. A log smelling odd is
carefully sniffed and probed with a finger. A leaf may be plucked, scrutinized with the
greatest care, and explored with the lower lip and gravely handed to others for a similar
examination and then thrown away. The group may visit a termite hill. On the way
there, an animal will break off a twig, trim it to a particular size and strip it of its leaves.
On arrival at the termite hill it pokes the twig into one of the holes. When it pulls it out
again, it is covered with soldier termites than have gripped it with their jaws in an attempt
to defend the nest against the intrusion. The Chimpanzee draws the stem through its lips,
taking off the insects and eating them with relish. Although other animals use tools,
Chimpanzees like humans make tools.
The move made so long ago by the early primates from a ground-based scent-dominated
often nocturnal existence, to a life in the trees, led to the development of grasping hands,
long arms, stereoscopic colour vision an increased brain size. With the aid of these
talents, the monkeys and ape have made a great success of their arboreal life. But those
of them that subsequently returned to the ground, whether it was because of an increase
in body size or some other reason, found that these very talents could be deployed in their
new situation in a manner that opened up fresh possibilities and led to further changes.
The enlarged brain led to an increase in learning and the beginnings of a group culture;
the manipulative hand and the coordinated eyes made possible the use and manufacture
of tools. The primates that are practising these skills today, however, are in essence
repeating a process that another branch of their family started soon after the ancestral
apes first appeared in Africa. It was this branch that eventually stood upright and
developed their talents to such a degree that they came to dominate and exploit the world
in a way that no animal had ever done before.
Assignments
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
Describe the differences between New World and Old World monkeys.
Describe the adaptive radiation of lemurs that has occured in Madagascar.
Describe why the primitive Primates (Prosimians) are generally nocturnal except for
lemur species occurring in Madagascar.
Briefly describe all the members of the Family Hominidae.
Discuss how similar humans are evolutionarily, biochemically and behaviourally to other
members of the Family Hominidae.
THE COMPULSIVE COMMUNICATORS
Homo sapiens has suddenly become the most numerous of all large animals. Ten
thousand years ago, there were about ten million individuals in the world. They were
ingenious, communicative and resourceful, but they seemed, as a species, to be subject to
the same laws and restrictions which govern the numbers of other animals. Then, about
eight thousand years ago, their number began to increase rapidly. Two thousand years
ago it had risen to three hundred million; and a thousand years ago, the species began to
overrun the earth. Today, there are over four thousand million. By the turn of the
century, on present trends, there will be over six thousand million. These extraordinary
creatures have spread to all corners of the earth in an unprecedented way. They live on
the ice of the Poles and in the tropical jungles of the equator. They have climbed the
highest mountains where oxygen is cripplingly scarce and dived down with special
breathing devices to walk on the bed of the sea. Some have even left the planet
altogether and visited the moon.
Humans evolved from ape-like creatures about the size of Chimpanzees. They were
descendants of a forest-living ape that had been widespread through not only Africa but
Europe and Asia about ten million years ago. The first fossils of the plains-living ape
were discovered in southern Africa and in was accordingly named Australopithecus,
Southern Ape, but now several more kinds have been discovered in other parts of Africa.
They were not abundant and their fossilised bones are rare, but enough have been found
to give a fairly clear idea of what they were like in life. Their hands and feet resembled
those of their tree climbing ancestors and were very good at grasping things with nails on
the digits, not claws. The limbs were not particularly well suited to running. Their skulls
also show clear signs of their forest dwelling past. The eyes, as can be judged from the
sockets, were well developed by contrast their sense of smell would have been relatively
poor since the nasal clefts were short. The teeth are small and rounded and not well
suited to grinding grass or pulping fibrous twigs nor did they have shearing blades, like
those of a carnivore. It is probable that they excavated for roots and gathered berries,
nuts and fruit, and despite the inadequacies of their anatomy, they became hunters.
The structure of their hip bones shows that they were well onto to evolving bipedalism
and being able to survive on the African plains. Although these ape men were small
defenceless and slow, compared with the predators of the plains, they were able to
compete with the other predators. The ape men had hands with a precise and powerful
grip, developed by their ancestors in response to the demands of a tree climbing life. If
they stood upright, these hands could be ready at all times to compensate for the lack of
teeth and claws. If the ape-men were threatened by enemies they could defend
themselves by hurling stones and wielding sticks. Faced with a carcass, they might not
have been able to open it with their teeth as a lion could do, but they could cut it open
using the sharp edge of a stone, held in the hand. They could even take one stone, strike
it against another and so shape it. Stones deliberately struck in such a way have facets on
them that are quite different from those on stone that have been chipped by rolling in
streams or split by frost. They can thus be identified and many such have been found
associated with the skeletons of ape-men. The animals had become tool-makers. So apemen claimed a permanent place for themselves in the community of animals on the
plains.
This state of affairs lasted for a very long time, probably as much as three million years.
Slowly, generation after generation, the bodies of one line of ape-men became better
adapted to the plains-living life. The feet became more suited to running, lost their
ability to grasp and acquired a slight arch. The hips changed, the joint moved towards the
centre of the pelvis to balance the upright torso, and the pelvis itself became more bowlshaped and broader to provide a base for the strong muscles running between the pelvis
and spine that were needed to hold the belly in its new upright position. The spine
developed a slight curve so that the weight of the upper part of the body was better
centred. Most importantly, the skull changed, the jaw became smaller and the forehead
more domed. The brain of the first ape-men was similar to that of a gorilla, around 500
cubic centimetres, but by this time had doubled in size and these ape-men had grown to a
height of over a metre and a half and were called Homo erectus, Upright Man.
Homo erectus was a much more skilled tool-maker than previous ape-men. Their stones
were carefully shaped with a tapering point at one end and a sharp edge on either side,
and were of a size that fitted neatly into the hand. Evidence of one of his successful
hunts has been unearthed at Olorgesailie in southwest Kenya. In one small area, lie the
broken and dismembered skeletons of a giant baboon species now extinct and with these
bones are the remains are hundreds of chipped stones and several thousand rough
cobbles. All are of rock that does not occur naturally within 30 kilometres of the site.
The fact that the stones come from a distant site suggests that the hunts were
premeditated and that the hunters had armed themselves long before they found their
prey. Baboons, even the smaller living species (Papio species), are very formidable
creatures with powerful fanged jaws. Few people today, without fire-arms, would be
prepared to tackle them. The numbers killed at Olorgesailie suggest that such hunts were
regular team operations demanding considerable skill. Homo erectus was clearly, a very
formidable hunter.
Although impossible to establish Homo erectus must have possessed a language to
discuss their plans and carry out such attacks? Attempts have been made to deduce from
their skulls and neckbones the structure of the soft parts of their throats and the current
view is that although they were probably capable of making noises considerably more
complex than the grunts and screams of modern apes, their speech, would probably have
been slow and clumsy. However, Homo erectus had another medium of communication
at their disposal - gestures - and we can make some confident guesses as to what they
were and what they meant. Human beings have more separate muscles in their face than
any other animal. They make it possible to move the various elements - lips, cheeks,
forehead, eyebrows - in a great variety of ways that no other creature can match. There is
little doubt, therefore, that the face was the centre of Homo erectus's gestural
communication.
One of the most important pieces of information it transmits is identity. We take it for
granted that all our faces are very different from one another yet this is a very unusual
characteristic among animals. If individuals are to cooperate in an organised team in
which each person has their own responsibility then it is crucial for those taking part to be
able to distinguish one from another immediately. Many social animals, such as hyenas
and wolves, distinguish each other by smell. Human's sense of smell, however, is much
less well-developed than their sight, so recognition should be based on the shape of the
face.
Since the features of the face are extremely mobile, they can also convey a great deal of
information about changing moods and intentions. We still have little difficulty in
understanding expressions of enthusiasm and delight, disgust, anger and amusement. But
quite apart from such revelation of emotion, we also send precise messages with our
faces. Are the gestures we use today arbitrary ones that we have learned from our parents
and share with the rest of the community simply because we have the same social
background? Or are they deeply embedded in us and are an inheritance from our
prehistoric past. Some gestures do vary between societies and are clearly learned yet
others appear to be more universal and deep-seated.
With this improved talent for communication and skill in making tools Homo erectus
became more successful. Their numbers increased and they spread from southeastern
Africa into the Nile valley and northwards to the eastern shores of the Mediterranean.
Their remains have been found further east in Java, and in China. Whether they migrated
into Asia from Africa or whether these people were the descendants of an Asiatic apeman is unknown. Some of the African groups reached Europe. A few crossed over a
land bridge that once connected Tunisia, Sicily and Italy. Others travelled eastwards
round the Mediterranean and up north through the Balkans.
Homo erectus was in Europe in some numbers about a million years ago. But about
600,000 years ago the climate changed. It started to get very cold. The shift was gradual
but the overall trend was of great cooling. With so much water being locked up in the ice
caps caused the sea-level to drop and land bridges connected the various continents, so
that eventually these people were able to spread into the Americas across the Bering
Straits and down the island chains of Indonesia towards New Guinea and Australia.
In Europe, Homo erectus must have felt the increasing cold very keenly. They had
evolved in the warmth of the African plain and did not have the protection of thick fur,
like the mammals that had lived in these cold regions for a long period. Doubtless, many
creatures, in such circumstances, would have moved to warmer parts or died out. These
humans being dexterous and inventive hunted for furred animals, stripped the skins from
their dead bodies and used the skin for themselves. They also found shelter in caves.
These human's living sites have been discovered in great numbers in southern France and
Spain. Along the great limestone valleys of central France such as the Dordogne and in
the foothills of the Pyrenees, the cliffs are riddled with caves. From the archaeological
evidence there appears to be no significant difference between these people who lived in
the caves of France 35 000 years ago and ourselves. Anthropologists, accordingly, have
given these people the same name as they use, somewhat immodestly, for all modern
humans - Homo sapiens, Wise Man.
The difference between the life of a skin-clad hunter leaving a cave with a spear over his
shoulder to hunt mammoth, and a smartly dressed executive driving along a motorway in
New York, London or Tokyo, to consult their computer print-out, is not due to any
further physical development of the body or brain during the long period that separates
them, but to a completely new evolutionary factor; culture.
People have credited themselves with several talents to distinguish themselves from all
other animals. Once we thought that we were the only creatures to make and use tools.
We now know that this is not so. Chimpanzees do so and so do finches in the Galapagos
that cut and trim long thorns to use as pins extracting grubs from holes in wood. Even
our complex spoken language seems less special the more we learn about the
communications used by chimpanzees and dolphins. But we are the only creatures to
have painted representational pictures and it is this talent which led to developments
which ultimately transformed the life of mankind. That skill is the use a written
information in order to communicate between ourselves and to create our own cultural
identities.
Assignments
IN YOUR OWN WORDS WRITE A FOUR PAGE ESSAY ON THE FOLLOWING
TOPIC
Discuss how communication, co-operation and tool-making contributed to the evolution
of the species Homo sapiens.