UNIVERSITY OF BUEA
FACULTY OF SCIENCE
--=-=-=-=-=DEPARTMENT OF ZOOLOGY AND ANIMAL PHYSIOLOGY
LECTURE NOTES - ZOO 202: Vertebrate Zoology
(6 Credits)
Recommended source material text:
1)Integrated Principles of Zoology. By Hickman CP, Roberts LS, Keen et al (2008). Fourteenth
edition, Mc Graw-hill publishers, New York.
2)Internet Websites
General objective: To understand the systematics, organisation and biology of chordates and their
evolutionary trends as well as their economic importance.
Introduction:
Amphibians, Reptiles, Birds and Mammals are all tetrapods (four limbs). Amphibia is a
Class of vertebrate animals living on land as well as inside water. They are the first terrestrial
vertebrates to invade and colonize the terrestrial environment. They are cold blooded vertebrates.
They lay eggs that have a gelatinous coat (non-amniotic). Their larvae are quite different from
the adults. Looking at the evolutionary trend, the class of Amphibia follows that of bony fishes
(Class Osteichthyes). The first true amphibians appeared in the Carboniferous Period. The study
of amphibians is called batrachology
The class of Reptilia follows that of Amphibia, and is followed by that of Aves, which is
also followed by that of Mammals. The origin of the reptiles lies about 320–310 million years
ago (Ma), when the first reptiles evolved from advanced reptiliomorph labyrinthodonts. They
are mostly terrestrial, though some are aquatic. [study of reptiles is herpetology]. Reptilia is a
Class of vertebrate Animals that move by creeping (reptation). Aves represent a Class of
vertebrate Animals that can move by flight with the aid of their wings. The fossil record
indicates that birds evolved from theropod dinosaurs during the Jurassic period, around 200–150
Ma, and the earliest known bird is the Late Jurassic Archaeopteryx. Birds are mostly aerial.
Mammalia represents a Class of vertebrate Animals that have hairs or fur on their skin and the
females can nourish their offsprings with breast milk. The first true mammals appeared in the
Triassic period.
The objective of this section is to study the general characteristics of this class (these classes) and
to understand their systematics, biology, evolutionary trends [as well as their economic
importance].
1. Class Reptilia
1.1.

General characteristics
Reptiles have their skin covered with corneoscutes (or scales), originating from their
epidermis.
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
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
The dermal glands are rare, thus they possess a dry skin.
They are poikilotherms.
The heart is shared in three cavities (except for crocodiles that have 2 ventricles and 2 atria).
They are amniotes.
The skull can bear temporal fenestrae or not.
The cloaca has a transverse opening (except for crocodiles that have a longitudinal one).
Modern reptiles inhabit every continent with the exception of Antarctica. The four currently
living orders are:

Sphenodontia/Rhynchocephala (tuataras from New Zealand)

Testudines/Chelonia (turtles and tortoises)

Squamata (lizards, snakes, and worm lizards)

Crocodilia (crocodiles, gavials, caimans, and alligators)
Scientific classification
Kingdom: Animalia
Phylum : Chordata
Subphylum: Vertebrata
Unranked: Amniota
Class: Reptilia
Some species names: Caiman crocodilus (Caiman), Chelonia mydas (Green Sea Turtle), Sphenodon punctatus
(Tuatara) and Emys pulchella (tortoise) . The Figure 1 shows some of those Reptiles
A sea Turtle
A Tegu lizard
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A tuatara (sphenodon).
Figure 1: Some examples of Reptiles
1.2.



Circulatory System
As shown in Figure 2, most reptiles have a three-chambered heart (two atria + one
variably partitioned ventricle), and two aortas that lead to the systemic circulation. The
degree of mixing of oxygenated and deoxygenated blood in the three-chambered heart
varies depending on the species and physiological state.
Some shunts are possible.
But there are some interesting exceptions to this general cardiophysiology: Cf.
crocodilians, pythons and monitor lizards
3
Figure 2 The two major types of heart and blood circulation in Reptiles
1.3.
Respiratory System
a) Reptilian lungs role
All reptiles breathe using lungs. Lung ventilation is accomplished differently in each main reptile
group. In squamates, the lungs are ventilated almost exclusively by the axial musculature. This is
also the same musculature that is used during locomotion. Because of this constraint, most
squamates are forced to hold their breath during intense runs. Some, however, have found a way
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around it. Varanids, and a few other lizard species, employ buccal pumping as a complement to
their normal "axial breathing."
There are some other specific cases like Tegu lizards, Crocodiles, turtles and tortoises.
a) The Palate role
Most reptiles lack a secondary palate, but some others like crocodiles have evolved a bony
secondary palate that allows them to continue breathing while remaining submerged (and protect
their brains against damage by struggling prey). Snakes took a different approach and extended
their trachea instead. Their tracheal extension sticks out, and allows these animals to swallow
large prey without suffering from asphyxiation.
1.4.
Digestive system
The internal anatomy of a reptile displays the various parts of his digestive system (Figure 3).
Most modern reptiles are carnivorous and have rather simple and comparatively short guts, meat
being fairly simple to break down and digest. Digestion is slower than in mammals, reflecting
their lower metabolism and their inability to divide and masticate their food. Being
poikilotherms, their energy requirement is about a fifth to a tenth of that of a mammal of the
same size. Large reptiles like crocodiles can live from a single large meal for months, digesting it
slowly. Turtles are the predominantly herbivorous reptile group, but several lines of agams and
iguanas have evolved to live wholly or partly on plants.
Herbivorous reptiles face the same problems of mastication as herbivorous mammals but,
lacking the complex teeth of mammals, many species swallow rocks and pebbles [cailloux in
French] (so called gastroliths) to aid in digestion.
1.5.
Excretory System
Excretion is performed mainly by two small kidneys. In diapsids (reptiles with two pairs of
fenestrae), uric acid is the main nitrogenous waste product; turtles, like mammals, excrete mainly
urea. Unlike the kidneys of mammals and birds, reptile kidneys are unable to produce liquid
urine more concentrated than their body fluid. This is because they lack a specialized structure
called a loop of Henle, which is present in the nephrons of birds and mammals. Because of this,
many reptiles use the colon to aid in the reabsorption of water. Some are also able to take up
water stored in the bladder (Cf. Figure 3). Excess salts are also excreted by nasal and lingual salt
glands in some reptiles.
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Figure 3
1.6.
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Internal anatomy of a Lizard (female)
Nervous system and sense
The reptilian nervous system includes a central nervous system and a peripheral nervous
system. The brain (cf Figure 3) contains the same basic part of the amphibian brain, but the
reptile cerebrum and cerebellum are slightly larger, as compared to amphibians of the same
size.
There are twelve pairs of cranial nerves.
Most typical sense organs are well developed with certain exceptions (see some examples).
1.7.
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
Reproductive System
There is an external sex dimorphism. For instance, males are bigger than females of the
same age. Males have pre-anal pores and hemipenal bulges while females have less and
smaller pores and do not have external bulges. Most reptiles have copulatory organs,
which are usually retracted or inverted and stored inside the body. In turtles and
crocodilians, the male has a single median penis, while squamates, including snakes and
lizards, possess a pair of hemipenes. Tuataras, however, lack copulatory organs, and so
the male and female simply press their cloacas together as the male excretes sperm.
The male possesses two testicles, housed inside the body. The male also has a copulatory
organ, either a single penis (turtles and tortoises, crocodilians) or a pair of hemipenes
(lizards, snakes) that can often be seen as two bulges behind the cloaca at the base of the
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tail. The penis or hemipenis is not connected to the urinary tract, and is strictly an organ
of reproduction.
Figure 5 Reptile copulation
Most reptiles reproduce sexually such as this Trachylepis maculilabris
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A female has ovaries which store eggs.
For fertilization, a male reptile inserts either one of his two hemipenes into the female's
cloaca, or the single penis is inserted.
Most reptiles reproduce sexually. All reproductive activity occurs through the cloaca
Most reptiles have copulatory organs, which are usually retracted or inverted and stored
inside the body. In turtles and crocodilians, the male has a single median penis, while
squamates, including snakes and lizards, possess a pair of hemipenes. Tuataras, however,
lack copulatory organs, and so the male and female simply press their cloacas together as
the male excretes sperm.
Figure 5: Eggs of extant Vertebrates
A- Non Amniotes egg: this would be the type of vertebrate egg found in basically aquatic species such as
frogs and fish. B- Amniotes egg: is an egg adapted for terrestrial environments having sacs (membranes)
for support, respiration and for storing waste. Organisms such as reptiles and birds that have this type of
egg are known as amniotes.
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Most reptiles lay amniotic eggs covered with leathery or calcareous shells. An amnion,
chorion, and allantois are present during embryonic life (see Figure 5). Such animals are
called Amniotes.
Some squamates can also reproduce asexually by parthenogenesis
There is also an important phenomenon : TDSD
1.8.
Skin
Figure 6 The hind leg of an iguana, showing iguanas' iconic scales.
Reptilian skin is covered in a horny epidermis (making it watertight impermeable) and enabling
reptiles to live on dry land, in contrast to amphibians. Compared to mammals’ skin, that of
reptiles is rather thin and lacks the thick dermal layer that produces leather in mammals. Exposed
parts of reptiles are protected by scutes (or scales), sometimes with a bony base. In
lepidosaurians such as lizards and snakes, the whole skin is covered in overlapping epidermal
scales (Figure 6). Such scales were first thought to be typical of the class Reptilia as a whole, but
are now known to occur only in lepidosaurians. The scales found in turtles and crocodiles are of
dermal, rather than epidermal origin and are properly termed scutes. In turtles, the body is hidden
inside a hard shell composed of fused scutes.
1.9.
Skeleton
The skeleton is made up of the cephalic skeleton, the axial skeleton, the zonal skeleton and the
appendicular skeleton (but snakes are peculiar in this class). The cephalic skeleton is represented
by skull. Three types of skulls can be found in reptiles and this has been giving rise to names
for their various groups. The first reptiles to evolve 300 million years ago were called anapsids.
They had no temporal fenestra (large hole in the back of the skull). It is believed that turtles are
surviving anapsids, although it is also possible that they re-evolved this skull configuration over
time. Later reptiles diverged into two groups. Synapsids include extant mammals and mammallike reptiles. They have a single temporal fenestra (per Skull side). Diapsids include all other true
reptiles. They have two temporal fenestra, which impart room for jaw muscles and a more
powerful bite.
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Figure 7: Types of skulls in Reptiles
A = Anapsid, B = Synapsid, C = Diapsid
Reptiles are better adapted to deal with the rigor of living on land than their amphibian ancestors.
They have more compact bones in the feet (appendicular skeleton) and a highly flexible
backbone (axial skeleton). Another feature that differs from amphibians is that modern reptiles
have several vertebrae connected to the upper bones of the pelvis (zonal skeleton), which afford
reptiles an expanded connection between hind limbs and skeleton.
In turtles, the skeleton presents some of the most interesting skeletal adaptations of any reptile.
In fact, its vertebral column and ribs are fused to the bones on the inside of the upper shell. The
vertebrae of the neck and tail are small enough to allow great flexibility so that most turtles can
pull back into the shell. The forelimbs are connected to the bottom of the shell by a pectoral
girdle that extends down the length of the turtle from the vertebral column.
The skeletal structure of the snake is unique. Because the snake's ribs are attached to all
vertebrae (which total between 130 to 500) except those in the tail (Figure 8). Some boas and
pythons retain a vestigial pelvis utilized for reproduction. Teeth point backwards; two rows are
on each upper jaw, and one row is on each lower jaw. Hollow, grooved fangs can inject venom
(Figure 9).
Figure 8 Snake skeleton
Snakes have an extremely long spine, with 130 to 500 vertebrae. (Humans have 32 to 34 vertebrae.)
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Figure 9 Skeleton of a venomous snake (head)
1.10.
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Defense mechanisms
Avoidance is the most common form of defense in reptiles.
Reptiles may also avoid confrontation through camouflage, fight or venomous attack.
1.11.
Economic importance
Reptiles have a significant economic value for food and ecological services (such as insect
control), and they are valued nationally and internationally for food, medicinal products, leather
goods, and the pet trade.
NB: There is unfortunately a sad consequence of these uses of Reptiles which is their progressive
disappearance. Thus, several countries now have laws to protect them.
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1.12.
Main Subgroups and phylogeny
 Classification
A recent and very interesting classification (adapted from Benton, 2004) is presented below.
 Series Amniota (unranked)

Class Reptilia
o SubClass Synapsida
 Order Pelycosauria
 Order Therapsida
o ``Class`` Sauropsida
 Subclass Anapsida
 Order Testudines (turtles)
 Subclass Diapsida
 Order Araeoscelidia
 Order Younginiformes
 Infraclass Ichthyosauria
 Infraclass Lepidosauromorpha
 Superorder Sauropterygia
 Order Placodontia
 Order Nothosauroidea
 Order Plesiosauria
 Superorder Lepidosauria
 Order Sphenodontia (tuatara)
 Order Squamata (lizards and snakes)
 Infraclass Archosauromorpha
 Order Prolacertiformes
 Division Archosauria
 Subdivision Crurotarsi
 Superorder Crocodylomorpha
 Order Crocodilia
 Order Phytosauria
 Order Rauisuchia
 Order Rhynchosauria
 Subdivision Avemetatarsalia
 Infradivision Ornithodira
 Order Pterosauria
 Superorder Dinosauria
 Order Saurischia

Phylogeny
a) Anapsids, synapsids and sauropsids (Diapsids and some other anapsids)
 The first reptiles were anapsids, and Turtles are believed to be surviving anapsids
 Very soon after the first reptiles appeared, they split into two branches. One branch, the
Synapsida ; the other branch, the Diapsida (Figure 10).
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Figure 10: Reptiles phylogeny
Reptiles (dark field) are a paraphyletic group comprising all non-avian and non-mammalian
amniotes.
The above diagram illustrates the "family tree" of reptiles (Cf. Laurin and Gauthier 1996).
b)- Permian reptiles
 While the terrestrial reptiliomorph labyrinthodonts still existed, the synapsids evolved the
first terrestrial megafauna (giant animals) in the form of pelycosaurs such as
Edaphosaurus and the carnivorous Dimetrodon.
 In the mid-Permian period the climate turned dryer, resulting in a change of fauna: The
primitive pelycosaurs were replaced by the more advanced therapsids (cf. Order
Therapsida).
 The anapsid reptiles, continued and flourished throughout the Permian. The pareiasaurs
reached giant proportions in the late Permian, eventually disappearing at the close of the
period (the turtles being possible survivors).
 Early in the period, the diapsid reptiles split into two main lineages, the archosaurs and
the lepidosaurs. Both groups remained lizard-like and relatively small and inconspicuous
during the Permian.
C)- The Mesozoic era, the "Age of Reptiles"
 Most of the earlier anapsid/synapsid megafauna disappeared, being replaced by the
archosauromorph diapsids.
 The archosaurs became the dominant group during the Triassic period, developing into
the well-known dinosaurs and pterosaurs, as well as crocodiles and phytosaurs.
 Some of the dinosaurs developed into the largest land animals ever to have lived, making
the Mesozoic era popularly known as the "Age of Reptiles".
 The dinosaurs also developed smaller forms, including the feather-bearing smaller
theropods. In the mid-Jurassic period, these gave rise to the first birds.
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2. Class Aves (Birds)
2.1.
General characteristics

Birds are also tetrapods, the forelimbs being converted as wings for flight aptitude and
the behindlimbs being covered with scales.
 They have their skin covered with feathers (plumage).
 They have "warm-blooded" (homeothermic or endothermic) metabolism.
 Their mouth is a beak with no teeth.
 They possess a four-chambered heart and a lightweight but strong skeleton.
 Only the left ovary and left oviduct are functional and the females lay hard-shelled eggs.
 They have lungs, but also air sacs for extra-air storage.
 They are mainly represented by the Subclass Neornithes including a large amount of
orders, as we will see later (Classification and phylogeny).
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Subphylum: Vertebrate
Superclass
Tetrapoda
Class:
Aves
The figure 11 shows the external morphology of a Bird.
Figure 11. External morphology of a bird
1 Beak, 2 Head, 3 Iris, 4 Pupil, 5 Mantle
6 Lesser feathers, 7 Scapulars, 8 Median feathers,
9 Tertials, 10 Rump, 11 Primaries,12 Vent,
13 Thigh, 14 Tibio-tarsal articulation, 15 Tarsus,
16 Foot, 17 Tibia, 18 Belly, 19 Flanks, 20 Breast,
21 Throat, 22 Wattle
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2.2.
Respiratory system and flight
Compared to other vertebrates, birds show many unusual adaptations, mostly to facilitate flight.
The skeleton consists of very lightweight bones. They have large air-filled cavities (called
pneumatic cavities) which connect with the respiratory system. Birds have one of the most
complex respiratory systems of all animal groups. Upon inhalation, 75% of the fresh air bypasses
the lungs and flows directly into a posterior air sac which extends from the lungs and connects
with air spaces in the bones and fills them with air. The other 25% of the air goes directly into
the lungs. When the bird exhales, the used air flows out of the lung and the stored fresh air from
the posterior air sac is simultaneously forced into the lungs. Thus, a bird's lungs receive a
constant supply of fresh air during both inhalation and exhalation. Sound production is achieved
using the syrinx, a muscular chamber incorporating multiple tympanic membranes which
diverges from the lower end of the trachea.
Figure 12. Internal anatomy of a bird
2.3.
Circulatory system
The bird's heart has four chambers (two atria and two ventricles) and the right aortic arch gives
rise to systemic circulation (unlike in the mammals where the left arch is involved). The postcava
receives blood from the limbs via the renal portal system. Unlike in mammals, the red blood cells
in birds have a nucleus.
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2.4.
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Digestive system, Diet and feeding
The digestive system of birds is unique, with a crop for storage and a gizzard (Figure 12) that
contains swallowed stones for grinding food to compensate for the lack of teeth.
Most birds are highly adapted for rapid digestion to aid with flight.
Birds' diets are varied and often include nectar, fruit, plants, seeds and various small animals,
including other birds.
Many birds glean for insects, invertebrates, fruit, or seeds. Some hunt insects by suddenly
attacking from a branch.
Those species that seek pest insects are considered beneficial 'biological control agents' and
their presence encouraged in biological pest control programs.
Water is needed by many birds although their mode of excretion and lack of sweat glands
reduces the physiological demands. Thus, there are also several adaptations for drinking.
2.5.
Excretory system
Like the reptiles, birds are primarily uricotelic, that is, their kidneys extract nitrogenous wastes
from their bloodstream and excrete it as uric acid instead of urea or ammonia via the ureters into
the intestine. Birds do not have a urinary bladder or external urethral opening and (with
exception of the Ostrich) uric acid is excreted along with feces as a semisolid waste.
However, birds such as hummingbirds can be facultatively ammonotelic, excreting most of the
nitrogenous wastes as ammonia. They also excrete creatine, rather than creatinine like mammals.
This material, as well as the output of the intestines, emerges from the bird's cloaca.
2.6.
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Reproduction
In nearly all species of birds, an individual's sex is determined at fertilization. However,
one recent study demonstrated TDSD
The two testes are functional in male birds, and this is in contrast to many (figures 13 and
14).
The bulk of the testis is composed of numerous convoluted seminiferous tubules. Sperm
cells are formed in the tubules.
Figure 13 Gallus domesticus testis with 59 weeks of age.
Right testis (RT) with lenght higher than the left testis (LT)
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A-Male reproductive system of Birds
B- Female reproductive system of Birds
Figure 14. Male and Female Reproductive systems of Birds
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Male and female birds mate by joining cloaca.
All female birds lay amniotic eggs with hard shells made mostly of calcium carbonate,
through the cloaca.
According to their help needs, chicks are termed altricial or precocial.
2.7.
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The nervous system is large relative to the bird's size.
The most developed part of the brain is the one that controls the flight-related functions,
while the cerebellum coordinates movement and the cerebrum controls behaviour
patterns, navigation, mating and nest building.
About their senses, there are some very interesting specificities (Cf. sight).
2.8.
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
Skin covers: Plumage, and scales
Feathers are a feature characteristic of birds. They have a variety of roles.
The scales of birds are composed of the same keratin as beaks, claws, and spurs. They are
found mainly on the toes and metatarsus, but may be found further up on the ankle in
some birds.
2.10.

Skeleton and flight
Bird skeleton (Figure 15) is specifically adapted for flight. Bones are light but resistant.
The ribs are flattened and the sternum is keeled for the attachment of two large flight muscles,
that are the pectoralis and the supracoracoideus (except in the flightless bird orders).
The forelimbs are modified into wings, that serve as an aerofoil. Modification for flight has
rendered avian forelimbs almost useless for any task other than flight (which distinguishes them
from almost all other vertebrate classes
2.9.


Nervous system and senses
Behaviour
Most birds are diurnal, but some birds, such as many species of owls and nightjars, are
nocturnal or crepuscular (active during twilight hours), and many coastal waders feed
when the tides are appropriate, by day or night.
One of the most interesting features in their behaviour is the communication.
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Figure 15. A Bird (Pigeon) skeleton
1. Skull, 2. Cervical vertebrae, 3. Furcula, 4. Coracoid, 5. Uncinate process, 6. Keel, 7. Patella,
8. Tarsometatarsus, 9. Digits, 10. Tibiotarsus (10 and 11), 11. Tibiotarsus (10 and 11), 12.
Femur, 13. Ischium (innominate bone), 14. Pubi (innominate bone), 15. Illium (innominate
bone), 16. Caudal vertebrae, 17. Pygostyle, 18. Synsacrum, 19. Scapula, 20. Lumbar
vertebrae, 21. Humerus, 22. Ulna, 23. Radius, 24. Carpus, 25. Metacarpus, 26. Digits, 27.
Alula.
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2.11.
Classification and phylogeny
 Classification
Modern birds are part of the subclass Neornithes. The subclass Neornithes has two superorders
a) Superorder Palaeognathae:
The Palaeognathae consists of two orders which comprise 49 existing species.

Struthioniformes—ostriches, emus, kiwis, and allies

Tinamiformes—tinamous
b) Superorder Neognathae:
The superorder Neognathae comprises 27 orders which have a total of nearly ten thousand
species.
The orders comprising the Neognathae are:

Anseriformes—waterfowl

Galliformes—fowl

Charadriiformes—gulls, button-quails, plovers and allies

Gaviiformes—loons

Podicipediformes—grebes

Procellariiformes—albatrosses, petrels, and allies

Sphenisciformes—penguins

Pelecaniformes—pelicans and allies

Phaethontiformes—tropicbirds

Ciconiiformes—storks and allies

Cathartiformes—New World vultures

Phoenicopteriformes—flamingos

Falconiformes—falcons, eagles, hawks and allies

Gruiformes—cranes and allies

Pteroclidiformes—sandgrouse

Columbiformes—doves and pigeons

Psittaciformes—parrots and allies

Cuculiformes—cuckoos and turacos

Opisthocomiformes—hoatzin

Strigiformes—owls

Caprimulgiformes—nightjars and allies

Apodiformes—swifts and hummingbirds

Coraciiformes—kingfishers and allies

Piciformes—woodpeckers and allies

Trogoniformes—trogons

Coliiformes—mousebirds

Passeriformes—passerines
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
Phylogeny
Aves
Archaeopteryx
Pygostylia
Ornithothoraces
Confuciusornithidae
Enantiornithes
Ornithurae
Hesperornithiformes
Neornithes
Figure 16 Basal bird phylogeny simplified after Chiappe, 2007
The subclass Neornithes is now known to have evolved into some basic lineages by the end of
the Cretaceous and is split into two superorders (the Palaeognathae and Neognathae).
2.12.
Relationship with humans and economic importance
Since birds are highly visible and common animals, humans have been having a relationship
with them for long now. Birds have become of great commercial importance.
2.13.
Some Diseases caused to human
Birds can act as vectors for spreading diseases such as psittacosis, salmonellosis,
campylobacteriosis, mycobacteriosis (avian tuberculosis), avian influenza (bird flu), giardiasis,
and cryptosporidiosis over long distances. Some of these are zoonotic diseases that can also be
transmitted to humans.
3. Class Mammalia
3.1.
General characteristics

Mammals are homeotherms (endotherms), air-breathing animals whose females are
characterized by the possession of mammary glands while both males and females are
characterized by hair and/or fur.
 They possess three middle ear bones used in hearing, and a very well developed brain.
 They have five fingered-limbs.
 The largest group of mammals, the placentals, use a placenta during gestation.
 The circulatory includes a four-chambered heart.
 Mammals are divided into two subclasses or into three main infraclass taxa depending on
how they are born.
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Subphylum:
Vertebrata
Class:
Mammalia
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3.2.
Circulatory system
The heart is well defined, showing four chambers (two atria and two ventricles). In the general
(or systemic) circulation, the left ventricle propels blood from the heart to organs towards the
aortic arc (that divides in left and right aortic vessels). The blood returns to heart (right atrium)
via the vena cava. In the small (or pulmonary) circulation, the pulmonary artery drives blood
from the right ventricle to lungs, and the pulmonary vein drives blood from lungs to left atrium.
Red blood cells do not have nucleus.
3.3.


Like birds, mammals have lungs for breathing. But they don’t have air sacs.
Breathing is largely driven by the muscular diaphragm and other respiratory muscles. As
a result, air is sucked into or expelled out of the lungs, always moving down its pressure
gradient.
3.4.




Respiratory system
Digestion and energy metabolism
As compared to the above classes, mammals have the most defined and specialized
digestive system.
Digestion is related to the energy metabolism. Endothermy requires plenty of food
energy, so at similar size, mammals eat more food than reptiles.
To maintain a high constant body temperature is energy expensive.
The digestive tract of an herbivores is more complex than that of the carnivores.
3.5.
Urinary system
The urinary system is well defined but simple. It includes paired kidneys (urinary organs), two
ureters, a single bladder and a urethra that opens at the edge of the penis (in male) or inside the
vulva (in female). In female it is totally separated from the reproductive system while in male the
semen and the urine flow throw the same urethra.
3.6.
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
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Reproduction
Certain glands of mammals known as mammary glands are specialized to produce milk, a
liquid used by newborns as their primary source of nutrition. The monotremes branched
early from other mammals and do not have the nipples seen in most mammals, but they
do have mammary glands.
Most mammals give birth to live young (vivipary), but a few lay eggs.
Marsupialia, or marsupials, have short gestation periods and give birth to undeveloped
young which are contained within a sac-like structure (marsupium) located in front of the
mothers' abdomen.
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
Placentals are mammals that give birth to complete and fully developed young. This is
usually characterized by long gestation periods.
In Eutherians, the reproductive system is quite specialized (Figures 17, 18).
a)- Female reproductive system
Fig.17 Dog female reproductive system
b)-Male reproductive system
Fig 18. Dog male reproductive system
3.7.


Nervous system
Mammals have the highest brain stage of development.
The mammalian nervous system includes mainly the central nervous system (CNS) and
the peripheral nervous system (PNS).
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
The CNS includes a well-developed brain and a spinal cord, while the PNS is made up of
nerves.
All mammalian brains possess a neocortex, a brain region that is unique to mammals.
3.8.


Skin
The integumentary system is made up of three layers: the outermost epidermis, the
dermis, and the hypodermis.
Although mammals and other animals have cilia that superficially may resemble it, no
other animals except mammals have hair. That is the reason why it is a definitive
characteristic of the class.
3.9.
Skeletal system
Mammals possess a cephalic skeleton, an axial skeleton, the zonal skeleton and the appendicular
skeleton (Figures 19, 20).
Fig.19 Goat skeleton
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A
B
Fig.20 Human skeleton (A-Front view, B-Back view)
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3.10.
Classification and phylogeny
The Class Mammalia includes the following main representatives:
 Subclass monotremes (or prototheria)
o Examples : Echidnas, Platypus , Ornithorhyncus
 Subclass marsupials (or Metatheria)
o Examples: Koala, Wombats, Kangaroos
 Subclass placentals (or Eutheria).
o Order Fissipeda
 Family canidae (dog, renard, loups)
 Family Ursidae (ursus)
 Family Viverridae (Mongoose)
 Family Hyenidae (Hyena)
 Family Felidae (cat, tiger, lion)
o Order Pinnipeda
 Family Phocidae (sea elephant)
 Family otariidae (otary or sea lion)
o Order Cetacea (Whale, Dolphin, baleen)
o Order Artiodactyla
 Sb.order Suiforma
 Family Hippopotamidae (Hippopotamus)
 Family Suidae (Porc, Sanglier, phacochere)
 Sb. Order Solenodonta
 Family Tylopoidae (camel)
 Family Elaphoidae (Cerf)
 Family Tauroidae (caw, goat, giraffe)
o Order Perissodactyla
 Sb.order Hippomorpha (horst, zebra)
 Sb. Order Ceratomorpha (Tapir, Rhinocerose)
o Order Pholidota (Pangolin)
o Order Lagomorpha (Pikas, Rabbits,Hares)
o Order Rodents
 (Rats, mice, castors, cabails, Scaly-tailed flying squirrel)
o Order Insectivores (Herissons, musaraignes)
o Order Primates
 Super Family Cynomorphes (Cercopithecidae like Macaque, Baboons)
 Super Family Anthropomorphes (Orang-Outan, Chimpanzees, Gorilla)
 Sub Order Hominians
 Family Australopithecidae
 Family Hominidae (Pithecanthropus, Homo neanderthalensis,
Homo sapiens)
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