Chapter 28
Mammals
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Features
• Hair is a critical sign of being a mammal.
• A few mammals, especially aquatic forms, may have very
few hairs but they are still present.
• Hair serves many functions: protection, concealment,
waterproofing and buoyancy, signaling, sensitive vibrissae
and especially thermal insulation.
• Mammals have other characteristic features.
– Most have a specialized placenta to feed the embryo.
– The mammal nervous system is more advanced than in other animal
groups.
– Mammary glands nourish the newborn.
– Convoluted turbinate bones in the nasal cavity provide a high
surface area for warming and moistening inspired air and for reducing
moisture loss during exhalation.
Diversity
• About 4600 living species are known.
• Nevertheless they are the most highly differentiated
group in the animal kingdom.
• They have been domesticated for use as food,
clothing, pets, beasts of burden and in research.
• Exotic mammal introductions have usually disrupted
the ecology.
• Habitat destruction is a factor in 300 species and
subspecies being considered endangered.
History
• The evolution of mammals from their earliest
amniote ancestors is very fully documented.
• Over the last 150 million years, small, ectothermic,
hairless ancestors evolved into today’s
endothermic, furry mammals.
• Skull structures, especially teeth, provide abundant
evidence of the evolutionary descent.
• The skull roof separates amniotes into synapsids,
anapsids and diaspids.
• Mammals derive from the synapsids that have a
pair of openings in the skull roof.
– The synapsids were also the first amniotes to radiate
widely into terrestrial habitats.
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The earliest synapsids radiated into
diverse herbivorous and carnivorous
pelycosaurs.
From one of the early carnivorous
synapsids arose the therapsids.
The therapsids were the only
synapsid group to survive beyond
the Paleozoic.
Therapsids were the 1st to have an
efficient erect gait with upright limbs
beneath the body.
This reduced stability required the
cerebellum to have an expanded role
in muscle coordination.
Most of the variety of herbivores and
carnivores disappeared in the
Permian extinction.
Only the last therapsids subgroup to
evolve, the cynodonts, survived into
the Mesozoic.
Therapsids
Lineage
Cynodonts
• This group evolved a high metabolic rate that supported a
more active life.
• They have more jaw musculature and skeletal changes for
greater agility.
• A secondary bony palate allowed them to breathe while
holding prey or chewing food.
• This secondary palate would be important later to mammal
evolution by allowing young to breathe while suckling.
• Bones improved in biomechanics and developed processes
for firmer muscle attachment.
• The number of ribs was reduced, making the spinal column
more flexible.
• Within the cynodont clade, a small carnivorous group, called
trithelodontids, resembles mammals.
• Loss of lumbar ribs in cynodonts is correlated with the
evoluion of a diaphragm and also may have provided greater
dorso-ventral flexibility of the spinal column.
Early Mammals of the Triassic Period
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The earliest mammals of the late Triassic were small and mouse- or
shrew-sized.
They were diphyodonts; teeth were replaced only once as deciduous and
permanent teeth.
They were almost certainly endothermic although cooler than modern
placental mammals.
Hair was essential and also indicates that sebaceous and sweat glands
were present.
There is no fossil evidence, but mammary glands must have evolved
before the end of the Triassic.
Young early mammals must have hatched from eggs and relied on
maternal milk.
Mammals, having developed in the mid-Triassic, had to wait 150 million
years to diversify.
All non-mammalian synapsid groups became extinct when the dinosaurs
became abundant.
Fig. 28.3
Cenozoic Radiation of Mammals
• Mammals survived first as shrew-like nocturnal animals, then
in a radiation in the Eocene Epoch.
• The radiation is attributed to the many habitats vacated by
extinction of many amniote groups at the end of the
Cretaceous.
• Mammals were agile, endothermic, intelligent, adaptable,
and gave birth to young they protected.
• With the evolution of a new jaw joint between the dentary
and squamosal (temporal) bones in mammals, bones of the
previous jaw joint, the articular and quadrate, continued
their gradual reduction in size and became relocated in the
middle ear, where they became the malleus and incus,
respectively.
Skin
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Mammals skin is generally
thicker than in other classes
of vertebrates.
As with all vertebrates, skin
is made of epidermis and
dermis.
In mammals, the dermis
becomes much thicker than
the epidermis.
The epidermis is thinner and
well protected by hair.
In places that are subject to
abrasion, the outer layers
become thicker and cornified
with keratin.
Hair
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Hair is characteristic of mammals; it is reduced on humans
and exists as a few bristles on whales.
The hair follicle is an epidermal structure, but is sunk into the
dermis of the skin.
A hair grows continuously by rapid proliferation of cells in the
follicle.
Cells in the hair shaft are carried upward away from their
source of nourishment and die.
The dense protein keratin is the same protein as is found in
nails, claws, hooves and feathers.
Dense and soft underhair serves for insulation by trapping a
layer of air.
Coarse and longer guard hairs protect against wear and
provide coloration.
Hair
• Hair consists of three layers
– The medulla or pith is in the center of the hair.
– The cortex with pigment granules is next to the
medulla.
– The outer cuticle is composed of imbricated scales
• Different mammals have unique hair structure.
• Hair stops growing at a certain length; it
remains in the follicle until new growth pushes
it out.
Hair
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In most mammals, there are periodic molts of the entire
coat.
– Foxes and seals shed once every summer.
– Most mammals molt twice, in the spring and in the fall, with the winter
coat much heavier.
– Some have white winter coats for camouflage and brown summer
coats.
– Arctic mammals are not genetically albino where eye and skin
pigments are also missing.
– White winter fur is leukemism; they have dark eyes, dark-colored ear
tips, noses, etc.
• Patterns including spots, stripes, salt-and-pepper, etc. are
disruptive and conceal the animal.
• Vibrissae or “whiskers” are sensory hairs; they provide a
tactile sense to nocturnal mammals.
• Porcupine, hedgehog, and echidna quills are barbed and
break off easily.
Glands
• Mammals have the greatest variety of
integumentary glands; all are derived from
the epidermis.
• Sweat glands are tubular, highly coiled glands
found in mammals but never in other
vertebrates
– Eccrine Sweat Glands
– Apocrine Sweat Glands
Scent Glands
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Present in nearly all mammals,
they vary greatly in location and
function.
They communicate with members
of the same species: mark territory,
warning and defense.
Scent-producing glands are located
in many different regions in
different mammals.
The scent glands of skunks, minks
and weasels open into the anus
and are very odoriferous.
Many mammals give off strong
scents during the mating season to
attract the opposite sex.
Source
Sebaceous Glands
• Most are associated with hair follicles
although some open directly onto the surface.
• Cells in the cellular lining accumulate fats,
then die and are expelled to form oily sebum.
• It does not turn rancid but serves as a
dressing to keep the skin and hair pliable and
glossy.
• Most mammals have sebaceous glands over
the entire body.
Mammary Glands
• Mammary glands are probably modified apocrine glands.
• They are rudimentary in males and occur on all female
mammals.
• The epidermis thickens to form a milk line along which
mammae appear.
• Human females develop mammary glands at puberty with fat
accumulation; additional development occurs during
pregnancy.
• Other mammals have swollen mammae periodically when
pregnant or nursing.
• Mammary glands increase their size at maturity (in most
mammals milk is secreted from mammary glands via nipples
or teats, but monotremes lack nipples and simply secrete
milk into a depression on the mother’s belly where it is
lapped up by the young).
Food and Feeding
• Mammals exploit a
wide variety of food
sources; some are
specialists and others
are generalists.
• Mammal structures are
closely associated with
adaptations for food
finding or capturing.
Source
Teeth
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Structure or teeth
reveal the life habits of
a mammal.
• Reptiles had
homodont dentition or
uniform tooth patterns.
• Differentiation of teeth
for cutting, seizing,
gnawing, etc. resulted
in heterodont
dentition.
Types of Teeth
• Incisors have sharp edges for snipping or
biting.
• Canines are specialized for piercing.
• Premolars have compressed crowns with one
or two cusps for shearing and slicing.
• Molars have larger bodies and variable cusp
arrangements for crushing and grinding.
Teeth
• A primitive mammal tooth formula is three
incisors, one canine, four premolars and
three molars.
• Mammals do not continually replace teeth;
they have one deciduous set and a
permanent set.
• Generally, the incisors, canines and
premolars are deciduous; molars are a single
permanent set.
Specializations for Feeding
• Insectivores
– Shrews, moles,
anteaters and most bats
are insectivores.
– They eat little fibrous
vegetable matter so their
digestive tract is short.
– Many other mammals
occasionally feed on
insects, making this
distinction blurred.
Specializations for Feeding
Herbivores
• Browsers and grazers include horses,
deer, antelope, cattle, sheep and goats.
• Herbivores have reduced or absent
canines, but molars are broad and highcrowned.
• Rodents have chisel-shaped incisors
that grow throughout life.
• Cellulose is a chain of glucose
molecules, but the chemical bonds are
difficult to break.
• Herbivores use anaerobic fermentation
chambers so microorganisms can
metabolize cellulose.
• A side pocket or cecum may also serve
as a fermentation chamber and
absorptive area.
• Hares, rabbits, and some rodents eat
fecal pellets in order to provide
additional fermentation.
Specializations for Feeding
Herbivores
• Ruminants have a huge
four-chambered stomach.
• Food is regurgitated, rechewed, and passed to the
rumen, reticulum, omasum
and abomasum.
• Herbivores generally have
long digestive tracts for the
prolonged time needed to
digest fiber.
Specializations for Feeding
Carnivores
• Most carnivores feed on
herbivores.
• This requires specialization
for killing the prey.
• A high protein diet is easily
digestible and therefore the
digestive tract is shorter.
• Carnivores do not have to
continuously graze and they
have more leisure time.
• Capturing prey also requires
more intelligence, stealth,
and cunning.
Other Feeding Specializations
• Predation has driven herbivores
to have keen senses and
escape behaviors.
• Some herbivores use size (i.e.
rhinos, elephants) or defensive
group behaviors.
Omnivores
• Omnivores feed on both plant
and animal tissues.
• Many carnivores will switch to
fruits, berries, etc. when normal
food is scarce.
• Some mammals cache food
stores during times of plenty, a
common behavior of rodents.
Body Weight and Food Consumption
• The smaller the animal, the greater is its metabolic rate and
the more it must eat per unit size.
• The amount of food varies in proportion to the body surface
area rather than the body weight.
– Surface area is proportional to about 0.7 power of body weight.
– Amount of food a mammal or bird eats is also about 0.7 power of
body weight.
– A 3 gram mouse will consumer per gram of body weight five times
more food than does a 10 kilogram dog and about 30 times more
food than does a 50,000 kilogram elephant.
• Small mammals must spend much more time hunting and
eating food than do large mammals.
– A small shrew weighing 2 grams must eat more than its body weight
each day and will starve if deprived of food for a few hours.
– In contrast, a mountain lion may kill an average of one deer a week.
Fig. 28.14
Reproductive Cycles
• Most mammals have mating seasons timed to
coincide with most favorable time to rear young.
• Female mammals usually restrict mating to a fertile
period during the periodic estrous cycle.
• This time of female receptivity is known as heat or
estrous.
• Some animals lengthen gestation period by delayed
implantation; the blastocyst remains dormant while
its implantation in the uterine wall is postponed to
align birth with a favorable season.
• Animals with only one breeding season a year are
monestrous; recurrent breeding is polyestrous.
Reproductive Patterns
Egg-Laying Monotremes
• Monotremes, such as the duck-billed
platypus, lay eggs with one breeding
season per year.
• Eggs are fertilized in the oviduct before
albumin and a thin, leathery shell are
added.
• She lays eggs in a burrow nest where
they are incubated for 12 days.
• Similar to reptiles and birds, there is no
gestation and the egg provides all
nutrients.
• However, after hatching, young suck
milk from the mother’s fur near her
mammary glands.
Source
Reproductive Patterns
Pouched Marsupials
• Marsupials are pouched, viviparous mammals.
• Although only eutherians are “placental
mammals,” marsupials do have a primitive
choriovitelline “placenta.”
• The embryo is first encapsulated by shell
membranes and floats free for several days.
• After “hatching” from shell membranes, the
embryo erodes a shallow depression in the
uterine wall and absorbs nutrient secretions by
a vascularized yolk sac.
• Gestation is brief and marsupials give birth to
tiny young that are still embryos.
• Early birth is followed by a prolonged interval
of lactation and parental care.
Source
Source
• In red kangaroos, the first pregnancy is followed by a 33-day gestation
and then birth.
• The mother immediately becomes pregnant, but the presence of a
suckling young arrests development of the new embryo at the 100-cell
stage.
• Such a period of arrest is called embryonic diapause.
• It is possible to stairstep three young with one external, one suckling, and
one embryonic.
Placental Mammals
• Eutherians are viviparous
placental mammals.
• They have an investment in a
prolonged gestation in contrast
to marsupials with an investment
in prolonged lactation.
• The embryo in the uterus is
nourished through a
chorioallantoic placenta.
• Gestation is longer than in
marsupials and is much longer
for large mammals.
• Gestation and body size are
loosely correlated because there
is variation in maturity at birth.
• Humans are slower developing
than any other mammal; this
contributes to our uniqueness.
Reproductive
Patterns
Source
Fig. 28.21
Reproduction Patterns
• The ultimate number of young produced per
year also depends on mortality rate.
• Small rodents that are prey for carnivores
usually produce more than one litter each
season.
• Meadow mice can produce up to 17 litters of
four to nine young each year!
• At the other extreme, an elephant produces
on average four calves during her 50-year
life.
The End.