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Chapter 20
MAMMALS
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Features and Diversity
• The Tell-Tale Hair
– Hair
• Protection, concealment, waterproofing and buoyancy,
signaling, sensory function, and thermal insulation
– Mammary glands nourish the newborn
– Most are placental
– Specialized teeth and jaws for processing food
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Features and Diversity
• Approximately 5400 living species
• Among the most highly differentiated groups in
animal kingdom
• Domesticated for use as food, clothing, pets, beasts
of burden, pets, and in research
• Alien mammal introductions have usually disrupted
the ecology
• In 2007, 512 species were listed as “critically
endangered” or “endangered”
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Origin and Evolution of Mammals
• Evolution of mammals from earliest amniote
ancestors is well documented
• Over last 150 million years, small, ectothermic,
hairless ancestors evolved into today’s endothermic,
furry mammals
• Amniotes classified as synapsids, anapsids, or
diapsids
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Origin and Evolution of Mammals
• Mammals evolved from synapsids that have a pair of
temporal openings in the skull
• Synapsids were first amniotes to diversify widely into
terrestrial habitats
• Anapsids have solid skulls and include turtles and
their ancestors
• Diapsids have two pairs of openings in the skull roof
and include dinosaurs, lizards, etc.
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Origin and Evolution of Mammals
• Earliest synapsids diversified into diverse herbivorous
and carnivorous pelycosaurs
• One of the early carnivorous synapsids gave rise to
the therapsids
• Therapsids were the only synapsid group to survive
beyond the Paleozoic
• Therapsids were first to have an efficient erect gait
with upright limbs beneath body
• Most of the variety of herbivores and carnivores
disappeared during the great extinction at the end of
the Permian period
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Origin and Evolution of Mammals
• The cynodonts were the only therapsid subgroup
that survived
• Evolved a high metabolic rate that supported a more
active life
• Specialized jaw musculature permitting a stronger
bite
• Several skeletal changes supporting greater agility
• Heterodont teeth -Improved food processing for variety of
foods
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Origin and Evolution of Mammals
• Turbinate bones the nasal cavity aided in the
retention of body heat
• A secondary bony palate permits breathing
while holding prey or chewing food
– Important later to mammal evolution by allowing
young to breathe while suckling
– Number of ribs reduced -Loss of lumbar ribs correlated
with the evolution of a diaphragm
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Origin and Evolution of Mammals
• Earliest mammals of late Triassic were small and
mouse- or shrew-sized
• Diphyodonts
– Teeth replaced only once as deciduous and permanent
teeth
• Transformation of three middle ear bones
– Malleus, incus, and stapes
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Origin and Evolution of Mammals
• Malleus and incus -Originated from articular and quadrate
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bones that previously served as jaw joint but became reduced
in size to better transmit sound vibrations
Believed to have been endothermic although cooler than
modern placental mammals
Hair was essential for insulation and indicates that sebaceous
and sweat glands were present
No fossil evidence, but mammary glands must have evolved
before end of the Triassic
Young early mammals would have hatched from eggs and
relied on maternal milk
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Origin and Evolution of Mammals
• Mammals survived first as shrew-like nocturnal
animals of the Late Triassic
• Diversification occurred in the Cretaceous period
during the Eocene Epoch
– Attributed to the many habitats vacated by extinction of
many amniote groups at the end of the Cretaceous
• Agile, endothermic, intelligent, adaptable, and gave
birth to young they protected
• 29 orders 1 order of monotremes (Prototheria), 7
orders of marsupials (Methatheria), and 21 orders of
placentals (Eutheria)
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Structural and Functional Adaptations of Mammals
Integument and Its Derivatives
• Mammal’s skin generally thicker than in other
vertebrates
• Composed of an epidermis and dermis
• Dermis thicker than the epidermis
• In regions subject to much contact and use, outer
layers of epidermis become thicker and filled with a
type of fibrous protein called keratin
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Structural and Functional Adaptations of Mammals
Hair
• Characteristic of mammals
– Reduced on humans and exists as a few bristles on whales
• Hair follicle is an epidermal structure, but lies in
dermis of skin
• A hair grows continuously by rapid proliferation of cells
in the follicle
• Cells in hair shaft are shifted upward away from their
source of nourishment, becoming filled with keratin,
and die
– Keratin is same protein as is found in nails, claws, hooves
and feather
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Structural and Functional Adaptations of Mammals
• Mammals have two kinds of hair forming their
pelage (fur coat): undercoat and guard coat
• Dense and soft underhair
– Serves as insulation by trapping a layer of air
• Coarse and longer guard hairs
– Protect against wear and provide coloration
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Structural and Functional Adaptations of Mammals
• Hair stops growing at a certain length
– Remains in follicle until new growth pushes it out
• 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
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Structural and Functional Adaptations of Mammals
• Patterns including spots, stripes, salt-and-pepper,
etc. are disruptive and conceal the animal
• Vibrissae or “whiskers” are sensory hairs
– Provide a tactile sense for nocturnal mammals
• Porcupine, hedgehog, and echidna quills are barbed
and break off easily
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Structural and Functional Adaptations of Mammals
Horns and Antlers
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True Horns
Occur in ruminants such as sheep and cattle
Hollow sheaths of keratinized epidermis
Surround a core of bone arising from skull
Normally not shed and are usually not branched, but
may be curved
• Grow continuously and occur in both sexes
– May be longer in males
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Structural and Functional Adaptations of Mammals
Antlers-Occur in the deer family
• Are branched and composed of solid bone when
mature
• Develop beneath an annual spring covering of highly
vascular soft skin or velvet
• When growth is complete just before breeding
season
– Blood vessels constrict in velvet
– Velvet removed by rubbing antlers against trees
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Structural and Functional Adaptations of Mammals
• Antlers are shed after breeding season and a new bud
appears for the next growth
• Each year, the new pair of antlers is larger than the
previous set
• Growing antlers may require a moose or elk to
accumulate over 50 pounds of calcium salts from its
vegetable diet
Rhinoceros Horn-hairlike keratinized filaments arise from
dermal papillae and are cemented together
• Not attached to the skull
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Figure 20_09
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Structural and Functional Adaptations of Mammals
Glands
• Mammals have the greatest variety of integumentary
gland
– All derived from epidermis
• Sweat glands are tubular, highly coiled glands found in
mammals
– Eccrine Glands
• Secrete a watery fluid that draws heat away from the skinand cools
it
• Found in hairless regions such as footpads in most mammals and
scattered all over body in horses and primates
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Structural and Functional Adaptations of Mammals
– Apocrine Glands
• Larger than eccrine glands with longer, convoluted ducts
• Secretory coil may be present in dermis or hypodermis
• Apocrine glands open into hair follicles
• In humans, they develop near puberty and are restricted
in distribution
• Secretion is milky and dries to form a film on the skin
• Do not function in temperature regulation and are
correlated with reproductive function
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Structural and Functional Adaptations of Mammals
– Scent Glands
• Present in nearly all mammals
• Vary in location and function
• Allow for communication with members of the same
species
– Mark territory, warning and defense signals
• Scent glands of skunks open into the anus and are very
odoriferous
• Many mammals release strong scents during the
mating season to attract opposite sex
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Structural and Functional Adaptations of Mammals
– Sebaceous Glands
• Most associated with hair follicles
– May open directly onto the skin surface
• Glandular cells produce an oily secretion, sebum
– Lubricates skin and hairs
• Most mammals have sebaceous glands over the entire
body
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Structural and Functional Adaptations of Mammals
– Mammary Glands
• Rudimentary in males and occur on all females
• Epidermis thickens to form a milk line along each side of
abdomen
• Mammae located along milk line
• Other mammals have swollen mammae periodically when
pregnant or nursing
• Mammary glands increase in size at maturity
• Human females develop mammary glands at puberty
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Structural and Functional Adaptations of Mammals
• In most mammals milk is secreted from mammary
glands via nipples or teats
• Exception: Monotremes lack nipples and simply secrete
milk into a depression on the mother’s belly where it is
lapped up by young
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Structural and Functional Adaptations of Mammals
Food and Feeding
• Mammals exploit a wide variety of food sources
– Some are require highly specialized diets and others are
opportunistic feeders
• Teeth
– Structure of teeth reveal the habits of a mammal
– Reptiles have homodont dentition or uniform tooth
patterns
– Differentiation of teeth for cutting, seizing, gnawing, etc.,
resulted in heterodont dentition
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Structural and Functional Adaptations of Mammals
– Incisors: Sharp edges for snipping or biting
– Canines: Specialized for piercing
– Premolars: Compressed crowns with one or two cusps for
shearing , slicing, crushing and grinding
– Molars: Larger bodies and variable cusp arrangements for
shearing , slicing, crushing and grinding
– Primitive mammal tooth formula
• 3 incisors, 1 canine, 4 premolars and 3 molars
– Mammals do not continually replace teeth
• Have one deciduous set or milk teeth and a permanent
set
– Generally, incisors, canines, and premolars are deciduous
– Molars are a single permanent set
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Structural and Functional Adaptations of Mammals
– Feeding Specializations
• Insectivores
– Shrews, moles, anteaters, and most bats
– Feed primarily on insects
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Structural and Functional Adaptations of Mammals
– Herbivores
• Browsers and grazers include horses, deer, antelope,
cattle, sheep, and goats
• Gnawers include rodents, rabbits, and hares
• Herbivores have reduced or absent canines, but molars
are broad and high-crowned
• Cellulose is a chain of glucose molecules, but the
chemical bonds are difficult to break
• Herbivores, which cannot produce cellulases, use
anaerobic fermentation chambers so microorganisms can
metabolize cellulose
• A side pocket or cecum may also serve as a fermentation
chamber and absorptive area
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Structural and Functional Adaptations of Mammals
– Rodents eat fecal pellets (coprophagy) in order to provide
additional fermentation
– Ruminants have a large, four-chambered stomach
• Food is regurgitated, re-chewed, and passed to the
rumen, reticulum, omasum, and abomasum
– Carnivores
• Most feed on herbivores
• Requires specialization for killing prey
• High protein diet is easily digestible and, therefore, the
digestive tract is shorter
• Do not have to continuously graze
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Structural and Functional Adaptations of Mammals
– Omnivores
• Feed on both plant and animal tissues
• Include pigs, raccoons, rats, bears, and most primates
including humans
• Many carnivores also eat fruits, berries, and grasses
when other food sources are scarce
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Structural and Functional Adaptations of Mammals
Migration
• Few terrestrial mammals make regular seasonal
migrations
– Most remain in a home range
• Migratory animals are especially common in
northern North America
• Caribou migrates twice each year, spanning 160–
1100 kilometers (100–700 miles)
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Structural and Functional Adaptations of Mammals
• The fur seal breeds on the Pribilof Islands and then
journeys to wintering grounds off the southern
California coast
• Although bats can fly and could migrate similar to
birds, most bats hibernate in winter
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Structural and Functional Adaptations of Mammals
Flight and Echolocation
• Bats can fly and some mammals glide from trees
• Bats are nocturnal or crepuscular (active at twilight)
• Echolocation, along with flight, allows bats to
navigate and eat insects in total darkness
• Bats inhabit totally dark deep caves, a habitat
ignored by other mammals and birds
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Structural and Functional Adaptations of Mammals
• Bats use frequencies from 30,000 to 100,000 Hz
(cycles per second), well beyond our hearing range
– Ten to 200 pulses of signals are sent to locate prey
– Echo is received before next pulse is sent
• External ears of bats are large to focus in on sound
location
• Bat navigation may allow bats to build mental image of
surroundings similar to visual images
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Structural and Functional Adaptations of Mammals
• All bats are nocturnal
• Fruit-eating bats use sight and olfaction to locate
food
• Vampire bat has razor-sharp incisors and
anticoagulant saliva
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Structural and Functional Adaptations of Mammals
Reproduction
Estrous Cycle
– Most mammals have mating seasons timed to coincide
with most favorable time to give birth and rear young
– Female usually restrict mating to a fertile period during the
estrus cycle
• Commonly called heat or estrus
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Structural and Functional Adaptations of Mammals
• Reproductive Patterns
Egg-Laying monotremes
– Monotremes, such as the duck-billed platypus, lay eggs
with one breeding season per year
– Embryos develop for 10-12 days in the uterus, a thin,
leathery shell is secreted around the embryo before the
eggs are laid
– Eggs are laid in a burrow and incubated for 12 days
– After hatching, young nourished by milk lapped off
mother’s fur near mammary glands
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Structural and Functional Adaptations of Mammals
Marsupials
– Pouched, viviparous mammals
– Although only eutherians are “placental mammals,”
marsupials do have a primitive choriovitelline or yolk sac
placenta
– Embryo is first encapsulated by shell membranes and
floats free in uterine fluid for several days
– After “hatching” from shell membranes, 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
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Structural and Functional Adaptations of Mammals
Placental Mammals
– Eutherians are viviparous placental mammals
– Embryo nourished in uterus through a yolk sac placenta
– Yolk sac placenta is usually replaced by a chorioallantoic
placenta
– Gestation is longer than in marsupials and is much longer
for large mammals
– Gestation and body size are loosely correlated due to
variation in maturity at birth
– Humans have slower developmental period than any other
mammal
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Mammalian Populations
– A population of animals includes all members of a species that
can potentially interbreed in a region
– All mammals live in ecological communities with other animal
and plant species
– Density-independent factors
• Affect animals regardless of their density
• Fires, severe weather, etc.
– Density-dependent factors
– Related to crowding of populations
• Infectious diseases
– The snowshoe hare cycles in population size, apparently due
to density-related causes
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Human Evolution
• Darwin devoted the book The Descent of Man and
Selection in Relation to Sex to human evolution
• At that time few human fossils had been found
• Primates have grasping fingers, flat fingernails, and
forward-pointing eyes
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Human Evolution
• Ancestral primates split into two major lineages
– One gave rise to lemurs, lorises, and tarsiers (traditionally called
prosimians)
– The other gave rise to monkeys and apes (traditionally called simians or
anthropods)
• Both were probably arboreal
– Required a large cerebral cortex (for precise timing of movements,
judgement of distance, etc.), and grasping limbs
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Human Evolution
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Highly developed special senses aided vision and coordination
Earliest simian fossils are from Africa about 40 million years ago
Many became day active rather than nocturnal
Vision became the dominant sense enhanced by color vision
Three major simian groups
– New World monkeys of Central and South America including howler
monkeys, spider monkeys, and tamarins
– Old World monkeys including the baboon, mandrill, and colobus
– Apes: gibbons, humans, orangutans, chimpanzees, bonobos,and gorillas
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Human Evolution
• Old World monkeys and apes form the sister group of
New World monkeys
• Old World monkeys
– Lack a grasping tail, have close-set nostrils, and opposable
thumbs
• Humans, orangutans, gorillas, and chimpanzees are
placed in a single family, Hominidae, and are called
hominids
• Apes first appear in 20-million-year-old fossils
• Standing upright (bipedalism) provided a better view
of the landscape and freed hands for using tools,
defense, carrying young, and gathering food
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Human Evolution
• In 2001, a remarkably complete skull of a hominid
dated at nearly 7 million years ago was discovered in
Chad
– Named Sahelanthropus tchadensis
– Most ancient hominid yet discovered
• Ardipithecus ramidus
– Dates to 4.4 million years ago
– Both ape and humanlike traits
• Australopithecus afarensis
– Short, bipedal hominid
– Brain size slightly larger than that of a chimpanzee
– Numerous fossils of A. afarensis have been discovered, including
“Lucy” by Johanson in 1974
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Human Evolution
• Australopithecus anamensis
– Intermediate between Ardipithecus ramidus and A. afarensis
– Considered a sister group of modern humans
• Bipedal australopithecine lineage produced
– A. garhi may be the missing link between Australopithecus
and Homo
– Australopithecus africanus, a side branch of Paranthropus
robustus
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Human Evolution
• The first species of Homo and the definition of the
genus are both controversial
• Homo habilis
– Had a larger brain than australopithecines
Used stone tools
• Homo erectus
- Low but distinct forehead
- Strong brow ridges
- Intermediate between the brain capacity of H. habilis and
modern humans
- Culture
- Limited speech
- Used fire
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Human Evolution
• H. floresiensis-species only 1m tall, from the
island of Flores, Indonesia
• Modern humans diverged from H. erectus
about 800,000 years ago
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Human Evolution
• H. neanderthalensis (Neandertals)
– More sophisticated stone tools
– Robust, heavily muscled bodies allowed them to
survive the cold climates of the Ice Age
– About 30,000 years ago, they were replaced and
by modern humans
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Human Evolution
• Modern Humans –product of the same processes
that have directed the evolution of every organisms
since life began
• Mutation, isolation, genetic drift, and natural
selection affect human populations
• We are unique with a non-genetic cultural evolution
that provides constant feedback between our past
and future experience
• Symbolic languages, conceptual thought, knowledge
of history and an ability to manipulate our
environment emerge from this cultural endowment
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