Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the...

Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education

MAMMALSChapter 20

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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 5700 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 2013, 643 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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• 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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• 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

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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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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• 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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• 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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• 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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Horns and AntlersTrue 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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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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• 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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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 skin and 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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– 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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– Scent Glands• Present in nearly all mammals• Vary in location and function • Allow for communicate 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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– 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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– 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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• 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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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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– 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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– Feeding Specializations • Insectivores

– Shrews, moles, anteaters, and most bats – Feed primarily on insects

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– 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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– 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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Lori Bradshaw

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– 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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Migration • Few terrestrial mammals make regular seasonal

migrations– Most remain in a home range

• Migratory animals are especially in northern North America

• Caribou migrates twice each year, spanning 160–1100 kilometers (100–700 miles)

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• 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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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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• 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

• Fruit-eating bats use sight and olfaction to locate food• Vampire bat has razor-sharp incisors and

anticoagulant saliva

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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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• 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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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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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 psychogenic 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 anthropoids)

• Both were probably arboreal– Required a large cerebral cortex (for precise timing of

movements, judgment of distance, etc.) and grasping limbs

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Human Evolution• 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

– Ceboids: New World monkeys of Central and South America including howler monkeys, spider monkeys, and tamarins

– Cercopithecoids: 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 6.5 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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• 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


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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