Class Mammalia

A. General Characteristics

1. Body covered with hair, but reduced in some spp..

2. Integument with sweat, scent, sebaceous, and mammary glands.

3. Skeletal features include:

a. skull with

I. bony palate
II. middle ear with three ossicles (malleus, incus, and stapes)

b. seven cervical vertebrae
c. fused pelvic bones.

4. Mouth with diphyodont teeth (milk, or deciduous, teeth replaced by a permanent set of teeth); teeth in most spp. are heterodont (teeth with various structures and functions; lower jaw a single enlarged bone (dentary).

5. Movable eyelids and fleshy external ears

6. four limbs (reduced or absent in some spp.) adapted for many forms of locomotion.

7. Circulatory system of 4-chambered heart, non-nucleated RBC's, and biconcave RBC's.

8. Respiratory system of lungs with alveoli, and voice box (larynx); a secondary bony palate separates air and food passages; muscular diaphragm separates thoracic and abdominal cavities.

9. Excretory system of metanephric kidneys and ureters that usually open into a bladder.

10. Brain highly developed; 12 pairs of cranial nerves.

11. Strictly endothermic homeotherms.

12. Cloaca present only in monotremes (egg-laying mammals, such as duck-billed platypus).

13. Separate sexes; specialized reproductive and copulatory organs.

14. Internal fertilization; egg develops in a uterus with placental attachment (except in monotremes); fetal membranes (amnion, chorion, allantois); sex determination by heterogametic males (females are isogametic).

15. Young are nourished by milk from mammary glands.

B. Classification

Class Mammalia

Subclass Prototheria ("primitive wild animal") - extinct mammals with simple level of complexity compared to many living and other extinct spp. of mammals; egg-laying monotremes (e.g. duck-billed platypus) were once classified in this taxonomic group, but a 1985 report regarding relatively recent, additional fossil evidence suggested a higher level of complexity of extinct monotremes, thus "placing" them in the more complex therian subclass.

Subclass Theria (wild animal) - complex mammals of three different infraclasses; contains both extinct and extant (living) mammals; we will discuss only the living ones.

Infraclass Orinthodelphia (bird; womb) - egg-laying mammals; one living order.

Order Monotremata - duck-billed platypus and spiny anteater of Australia; oviparous.


Infraclass Metatheria (middle animal) - ovoviviparous mammals; one living order.

Order Marsupialia - marsupials, pouched mammals; kangaroo, koala, opossum, Tasmanian wolf (devil).


Infraclass Eutheria (true wild animal) - placental mammals; viviparous; several orders.

Order Xenarthra (Edentata, toothless) - anteaters, armadillos, and sloths; 29 spp. which are toothless (anteaters) or have simple, rootless molars that grow throughout life (sloths and armadillos). Mostly live in South and Central America; the nine-banded armadillo is common in the southern US.
Order Lagomorpha - rabbits; have constantly growing incisors, like rodents, but unlike rodents, they have an additional pair of incisors growing behind the first pair; all are herbivores.
Order Pholidota - pangolins. An odd group of mammals whose bodies are covered with overlapping horning scales that have arisen from fused bundles of hair. Found in tropical Asia and Africa; seven spp.
Order Rodentia - rodents; gnawing mammals: squirrels, rats, etc.; 6 families:

1. squirrel family
2. rat and mouse family
3. beaver family
4. porcupine family
5. gopher family
6. hamster family


Order Insectivora - insect-eating mammals: shrews, hedgehogs, moles; most primitive of placental mammals; distributed world wide except Australia and New Zealand; most are burrowers; shrews are among the smallest of all mammals.
Order Chiroptera - (cheir, hand; pteron, wing): bats; the only true flying mammals; wings are modified forelimbs.
Order Primates - monkeys, apes, humans: most highly-developed brain of all organisms; especially large cerebral hemispheres. Most of the 180 spp. are arboreal. This order consists of two suborders; suborder Anthropoidea includes humans (superfamily Hominoidea, family Hominidae).
Order Carnivora - flesh-eating mammals: dogs, wolves, cats, bears, and weasels. This order consists of 5 families:

1. Canidae - dog family
2. Felidae - cat family
3. Ursidae - bear family
4. Procyonidae - raccoons
5. Mustelidae - fur-bearing family; skunks, weasels, otters, badgers, minks, and wolverines


Order Pinnipedia - sea lions, seals, and walruses: fore- and hind limbs modified (flippers) for swimming; entirely marine.
Order Tubulidentata tube teeth - aardvark, only one sp. (Orycteropus afer); aardvark is Dutch for earth pig; pig-like body; found in Africa.
Order Sirenia - sea cows; large aquatic mammals with no hind limbs; forelimbs modified for swimming (flippers).
Order Proboscidea - proboscis (trunk, nose) mammals: elephants; largest of living land animals; the two upper incisors are elongated as tusks, and the molar teeth are well developed. There are two spp.: the African elephant (Loxodonta africana), which has large, floppy ears; and the Indian elephant (Elephas maximus), which has relatively small ears.
Order Perissodactyla - odd-toed, hoofed mammals: horses, zebras, and rhinos; along with the order Artiodactyla, are considered the ungulates (hoofed mammals; teeth adapted for chewing; 18 spp.; horses belong to family Equidae; all are herbivores.
Order Artiodactyla - even-toed, hoofed mammals: swine, camels, deer, hippos, antelopes, cattle, sheep, and goats; most are ruminants most are strictly herbivorous; many domesticated animals of economic significance; three suborders: Suina - pigs and hippos; Tylopoda - camels; and Ruminantia (deer, giraffes, sheep, cattle, etc.).
Order Cetacea - whales, dolphins, porpoises; 78 spp. well-adapted for aquatic life; anterior limbs are modified into broad flippers; posterior limbs are absent; nostrils are represented by a single blowhole on top of the head; no hair except for a few hairs on the muzzle, no skin glands except the mammary glands and glands associated with the eyes; no external ear; relatively small eyes. The order is divided into the toothed whales - dolphins, porpoises, and sperm whales; and the baleen whales - gray whale, blue whale, and other filter-feeding, plankton eating whales; the blue whale is the largest the largest animal known (living or extinct).

Order Macroscelidea - elephant shrews; secretive with long legs, a snout-like nose adapted for foraging for insect, large eyes, and are widespread in Africa; 15 spp..

Order Dermoptera - flying lemurs; similar to true bats and consist of a single genus, Galeopithecus; found in the East Indies; not true lemurs, either - lemurs are primates (suborder Prosimii); also not true bats because they do not truly fly, but rather, glide like flying squirrels; two spp..

Order Scandentia - tree shrews; small, squirrel-like, found in tropical rain forests of southern and southeastern Asia. Despite their name, they are not well adapted for life in trees, and some are completely terrestrial; 16 spp..

Order Hyracoidea - hyraxes; herbivores that are restricted to Africa and Syria. They have similarities with short-eared rabbits, but have teeth like members of the order Perissodactyla (odd-toed hoofed mammals); have four, hoofed toes on front feet and three on the back feet; 7 spp..

C. Structural and Functional Adaptations of Mammals

1. Integument - skin and its modifications; thickest of all vertebrates; composed of epidermis and dermis.
1. Epidermis - is very thin and protected by hair in most spp.; is thick in areas of contact which lack hair (e.g. surfaces of hands and feet; keratin deposits common).
2. Dermis - very thick with many specialized glands in most spp..
3. Hair of collagen and keratin - also called the pelage, the fur coat in some spp. or collectively the total body hair of a mammal; two types:
1. Underhair - dense and soft; provides insulation.
2. Guard hair - long and coarse for protection and coloration.
4. Horns and antlers: composed of mostly keratin; three kinds:
1. True horns - found only in ruminants (cud-chewers such as sheep and cattle; 4-chambered stomach); composed of sheaths of keratin surrounding a core of bone derived from the skull; usually not shed; unbranched; usually curved and frequently pointed; present on both males and females.
2. Antlers - found on members of the deer family; composed entirely of bone when mature; during annual growth, the bone is covered with highly-vascularized skin called velvet; when the growth is complete, the blood vessels constrict and the velvet is rubbed off by scraping the antlers against trees.
3. Rhinoceros horns - hair-like fibers which arise from the dermis; the fibers fuse together to form a single horn-like structure.
5. Glands:
1. Sweat - present only in mammals; two kinds:
• Eccrine - secrete watery sweat that evaporates and cools the body; located in middle portion of dermis and open to the outside through sweat pores.
• Apocrine - associated with certain hair follicles and open directly into region of the follicles at the epidermis - dermis interface; are located deep within the dermis.
2. Scent - secrete volatile chemicals which are involved in certain types of communication among members of certain mammalian spp.
3. Sebaceous - associated with many hair follicles; discharge their cellular lining during secretion of oils, therefore, these glands continually die and have to be replaced; located in shallow portion of dermis.
4. Mammary glands - present in both males and females; functional only in females; produce milk.
2. Feeding:


Trophic groups - nutritional (feeding) groups; three groups:

1. Insectivores - small mammals, usually opportunistic feeders; feed on a variety of small invertebrates including some non-insect ones, such as worms and crustacea; specific insectivore examples include shrews, moles, anteaters, and most bats; high protein and carbohydrate diet, low fiber content; short intestine; no cecum.
2. Herbivores - mammals that feed on grasses and other vegetation, and even wood; two groups:
• Browsers and grazers - hoofed mammals; browsers eat twigs and leaves of woody plants; grazers eat grasses and other herbaceous plants
• Gnawers - rodents and rabbits which eat wood; incisors that grow continuously and are worn away by gnawing.


Herbivores have several adaptations for digesting the cellulose of their high-fiber diet; cellulose, the structural carbohydrate of plants is not directly digested by vertebrates due to the lack of cellulose-degrading enzymes, such as cellulase; therefore, strict herbivores harbor microoganisms in large fermentation chambers and regions of the digestive tract, which can metabolize cellulose (e.g. some bacterial and protozoan spp.); the animal forms a mutualistic symbiosis with the cellulose-degrading bacteria and protozoa:

• Some herbivores, such as horses, zebras, rabbits, elephants and many rodents (non-ruminants) have a large cecum which serves as a fermentation structure where the cellulose-digesting bacteria and protozoa degrade the cellulose.
• The ruminant herbivores, such as cattle, goats, sheep, deer, etc., have a large 4-chambered stomach specialized for harboring the symbiotic microorganisms and metabolizing cellulose; also have a large cecum for additional fermentation and digestion of cellulose; the chambers of the stomach include the:
• rumen - largest chamber of the stomach where cellulose-degrading bacteria live in mutualistic symbiosis and breakdown cellulose into oligosaccharides, the disaccharide cellobiose, and glucose.
• reticulum - further digestion of oligosaccharides into cellobiose
• omasum - digestion of cellobiose into glucose
• abomasum (true, acid stomach) - acidic and enzymatic digestion of proteins and some undigested carbohydrates; also digestion of nucleic acids.
3. Carnivores - mammals that feed on herbivores, primarily; have specialized teeth and, usually, claws for tearing flesh; high protein, low fiber diet; short intestine and colon; small cecum.
4. Omnivores - mammals that eat plants (and plant products, such as fruits, seeds, etc.) as well as animals; e.g. primates; have relatively long digestive tract with small and large intestines, no cecum.
3. Reproduction in mammals:
   Most mammals have definite mating seasons, usually in winter or spring, and timed to coincide with the most favorable time of the year for rearing young.

Most female mammals mate only when fertile and undergo an estrus cycle during the maturation of the egg and eventual ovulation.

Stages of the estrus cycle:

1. Proestrus - preparation time during which a new ovarian follicle grows and forms the mature egg.
2. Estrus - time during which mating occurs; the mature follicle ruptures and releases the egg (ovulation); the mature egg can now be fertilized.
   In all placental mammals, the fertilized egg (zygote) implants in the uterine wall and pregnancy follows.

The number of times during the breeding season that estrus occurs varies from:

1. once per breeding season - monoestrus (e.g. dogs foxes, bats), to
2. many times per breeding season - polyestrus (e.g. mice, squirrels, rabbits).

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