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.
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
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:
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:
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
Trophic groups - nutritional (feeding) groups; three groups:
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:
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:
The number of times during the breeding season that estrus occurs varies from: