mammal, an animal of the highest class of vertebrates, the Mammalia. The female has mammary glands, which secrete milk for the nourishment of the young after birth. In the majority of mammals the body is partially or wholly covered with hair; the heart has four chambers, and only the left aortic arch is present; and a muscular diaphragm separates the chest from the abdominal cavity. Mammals are warm-blooded; that is, they have a relatively constant body temperature independent of the temperature of their surroundings. The mature red blood cells (erythrocytes) usually lack a nucleus. Except for the egg-laying monotremes (the platypus and the echidna, or spiny anteater), mammals give birth to live young. A marsupial is born in a more undeveloped state than the young of other mammals, although all are relatively helpless at birth. In some marsupials and in higher mammals the young receive prenatal nourishment through a placenta. The order Carnivora, or flesh-eating animals, includes terrestrial families such as the cat, dog, and bear as well as the aquatic seal, sea lion, and walrus. Other aquatic mammals are the whale, porpoise, and dolphin of the order Cetacea and the manatee and dugong of the order Sirenia. Unusual adaptations are also found in the bat (order Chiroptera); in the elephant (order Proboscidea); in the sloth, armadillo, and anteater (order Edentata); and in the beaver, woodchuck, porcupine, and squirrel (order Rodentia). The order Soricomorpha includes the shrew and the mole, and the spiny and hairy hedgehogs form Erinaceomorpha; both orders were formerly classed as Insectivora. There are two groups of ungulates, or hoofed mammals: most members of the order Perissodactyla, including the horse and the rhinoceros, are odd-toed, with the third digit the largest; those of the order Artiodactyla, including the deer, antelope, camel, pig, and cow, are even-toed, with the third and fourth digits symmetrical and functional. Humans, monkeys, apes, and lemurs belong to the order Primates. Some remains of mammals are identified as from the Jurassic period of the Mesozoic era, but mammals remained small creatures during the Mesozoic. The group became diversified relatively rapidly in geological terms in the Tertiary period of the Cenozoic era after the dinosaurs had become extinct.
See E. P. Walker et al., Mammals of the World (2 vol., rev. ed. 1968); S. Anderson, ed., Simon & Schuster's Guide to Mammals (1984); G. B. Corbett and J. E. Hill, World List of Mammalian Species (1986); H. H. Genoways, ed., Current Mammalogy (2 vol., 1987–89).