mammal

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 Insectivora includes the shrew and the mole. 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; the group became diversified in the Tertiary period of the Cenozoic era.

Bibliography: 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).

mammal XIX; animal of the class mammalia (XVIII), who suckle their young. modL., n. pl. of L. mammālis, f. mamma; see prec., -AL1.

mammal Any member of the class Mammalia of vertebrate animals characterized by mammary glands in the female and full, partial or vestigial hair covering. Mammals are warm-blooded. They have a four-chambered heart with circulation to the lungs separate from the rest of the body. As a group, mammals are active, alert, and intelligent. They usually bear fewer young than other animals, and give them longer and better parental care. Most mammals before birth grow inside the mother's body and are nourished from her by means of a placenta. When born, they continue to feed on milk from the mother's mammary glands. There is a wide range of features, shapes, and sizes among mammals. Mammals include 17 orders of placentals, one marsupial order – all live-bearing – and an order of egg-laying monotremes. They probably evolved c.180 million years ago from a group of warm-blooded reptiles. Today, mammals range in size from shrews weighing a few grams to the blue whale, which can weigh as much as 150 tonnes.