Mammal
Common name for any warm-blooded animal belonging to the class Mammalia, including humans and all other animals that nourish their young with milk, that are covered with varying amounts of hair, and that possess a muscular diaphragm. Mammals have the most highly developed nervous systems of all animals. Most members of the group have four appendages, usually legs. These may be adapted for use as swimming appendages, as in seals, or as wings, as in bats. Some types, however, have two limbs that have been reduced to small vestiges beneath the skin, as in whales, or have been lost altogether, as in sea cows. All mammals, except the egg-laying monotremes, produce live young that undergo the early stages of development within the body of the mother . Mammalians are helpless at birth. The largest mammal, the blue whale, often exceeds 30 m (100 ft) in length, and the smallest shrews, mice, and bats are often less than 5 cm (less than 2 in) in length, excluding the tail.
The outer layer of the mammalian body, called the integument, consists of the skin and its derivatives. The skin, equipped with varying amounts of hair, serves as a protective layer against mechanical injury or invasion by germs, and prevents excessive loss or gain of body heat and moisture. In many mammals the color of the skin or fur blends with the animal's natural surroundings. The skin also functions as a sensory, excretory, and respiratory organ and contains specialized glands.
Mammary glands, which are present in fully developed form in all adult female mammals, and only in rudimentary form in most male and young female mammals, secrete milk to nourish the young. Sweat glands have been detected in almost all terrestrial mammals but appear to be lacking in some, notably the Cape mole rat and the two-toed sloth. Aquatic mammals, such as whales, dolphins, and sea cows, have no sweat glands. Sweat glands are usually located at the base of hairs, except those in areas of skin bordering mucous membranes, such as the sweat glands around the edges of the lips and covering the genitalia. Many mammals have few functional sweat glands; in dogs and cats, for example, only the glands on the soles of the feet are functional. Meibomian glands, located on the edges of the eyelids, secrete an oily film that covers and protects the space between the eyeball and eyelid and retains the film of tears that moistens the eyeball. A secretion from the wax glands of the ear prevents the intrusion of dust particles and small insects into the inner ear. Many mammals have scent glands located in the integument in various parts of the body; the fluids secreted by the glands repel enemies or attract the opposite sex.
The eyes, ears, and nose of mammals also have their external endings in the integument. All mammals have two eyes, but the eyes of several burrow-dwelling mammals, such as moles, have lost their function partially or completely or have become covered with skin. The ears of terrestrial and arboreal mammals are visible superficially as projecting cartilaginous processes, but the external ears of aquatic mammals, such as seals and walruses, are reduced to little more than small protective flaps. In whales' ears, the external openings are merely small holes.
The internal organs of mammals are essentially the same from the most primitive to the most complex. The internal features that distinguish mammals from lower vertebrates are the presence of two to four optic lobes in the brain, the presence of a muscular diaphragm separating the heart and lungs from the abdominal cavity, the presence of a single aortic arch located on the left side of the body, the possession of a four-chambered heart, including two auricles and two ventricles, and the absence of nuclei in red blood corpuscles after birth. All mammals except sea cows and certain sloths have seven cervical (neck) vertebrae; this characteristic is possessed by the long-necked giraffe and the largest whales as well as by mice and other tiny mammals. Other unique skeletal features common to mammals are the articulation between the tibial and tarsal bones, the chain of small separate bones in the ear, and the articulation of the mandible and squamosal bones of the skull.
All mammals reproduce sexually, and two types of reproductive acts are used to bring about sexual conjugation. In the primitive egg-laying mammals, excretory and genital organs open into a common orifice, called the cloaca. Transfer of sex cells from the male to the female is accomplished by bringing the cloacae into apposition. In all other mammals, however, the male sex cells are transmitted by copulation. After fertilization, development of offspring takes place entirely within the body of the mother in all mammals except the monotremes, which produce leathery-shelled eggs with large yolks. After birth, mammalian young are not prepared to pursue an independent existence but must be taken care of during infancy.
Mammals are classified differently by many zoologists. The class Mammalia, which is represented by about 4500 living species, is usually divided into three subclasses: the Monotremata (Prototheria) or egg-laying mammals , the Marsupialia (Metatheria) or marsupial mammals , and the Placentalia (Eutheria) or placental mammals.
The monotremes include the platypus and spiny anteaters of Australia. The marsupials include the opossums of the New World and many Australian mammals, such as the kangaroos and the koalas.
The placentals include the bulk of mammalian species, usually divided into 17 orders: (1) Insectivora, including small mammals such as moles, shrews, and hedgehogs, and formerly including the following order; (2) Dermoptera, the colugos, or so-called flying lemurs; (3) Chiroptera, the bats; (4) Carnivora, including the cats, viverrids (civets and mongooses), dogs (including wolves, foxes, and coyotes), hyenas, raccoons, bears, and mustelids (weasels, martens, otters, badgers, and skunks), and formerly including the following order of aquatic mammals; (5) Pinnipedia, including the seals, sea lions, and walrus; (6) Primates, including the tree shrews, lemurs, lorises, tarsiers, monkeys, apes, and humans; (7) Edentata, including armadillos, sloths, and the three anteaters of the New World, and formerly including the following two orders; (8) Pholidota, the pangolins; (9) Tubulidentata, the aardvark; (10) Rodentia, including squirrels, beavers, pocket gophers, pocket mice, rats, mice, mole rats, dormice, jerboas, porcupines, guinea pigs, and chinchillas, and formerly including the following order; (11) Lagomorpha, including rabbits, hares, and pikas; (12) Sirenia, including the manatees and dugong; (13) Cetacea, the whales and dolphins; (14) Hyracoidea, the hyraxes; (15) Artiodactyla (even-toed ungulates), including pigs, hippopotamuses, camels, llamas, chevrotains, deer, giraffes, the pronghorn, cattle, antelopes, goats, and sheep; (16) Perissodactyla (odd-toed ungulates), including horses, rhinoceroses, and tapirs; and (17) Proboscidea, the elephants. The four last-named orders were formerly included in one single order called Ungulata.
Mammals probably appeared on the earth during the early Mesozoic era. Most zoologists believe that mammals evolved from a group of extinct mammallike reptiles, Theriodontia, which existed during the Triassic period. The earliest animal fossils that have definitely been identified as mammals were found in rocks from the Jurassic period.
During the Jurassic period, four distinct orders of mammals existed: the Multituberculata and Triconodonta, both belonging to the subclass Allotheria, and the Panthotheria and Symmetrodonta, both of the subclass Theria. The Multituberculata were small, rodentlike mammals, having gnawing front teeth and grinding teeth with several cusps, that became extinct in the Eocene epoch. The Triconodonta were small, carnivorous mammals, having molar teeth equipped with three simple, conelike cusps, that became extinct before the end of the Eocene epoch. The Pantotheria, a group of small insectivorous mammals, are the probable ancestors of present-day mammals.
Of the mammalian subclasses that still exist, the monotremes are unrepresented by fossil remains; the earliest marsupial and placental fossils were found in rocks of the Cretaceous period. The marsupials were apparently unsuccessful in competition with the placentals, and by the beginning of the Eocene epoch were restricted to the opossum family in North America, several families (now mostly extinct) in South America, and several families in Australia. The earliest fossil remains of placentals discovered thus far have been found in western North America and western Europe; the placentals originated in the Eocene, but fossil records indicate that they spread rapidly throughout the remaining epochs of the Cenozoic era to form the dominant mammalian group all over the world except in Australia. The Insectivora, considered the oldest order of placental mammals, strongly resemble primitive fossil placentals.
Most mammals are land dwellers, living in such diverse habitats as the bare areas of deserts, tundras, and mountains and the thickly vegetated areas of the Tropics. Two placental orders, Sirenia and Cetacea, and several genera of a third order, Carnivora, are aquatic. Monotremes are restricted to Australia, Tasmania, and New Guinea. Marsupials are dominant in the same region as monotremes, but two groups of marsupials, the opossums and the Caenolestes, are native to the New World. Two orders of placental mammals, Chiroptera and Rodentia, are represented in the fauna of all continents except Antarctica. In Australia the placentals are represented by a few species of rats, the wild dog (or dingo), and a few bats; the dingo probably was introduced by humans.
The primates are native to most tropical and subtropical regions except those of Australia. Insectivora, Lagomorpha, and Artiodactyla are native to all continents except Australia and Antarctica, and the Perissodactyla are native to Eurasia, Africa, and South America. The Edentata are found only in the New World. The Dermaptera are restricted to the Malay Peninsula. The Pholidota, or pangolins, are found in Africa and in Asia. The Tubulidentata are exclusively African. Two small orders, the Proboscidea and Hydracoidea, are represented only in the faunas of Asia and Africa.