I.

Introduction

Mammal, common name applied to any warm-blooded animal belonging to the class that includes human beings and all other animals that nourish their young with milk, are covered with varying amounts of hair, and possess a muscular diaphragm. The class Mammalia, which is represented by about 4,600 living species, is usually divided into three subclasses: the monotremes (egg-laying mammals), the marsupials (usually mammals with pouches), and the placental mammals. The majority of mammals are placental mammals. Mammals have the most highly developed nervous systems, including the brain, 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 monotremes, produce live young that undergo the early stages of development within the body cavity of the mother (see Development). Some mammals are helpless at birth; others are able to walk and even run immediately, and may be born fully furred and with their eyes and ears open. The range in size of mammals is extraordinarily wide; the largest mammal, the blue whale, often exceeds 30 m (100 ft) in length, and the smallest shrews, mice, and bats are less than 5 cm (2 in) in length, excluding the tail.

II.

Anatomy

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 injuries and invasion by germs, and prevents excessive loss or gain of body heat and moisture. In many mammals the colour of the skin or fur blends with the animal's natural surroundings. In others there is great contrast with the natural surroundings to favour visual signals that provide information about the identity of a species, and about the gender, age, or social status of an individual. The skin also functions as a sensory and excretory 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 bases of hairs, except those in regions of skin bordering mucous membranes, such as the sweat glands surrounding the edges of the lips and covering the genitalia. Many mammals, however, 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 (outer layer) 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 partly or completely or have become covered with skin. The ears of terrestrial and arboreal (tree-dwelling) mammals are visible 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 and anteaters 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 at the ankles, the chain of small separate bones in the ear, and the articulation of the mandible (lower jaw) and squamosal bones of the skull.

III.

Reproduction

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 next to each other. 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, and in many marsupials, in which the gestation period lasts only about 10 to 15 days, with most of the development taking place in the mother's pouch after birth. Mammalian young are not sufficiently well developed to pursue an independent existence immediately following birth but must be nursed during infancy. See Reproduction.

IV.

History

Mammals probably appeared on the Earth during the early Mesozoic era (245 million to 145 million years ago). Most zoologists believe that they evolved from a group of extinct mammal-like reptiles, Theriodontia, which existed during the Triassic period (245 million to 208 million years ago). The earliest animal fossils that have definitely been identified as mammals were found in rocks from the Jurassic period (208 million to 145 million years ago).

During the Jurassic period, five distinct orders of mammals existed. One order was made up of small, rodent-like mammals, having gnawing front teeth and grinding teeth with several cusps (molars and premolars), that became extinct in the Eocene epoch (56 million to 35 million years ago). A second order consisted of small, carnivorous mammals, having molar teeth equipped with three simple, conical cusps, that became extinct before the end of the Eocene epoch. A third 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 (145 million to 65 million years ago). 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 late Cretaceous period, and fossil records indicate that they spread rapidly throughout the Cenozoic era (65 million to 10,000 years ago) to form the dominant mammalian group throughout the world except in Australia. The insectivores, considered the oldest order of placental mammals, strongly resemble early fossil placentals.

V.

Distribution

Most mammals are land dwellers, living in such diverse habitats as the bare areas of deserts, tundras, and mountains, and the forested areas of the tropics. Two placental orders and several genera of a third order 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 are native to the Americas. Two orders of placental mammals—bats and rodents—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 species of bats; the dingo probably was introduced by human beings.

The primates are native to most tropical and subtropical regions except those of Australia. Insectivores, rabbits, hares, pikas, and even-toed ungulates are native to all continents except Australia and Antarctica, and the odd-toed ungulates are native to Eurasia, Africa, and South America. The sloths, armadillos, tamandua, and anteaters are found only in the Americas. The colugos are restricted to the Malay Peninsula, Indonesia, Borneo, and the Philippines. The pangolins are found in Africa and in Asia. The aardvarks are exclusively African. Two small orders, the elephants and hyraxes, are represented only in the faunas of Asia and Africa.

Scientific classification: Mammals are classified differently by many zoologists. The class Mammalia 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 Monotremata subclass has one order, which includes the platypus and spiny anteaters of Australia. Marsupials are split into seven orders: (1) Didelphimorphia, American opossums; (2) Pancituberculata, American shrew opossums; (3) Microbiotheria, colocolos (or Monito del Monte) from Chile and Argentina; (4) Dasyuromorphia, carnivorous Australasian mammals, including the numbat; (5) Peramelemorphia, bandicoots and bilbies from Australasia; (6) Notoryctemorphia, two species of marsupial mole from Australia; and (7) Diprotodontia, koala, wombat, opossums, kangaroos, and wallabies.

The placentals include most mammalian species, usually divided into 18 orders: (1) Insectivora, including small mammals such as moles, shrews, tenrecs, and hedgehogs; (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), seals, sea lions, and walrus; (5) Macroscelidea, the elephant shrews; (6) Primates, including the lemurs, lorises, tarsiers, monkeys, apes, and humans; (7) Xenarthra, including armadillos, sloths, and the three anteaters of the Americas; (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; (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; (17) Scandentia, the tree shrews; and (18) Proboscidea, the elephants.