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The class Mammalia has been the dominant group of vertebrates since the extinction of the dinosaurs 65 million years ago. There are over 4200 living species, classified into over 1000 genera, 140 families, and 18 orders. The number of extinct mammals is at least five times that. Most living mammals are terrestrial. However, many groups of mammals moved to the water from land-dwelling ancestors. These included manatees and dugongs (which are distantly related to elephants), otters (which are related to weasels), seals, sea lions, and walruses (which are distantly related to bears), and whales (which are distantly related to even-toed hoofed mammals), as well as numerous extinct groups. Mammals have also taken to the air, with over 920 living species of bats, as well as numerous gliding forms such as the flying squirrels, phalangerid marsupials, and flying lemurs or colugos. Mammals are even more successful at small body sizes, with hundreds of small species of rodents, rabbits, and insectivores.
Mammals are distinguished from all other animals by a number of unique characteristics. These include a body covered with hair or fur (secondarily reduced in some mammals, particularly aquatic forms); mammary glands in the female for nursing the young; a jaw composed of a single bone, the dentary; and three middle ear bones, the incus, malleus, and stapes. All mammals maintain a constant body temperature through metabolic heat. Their four-chambered heart (two ventricles and two atria) keeps the circulation of the lungs separate from that of the rest of the body, resulting in more efficient oxygen transport to the body tissues. They have many other adaptations for their active life-style, including specialized teeth (incisors, canines, molars, and premolars) for biting, tearing, and grinding up food for more efficient digestion. These teeth are replaced only once in the lifetime of the animal (rather than continuous replacement, found in other toothed vertebrates). Mammals have a unique set of muscles that allow the jaw to move in many directions for chewing and for stronger bite force. Their secondary palate encloses the internal nasal passage and allows breathing while they have food in the mouth. Ribs (found only in the thoracic region) are firmly attached to the breastbone (sternum), so that expansion of the lung cavity is accomplished by a muscular wall in the abdominal cavity called the diaphragm. See Cardiovascular system, Dentition, Ear (vertebrate), Hair, Lactation, Mammary gland, Thermoregulation, Tooth
All mammals have large brains relative to their body size. Most mammals have excellent senses, and some have extraordinary senses of sight, smell, and hearing. To accommodate their large brains and more sophisticated development, mammals are born alive (except for the platypus and echidnas, which lay eggs), and may require considerable parental care before they are ready to fend for themselves. Juvenile mammals have separate bony caps (epiphyses) on the long bones, separated from the shaft of the bone by a layer of cartilage. This allows the long bones to grow rapidly while still having a strong, bony articulation at the end. When a mammal reaches maturity, these epiphyses fuse to the shaft, and the mammal stops growing (in contrast to other vertebrates, which grow continuously through their lives). See Brain, Nervous system (vertebrate), Skeletal system
The living mammals are divided into three major groups: the monotremes (platypus and echidnas), which still lay eggs, retain a number of reptilian bones in their skeletons, and have other primitive features of their anatomy and physiology; the marsupials (opossums, kangaroos, koalas, wombats, and their relatives), which give birth to an immature embryo that must crawl into the mother's pouch (marsupium), where it finishes development; and the placentals (the rest of the living mammals), which carry the young through a long gestation until they give birth to relatively well-developed progeny. In addition to these three living groups, there were many other major groups, such as the rodentlike multituberculates, now extinct. The most recent classification of the mammals can be summarized as follows:
- Class Mammalia
- Subclass Prototheria (monotremes)
- Subclass Theriiformes
- Infraclass Holotheria
- Cohort Marsupialia (marsupials or pouched mammals)
- Cohort Placentalia (placentals)
- Magnorder Xenarthra (sloths, anteaters, armadillos)
- Magnorder Epitheria
- Grandorder Anagalida (= Glires) (rodents, rabbits,
- elephant shrews)
- Grandorder Ferae (carnivores, pangolins, many extinct
- Grandorder Lipotyphla (hedgehogs, shrews, moles,
- tenreces, and kin)
- Grandorder Archonta
- Order Chiroptera (bats)
- Order Primates (lemurs, monkeys, apes, humans)
- Order Scandentia (tree shrews)
- Grandorder Ungulata (hoofed mammals)
- Order Tubulidentata (aardvarks)
- Order Artiodactyla (even-toed hoofed mammals:
- pigs, hippos, camels, deer, antelopes, cattle,
- giraffes, pronghorns, and relatives)
- Order Cete (whales and their extinct land relatives)
- Order Perissodactyla (odd-toed hoofed mammals:
- horses, rhinos, tapirs, and extinct relatives)
- Order Hyracoidea (hyraxes)
- Order Tethytheria (elephants, manatees, and extinct
This classification does not list all the extinct groups, which include at least a dozen more ordinal-level taxa. See separate articles on each group. See Reproductive system
Mammals evolved from the Synapsida, an early branch of the terrestrial amniotes that has been erroneously called the mammallike reptiles. (This name is inappropriate because synapsids were never related to reptiles.) The first undoubted mammals appeared in the Late Triassic (about 210 million years ago), and were tiny insectivorous forms much like living shrews. Through the rest of the age of dinosaurs, a number of different groups evolved over the next 145 million years of the Jurassic and Cretaceous. Most remained tiny, shrewlike animals, hiding from the dinosaurs in the underbrush and coming out mostly at night. The first two-thirds of mammalian history had passed before the dinosaurs became extinct 65 million years ago, and this allowed mammals to emerge from their shadow. Between 65 and 55 million years ago, a rapid adaptive radiation yielded all the living orders of placental mammals and many extinct forms as well.
a class comprising the most highly organized animals; belongs to the phylum Chordata.
The skeletal characteristics of mammals include simplification and strengthening of the cranium, which has two occipital condyles that articulate with a distinctly modified first cervical vertebra (atlas). The mandible consists of a single (dental) bone, which in most animals articulates with the squamosa; in some animals it articulates with a process of the squamosa. The dental system and skeleton of the extremities are improved.
Mammals have a hairy covering and a more or less constant body temperature. The heart is four-chambered and separated completely into venous (right) and arterial (left) halves; the aortic arch is connected with the latter. (Only the left half of the fourth arterial arch is preserved.) The red blood cells are flat, round, and, when mature, anuclear. The auditory apparatus consists of an external, middle, and inner ear; in most mammals, the external auricle is well developed.
The oral cavity is separated by a secondary palate from the nasal cavity, the site of the convoluted nasal conchae with the lining of olfactory epithelium. The teeth, which are rooted in sockets (alveoli), are usually differentiated into incisors, canines, premolars, and molars. All the teeth, except the molars, are replaced once or more during the animal’s life.
The excretory parts of the digestive and urogenital systems are separate. There is no cloaca (except in egg-laying mammals). The bodies of the vertebrae have flat jointed surfaces (platycelous). There are seven cervical vertebrae (very rarely, in some manatees, six; or, in some sloths, eight or nine). The extremities of most mammals are pentadactyl, but they are considerably modified in many: they are monodactylous in horses, flipper-like in seals and whales, and wing-shaped in bats. Some mammals, for example, whales and manatees, have no posterior extremities, except insignificant rudiments not visible externally. Instead, they have a broad horizontal fin.
The thoracic and abdominal cavities are separated by a diaphragm. The cerebral hemispheres are well developed, with the structures that perform vital mental functions concentrated in the cortex. The skin consists of a highly developed connective-tissue (mesodermal) layer and an epidermis of ectodermal origin with numerous secondary formations. These formations include hair (characteristic of mammals) of several types: vibrissae (large sensory hairs), whiskers, spines, and lanugo. The absence of hair in some mammals (hippopotamuses and whales, for example) is a secondary phenomenon.
The coloration of mammals, which is determined by the pigment of the hairs, may be uniform or patterned with stripes or spots of different sizes and shapes. Other cutaneous structures include quills (a modification of hairs), scales, calluses, digital suction pads, claws, nails, hooves, and horns. The armor of armadillos and pangolins is related to the connective-tissue layer (skin ossifications) and only partly to the epidermis.
The numerous skin glands in mammals play a role in thermoregulation and metabolism (sweat glands). Most, however, are odoriferous glands, serving a variety of signal purposes— scent of the track, marking of territory, searching for and attracting a mate, or defense, by spraying pungent secretions. The glands are located in different parts of the body, such as the head (anteorbital, occipital), legs (between the hooves, “fetlocks,” front and hind legs), side, belly, and groin. Some mammals have different kinds of anal and preanal glands. A delicate sense of smell, which plays an important part in intraspecific and some interspecific contacts, usually parallels the development of glands. In some cases, for example, in twin species of some rodents, scent serves as the principal sign of recognition.
The mammary glands, a mammalian characteristic, are of particular importance. Females give birth to live young, which develop in the uterus. The embryo is connected to the uterus by a placenta. Monotremes lay eggs. In marsupials, a true placenta does not form during uterine development.
The young are born at various stages of development. In marsupials, fetal development continues in an external protective pouch, whereas in some ungulates the young are capable of following after their mother within a few hours of birth. The young of mammals are raised on milk for varying periods of time, ranging from a few weeks to several years. The gestation period varies from 16–18 days (in some rodents) to 22 months (in elephants). In certain species (monestrous), young are born once a year or only once in as many as three years. Many mammals, for example rodents, give birth several times a year (polyestrous). Some rodent species and whales can be fertilized immediately after giving birth, so that gestation and rearing may go on simultaneously. Sexual maturity in the young of many small mammals, especially females, arrives very quickly—well before the animals achieve the general physical development and size of the adults. The sharp year-to-year fluctuations in the population of many mammals (rodents, hares) are due to this factor. The populations of other species, especially the larger species, remain relatively stable or change less significantly.
Attachment to a particular area, or territoriality, is characteristic of individuals, families, or other groups, such as wolf packs, prides of lions, or herds. The animals mark their territory in some way, chiefly with secretions from the odoriferous glands or with urine or feces, and defend it against intrusions by individuals of the same species. Regular seasonal migrations are characteristic of a number of mammals, including reindeer, arctic fox, cetacaeans, bats, and antelopes. Certain species, such as tree squirrels and lemmings, migrate en masse in some years beyond their normal range of distribution because of lack of food or because of overpopulation. Outside their habitat, they die. A complex internal structure whereby individuals or groups are “ranked” exists in packs and herds.
The lower mammals differ little from other vertebrates—birds and some reptiles—in the development of higher nervous activity, but the more highly organized groups, such as predators (wolves, dogs), cetaceans (dolphins and porpoises), and primates (especially anthropoid apes) reach the highest level of development of all animals.
The mammals are descended from the Mesozoic mammal-like reptiles Therapsida. Mammals existed as early as the Triassic period (160–170 million years ago), and by the Upper Triassic there were three orders—Docodonta, Triconodonta, and Eupantotheria. During the Jurassic period, the class Mammalia was represented by five orders, which encompassed 11 families. These orders, with the exception of Multituberculata, which survived to the Eocene epoch, became extinct during the middle Cretaceous period. Marsupials appeared during the early Cretaceous period, as did the insectivores, the first eutherians (placental mammals).
By the Paleocene epoch, the eutherians (early ungulates, lagomorphs, early carnivores, rodents, bats) were predominant over the marsupials. There were 28 orders of eutherians by the Eocene epoch, the period of the most rapid development of the mammals; 16 of these orders survive today. Beginning with the end of the Triassic period, according to various scientists, there were 36–40 orders of Mammalia, embracing 258–312 families of more than 3,000 genera with 12,000–13,000 species. Today, the class Mammalia comprises about 3,500 species (according to some zoologists, 4,250), which constitute about one-third of all mammals that ever existed.
The modern class Mammalia embraces 19 orders, usually divided into two subclasses: Prototheria (egg-laying mammals) and Theria (live-bearing mammals). Prototheria comprises the single order Monotremata. Theria is divided into two infra-classes: Metatheria (pouched mammals), with the single order Marsupialia, and Eutheria (placental mammals). Eutheria comprises 17 orders: Insectivora (insect-eating mammals), Dermoptera (flying lemurs), Chiroptera (bats), Primates, Edentata (anteaters, sloths, armadillos), Pholidota (pangolins), Lagomorpha (hares, rabbits, pikas), Rodentia, Cetacea (whales), Carnivora (flesh-eating mammals), Pinnipedia (seals, sea-lions, walruses), Tubulidentata (aardvarks), Proboscidea (elephants), Hyracoidea (hyraxes), Sirenia (manatees), Perissodactyla (odd-toed ungulates), and Artiodactyla (even-toed ungulates). Pinnipedia is often regarded as a suborder of Carnivora. Cetacaea is sometimes divided into two suborders: Odontoceti (toothed whales) and Mysticeti (whalebone whales). Insectivora is usually divided into either two or four suborders, Marsupialia into five suborders, and Artiodactyla into two suborders.
In terms of the number of biological types and adaptive specializations, the Mammalia are extremely varied. They have the most pronounced adaptive radiation of all animals. The principal specializations, which are related to radical morphophysiological reconstructions, include rapid locomotion on the ground using the two hind legs only or running and leaping using all four extremities. The latter is a form of locomotion usually associated with reconstruction of the extremities and a reduction in the number of digits to two or even one. Adaptation to life in trees (tree shrews, tree squirrels, monkeys), which is associated with the development of prehensile extremities or a prehensile tail, is another of the principal mammalian specializations. Some mammals (flying squirrels, lesser flying possums, flying lemurs) have developed the capacity for gliding, in an arboreal mode of life, a result of the formation of webs between the trunk and extremities. Others, including various bats, have adapted to free flight, which is associated with the transformation of the anterior extremities into true wings and the reconstruction of other systems. Also among the principal mammalian specializations is a shift to the aquatic mode of life. In aquatic mammals such as seals, some connection with the land is retained, and there is comparatively incomplete reconstruction of the extremities and other systems. Mammals such as cetaceans and manatees adapted completely to life in the water. This adaptation was accompanied by radical reconstruction of the organisms. Another major mammalian specialization is adaptation to digging and a completely underground mode of life, with a reconstruction of the locomotor apparatus and skull and a loss of vision (moles, mole rats, golden moles).
In addition to these basic adaptive specializations, there are more specific adaptations (for example, in feeding habits, reproduction, group structure) in certain species or groups of species. These are often duplicated in different orders.
Mammals are highly diverse and flexible ecologically. They are found everywhere except in deep water, on the ocean floor, on solid glacial land areas (central Greenland, Antarctica), or on snow-covered mountain peaks above 5,000 m. The ecological flexibility of certain mammalian species is very great; for example, the Old World badger is omnivorous, inhabits all areas from desert to northern taiga, and hibernates about seven months in the north but not at all in the south. On the other hand, many species are strictly specialized ecologically. The koala is restricted to forests with certain eucalyptus species, on whose leaves it feeds. The mole inhabits soils with some moisture content since it feeds on earthworms. Mammals are found in all the seas and oceans as far as the North Pole. Since the terrestrial mammals, except bats, have fairly limited means of dispersal (they cannot pass oceans), they characterize zoogeographic land regions, which are distinguished largely on the basis of the distribution of these mammals. Historical (geological) changes in the earth’s surface (for example, the formerly joined continents in the Bering Sea region) can be clearly traced in the areas of distribution of the mammals. This, together with a comparative wealth of paleontological evidence, provides important material for the study of the earth’s history.
From a practical standpoint, mammals constitute one of the most important animal groups in the world. They include most of the domestic animals, some of which have played an important role in the development of human society—wolves and dogs in the Mesolithic; sheep, goats, and aurochs, which provided man with food, in the Neolithic; the horse, the principal means of travel on land until the mid-19th century. Progress is still being made in breeding new species, for example, silver fox, nutria, chinchilla, mink, and other fur-bearing animals. Wild mammals provide man with fur, hides, meat, fat, panty (young, unossified deer horns used to manufacture medicinal preparations), musk, spermaceti, and ivory. Mammals are the main objects of commercial trapping and hunting. They are protected in game preserves and exhibited in zoos.
Some mammals, such as wolves, are harmful to livestock, and many species, chiefly rodents, do great harm to crops (especially grains) and meadows. Mammals transmit or serve as reservoirs for certain dangerous infectious diseases of man and domestic animals, including tularemia, plague, encephalitis, and rabies. Certain species—foxes and small predators—regulate the number of rodent pests. Others, for example monkeys, deer, squirrels, are used to populate parks and zoos. Increasingly more mammals are being used as laboratory animals in experiments.
Because of unrestricted trapping by man and changes in natural conditions, the populations of many mammalian species are rapidly declining, and many species are threatened with extinction, including large whales, lions, tigers, cheetahs, almost all lemurs, anthropoid apes, and the Asiatic wild ass and a number of other ungulates. Many species have become extinct in recent centuries, for example Steller’s sea cow and thysaline. According to the Red Book of the International Union for Conservation of Nature and Natural Resources, about 300 species and subspecies are now threatened with extinction. However, the experience of the USSR and other countries in protecting and restoring the populations of the sable, moose, the antelope Saiga tatarica, and certain other species shows that many endangered species can be saved from extinction by a rational approach to the problem.
The branch of zoology that studies mammals is called theriology.
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V. G. GEPTNER