{mam'-ul} The class Mammalia includes about 19 extinct orders and 19 living orders, the latter containing some 4,060 living species. This number of living species is insignificant if compared to the tremendous number within some invertebrate groups, such as the estimated 750,000 species of insects; however, if importance is measured by ability to modify the environment, mammals are the most important terrestrial animals. Over much of the world, the character of the landscape--including the vegetation, the soil, and even the air--has been transformed or modified by the activities of humans and other mammals. Mammals today are widely distributed, from the edges of the Arctic ice cap in the north, down through temperate, desert, and tropical areas, to the edge of the Antarctic continent in the south; they also occupy the oceans and major river systems of the world. Mammals span an incredible range of sizes, from shrews and bats that weigh but a few grams (a fraction of an ounce) to the blue whale, weighing more than 100 tons, the largest mammal that has ever lived. CHARACTERISTICS Mammals as a group can be defined by certain diagnostic features, even though some of these features are shared by other groups, and not every feature is found in all mammals. Viewed broadly, mammals are vertebrate animals that (1) are endothermic (warm-blooded), able to maintain a constant body temperature by internal regulation of either heat production or heat loss, or both; (2) bear living young that are nourished by milk; (3) have large brains, allowing for quick learning and comparatively flexible behavior; (4) produce at least a partial covering of hair; (5) contain sweat glands in the skin; (6) develop external ears (pinnae), which increase hearing acuity; (7) have a single bone making up each side of the lower jaw; (8) possess a four-chambered heart; and (9) have three middle-ear bones (auditory ossicles). Mammals are alone in possessing mammary glands, from which the term mammal is derived. Mammary glands, believed to be highly modified sweat glands, produce nutritious milk that allows the young to develop rapidly. The extended social bond between the mother and her young during and often after the suckling period facilitates the early training of the young. During this time the rudiments of social behavior are learned, and the young's ability to survive is enhanced by indoctrination into foraging techniques and choice of food. Sweat glands wet the surface of the skin and allow for evaporative cooling, especially under conditions of high temperatures or exertion. Hair in most mammals forms an insulating coat (the pelage) that entraps air and reduces both heat loss from the body and heat gain from the environment. Compared to reptiles and amphibians, mammals are unusually active, and the cost in energy of maintaining an even body temperature is high. These energetic demands are partly met by several features that increase efficiency of circulation and respiration. Unlike the arrangement in fishes, amphibians, and most reptiles, in mammals oxygenated and deoxygenated blood do not mix in the heart: oxygen-depleted blood from the body enters the right auricle and is pumped to the lungs by the right ventricle; oxygen-rich blood from the lungs enters the left auricle and is pumped by the left ventricle to the body. The thoracic cavity (the chamber that holds the lungs and heart) is separated from the abdominal cavity by a muscular sheet, the diaphragm. When relaxed, the diaphragm bows into the thoracic cavity, decreases the volume of the cavity, and with contraction of abdominal muscles forces air out of the lungs; when the muscles of the diaphragm contract, it becomes taut and flat, increasing the volume and lowering the pressure of this cavity, drawing air into the lungs. To support their high energy demands for oxygen, mammals must be able to breathe while they suckle, chew food, or clutch prey in their jaws. This capacity is provided by the secondary palate, a shelf of bone forming the roof of the mouth, which separates the mouth from the nasal passages. The skeleton of mammals differs most markedly from the skeletons of other vertebrates in being highly simplified. The mammalian skull lacks bones present in the reptilian skull. The lower jaw, which in other vertebrates consists of a mosaic of several bones on each side, is formed by one bone, the dentary, on each side in mammals. The jaw-skull joint in mammals is between the dentary and the squamosal, the skull bone immediately in front of the ear opening. The two bones that formed the jaw-skull in reptiles became modified during evolution into part of the mammalian hearing apparatus. These bones, the quadrate of the skull and the articular of the lower jaw, teamed with the stapes, a bone used for sound transmission in reptiles, to form a short chain of bony ossicles across the mammalian middle-ear chamber. This chain, consisting of the malleus, or hammer (articular), the incus, or anvil (quadrate), and the stapes, or stirrup, occurs only in mammals. The bony protective casing (auditory bulla) enclosing the chain is itself derived in part from another reptilian jawbone, the angular, and also is found only in mammals. Human beings share the diagnostic mammalian structural features with their fellow mammals and resemble their relatives the great apes (family Pongidae) extremely closely. Only minor structural differences, such as a reduction of body hair, changes in the posture and proportion of the limbs, and a differently proportioned skull and dentition, distinguish humans from gorillas or chimpanzees. The large size and distinctive organization of the human brain, however, confer upon human beings abilities to communicate, to think abstractly, and to create--abilities unmatched by the great apes or any other animals. Ecological Adaptation A primary key to the success of mammals is their ability to adapt to and occupy a spectrum of environments ranging from Arctic seas to deserts. Some mammals that occupy Arctic areas or frigid seas have greatly increased their insulating coat of fur or their envelope of blubber (fat) to reduce dissipation of heat from the body. The oryx, an antelope that inhabits almost waterless deserts of Africa, has a system whereby the blood flowing to the brain on a hot day is cooled by inhaled air, thereby maintaining the brain at a temperature lower than that of the overheated body, avoiding brain damage. The kangaroo rats of the deserts of the southwestern United States are known for their ability to live on a diet of dry seeds and no drinking water. This ability depends in part on three adaptations: (1) specialized kidneys that produce highly concentrated urine containing low amounts of water, (2) a lack of sweat glands, and (3) the ability of the nasal passages to withdraw some moisture from air exhaled from the lungs before this moisture is lost from the body. Deep-diving whales have adapted to their own unique problems, developing intricate physiological and anatomical systems that conserve energy, and thus oxygen, during prolonged dives by slowing down the heart and metabolic rate, by reducing blood flow to certain parts of the body, and by allowing the muscles to store oxygen. Some small mammals can avoid the intense cold and food shortages of winter by hibernating. Some species of ground squirrel store much of the energy derived from feeding in the summer as fat, which provides the energy that sustains the squirrels during six months of winter dormancy. Conversely, a similar adaptation to the harshness of some summer habitats is estivation, or summer dormancy. Some animals both hibernate and estivate and are active only during the relatively mild seasons in between. Social Behavior A remarkably interesting aspect of the biology of mammals is their complex social behavior. For example, in some species the social order is controlled by a dominance hierarchy. One individual, usually a male, is the dominant animal; remaining members are arranged in a hierarchy according to their ability to dominate others. Typically, males have one hierarchy and females another, and each animal recognizes its place in the system. The establishment and maintenance of the hierarchy is often by means of ritualized combat and threat displays. Such threats include the threat yawn of the baboon, which displays the huge canine teeth, and the high-head posture of the Grant's gazelle, which displays the animal's powerful neck and horns. Facial markings in some antelopes serve to increase the conspicuousness of such displays. Communication between individuals or groups takes place by means of such visual displays, by scents produced by specialized scent glands, or by vocalizations. Displays have seemingly evolved for a purpose. By means of threats, an animal can assert or demonstrate its dominance without risking the serious injury that often accompanies fighting. An injured animal under natural conditions is generally doomed to a short life with little chance to reproduce. A variety of types of social groups occur among social mammals. In some mammals the male and female and their young are the social unit. In lions the pride, an expanded kinship group, is the unit. The pride is usually controlled by two or three adult males that are brothers, and includes a number of adult females, most of which are closely related, and their offspring. Similarly, in savanna baboons, several adult males dominate the social group, which includes adult females and their young. In the hyena "clan," by contrast, the adult females are the dominant animals, and the social units include up to 30 or more assorted adult females, adult males, and young. Dominance relationships and close "friendship" bonds between females give organization to the clan. As a rule, male mammals compete strongly among themselves for access to females, and the largest and strongest individuals are usually the most successful breeders. The reproductive life of the male may be short, however, relative to that of the female. Dominant male lions, for example, generally maintain their position in a pride for no more than 2 or 3 years, after which they are deposed by younger and more vigorous males. The effective reproductive life of the male does not begin until he becomes a dominant member of the pride, and it ends when he is deposed. Thus, the reproductive life of the male may be as short as 2 years, whereas that of the female is usually more than 10 years. A brief reproductive life also commonly occurs in the Indian langur monkey. A single male forcibly takes over a troop of females and young from a previous leader and mates with the females; however, this new male is generally forced from the troop within 2 years by yet another male. FOSSIL HISTORY Mammals are an ancient group that appeared more than 190 million years ago in the Triassic Period, and their ancestry goes even further back. Mammallike reptiles, from which the mammals evolved, dominated the terrestrial scene about 280 million years ago during the Permian Period, long before the first dinosaurs arose. These reptiles were members of the subclass Synapsida, characterized by the presence of a single opening on each side of the skull behind the eye to allow more room for the jaw muscles. Synapsids, especially those of the order Therapsida (therapsids) and the suborder Theriodontia (theriodonts), were active reptiles that showed indications of development of a mammallike ability to maintain a constant body temperature. Their limbs developed a mammallike posture, and their dentition showed the mammalian trend toward the division of labor between front teeth adapted to grasping and killing prey and back teeth adapted to crushing or chopping food. Synapsid reptiles gradually became extinct as dinosaurs gained dominance over the Earth. Their unspectacular descendants, the mammals, survived. The earliest mammals were no larger than mice and probably ate insects. As do all mammals to this day, these primitive mammals had lower jaws made of a single bone on each side and used the three bony ossicles as transmitters of sound in the middle ear. Four orders of mammals are known from the middle and late Jurassic, from 170 million to 135 million years ago: the Triconodonta, Pantotheria (or trituberculates), Symmetrondonta, and Multituberculata. In some classifications the docodonts of the late Jurassic, here treated as pantotheres, are regarded as a separate order. The Pantotheria, Symmetrodonta, and Triconodonta survived into the following Cretaceous Period, and the Multituberculata remained until the early Eocene of the Cenozoic Period. For more than 100 million years--the vast sweep of time when dinosaurs dominated the Earth--mammals persisted, perhaps by being inconspicuous and nocturnal. But mammalian evolution continued, and refinements in reproduction, tooth, skull, and brain design were incorporated into the mammalian structural plan. Many authorities believe that the primitive pantothere mammals were the direct ancestors of the marsupials, represented by the opossum and kangaroo, and the placental mammals, represented by dogs and humans. The egg-laying monotremes, like the duck-billed platypus, are thought to have originated from a yet unknown group tracing back to the Jurassic. When the dinosaurs became extinct approximately 65 million years ago, at the end of the Cretaceous Period, mammals inherited the earth. Before long, different groups of mammals had become adapted to a great variety of ecological niches, many previously occupied by dinosaurs. The key to mammalian success in their wide variety of life-styles was an amazing structural and functional plasticity. Limbs were variously modified into flippers, shovels, wings, and pillars; teeth became adapted to dealing efficiently with diets ranging from a myriad of plant material through a variety of animal material, including insects and marine invertebrates as well as other mammals. Sensory systems were modified so that some dwellers of murky waters or those active in darkness could use the interpretation of echoes of their own voices (echolocation) for perceiving their environment. The Cenozoic Era, the period from about 65 million years ago to the present, has been termed the Age of Mammals. During this time the Earth has been populated by a succession of assemblages of mammals: as environments have changed, species unable to adapt have been replaced by species able to exploit the new conditions. Plant eaters have continually adapted to changing assemblages of plants, and, as plant eaters have become swifter or more clever, the carnivores that prey on them have, in turn, increased their speed or intelligence. The Cenozoic history of mammals can best be viewed against the background of the environmental conditions of the times. During the early parts of the Cenozoic, climates over broad regions of the world were warm and rainfall was high. The widespread forests in North America supported many subtropical species of plants. Under these conditions all of the orders of mammals that survive today underwent their early evolution; however, many of the early representatives would seem unusual and unfamiliar. Small primates called lemurs, early ancestors of man, inhabited the forests. Horses were the size of small dogs; they were forest dwellers and had four toes on the front feet and three toes on the hind feet. Sharing early Cenozoic forests with primitive horses were huge rhinoceros-like beasts called titanotheres; they stood 2.4 m (8 ft) at the shoulder and bore an imposing pair of horns on the front of the skull. Species ancestral to the living members of the dog family (Canidae) and cat family (Felidae) preyed on their fellow mammals. Cats, especially, underwent rapid evolution, and early types of cats had highly specialized dentitions characterized by large, saberlike upper canine teeth. Not all of the early Cenozoic mammals, however, would be unfamiliar today. Bats, identical in general appearance to the bats of the present, flew erratically across the twilight sky, and rodents looking essentially like present-day mice and rats occupied a broad array of habitats. The mild conditions of the early Cenozoic persisted for about 30 million years, but they were gradually replaced toward mid-Cenozoic by more temperate conditions. Of particular importance was the rise in the importance of grasses and the spread of plains and savannas. With the expansion of grasslands came possibilities for new ways of life and new food sources. To horses, the ability to move fast and far to avoid predators and to seek widely separated water holes or concentrations of food became important. Correspondingly, the side toes of the three-toed feet of the plains-dwelling Merychippus were reduced, and when moving on firm ground this animal was functionally one-toed. Appearing at this time in Europe, and later spreading to the New World, were the first representatives of the family Bovidae, which today includes antelope, bison, and sheep. The long, slender limbs and cloven-hoofed feet of these animals were well adapted to moving rapidly and efficiently over open grassland, and these animals prospered and diversified. In Africa some primates began a new way of life. They occupied grasslands or savannas, assumed a ground-dwelling style of life, and founded the evolutionary line that eventually led to humans. By 7 million years ago, in the middle Pliocene, mammals had become "modernized." Roughly 80 percent of the families of mammals of that time still have living representatives. Elephants and deer became widespread in northern latitudes, including those in the New World, and pronghorn antelope of several kinds occupied the plains of North America. Gone were the titanotheres and other "archaic" mammals, and the mammals that replaced them were fairly closely related to species living today. A spectacular segment of mammalian history occurred during the Pleistocene Ice Age. This relatively brief geological epoch, from about 2.5 million years ago to approximately 10,000 years ago, was characterized by mammalian faunas that were not only more diverse than those today but also contained many large and spectacular species. During the greatest Pleistocene glacial advance an estimated one-third of the Earth's surface was covered with ice, and ice flowed as far south as New Mexico. Four glacial advances occurred, separated by relatively warm periods when the glaciers receded. With each advance, mammals were pushed southward before the front of ice, and with each retreat they moved northward again. The musk ox, for example, an animal that now occupies arctic parts of Canada and Alaska, occurred as far south as Colorado and Arkansas in the United States during one Pleistocene glacial advance. Many of the species alive today had closely related Pleistocene counterparts, but in addition to these somewhat familiar types there were a number of spectacular giants. Elephants were widely distributed in Europe and North America, some larger than living elephants and bearing enormous tusks with tips that crossed in some old males. Huge bison, with horns spanning more than 1.8 m (6 ft), and beavers the size of bears inhabited North America; in Europe, "Irish elk," great deer often with a spread of antlers approaching 2.4 m (8 ft) and sometimes 4 m (13 ft), and enormous single-horned rhinoceroses were widespread. The La Brea tar pits of Los Angeles entrapped and preserved thousands of Pleistocene animals; this site produced the most complete record known of a Pleistocene mammalian community. Virtually the same array of mammals that lives in the hills surrounding Los Angeles today occupied the area in the late Pleistocene, but in addition the tar pits have yielded many impressive species that are now extinct. The carnivores included a huge lion larger than the living African lion, the dire wolf, a saber-tooth cat, and, largest of all, the enormous short-faced bear. Among the plant eaters were a type of horse, a javelina, a large camel, a small pronghorn antelope, a tremendous bison, three kinds of elephants, and three species of massive ground sloths. With the end of the Pleistocene, the huge mammals became extinct over most of the world. Only in Africa does a mammalian fauna resembling that of the Pleistocene remain, but even there a number of large Pleistocene species have become extinct. The factors causing these extinctions are not known, but climatic change and heavy hunting by human beings may have been critically important. CLASSIFICATION The class Mammalia is often divided into two subclasses, the Prototheria and the Theria. The subclass Prototheria, in turn, is divided into three infraclasses, the extinct Eotheria and Allotheria and the living Ornithodelphia, which contains the order of egg-laying monotremes. In some classifications the infraclass Allotheria (the multituberculates) is placed in the subclass Theria or forms a subclass of its own. The subclass Theria is also divided into three infraclasses, the extinct Pantotheria and the living Metatheria (marsupials) and Eutheria (placental mammals). The marsupials and placental mammals are separated, as the name implies, on the general basis of the presence or absence of a placenta, a special organ that develops between the mother and the fetus during pregnancy. All living mammals, except the marsupials and egg-laying monotremes, are placental mammals. About 19 orders of living mammals are commonly recognized. Subclass Prototheria This subclass is represented by only one living order, containing the most primitive mammals. Infraclass Ornithodelphia. The order Monotretata (2 families and 3 to possibly 6 species) includes the duck-billed platypus and the spiny anteaters, or echidnas. Monotremes are the only mammals that lay eggs, but they nourish the young with milk. They are largely insectivorous. Adults have no teeth and have birdlike beaks. They live in Australia, Tasmania, and New Guinea; the platypus is amphibious, and echidnas are terrestrial. Subclass Theria This subclass comprises all living mammals that bear the young alive. Infraclass Meratherie. The Marsupialia (12 families and 242 species) contain the opossums, kangaroos, koalas, wombats, and bandicoots. Baby marsupials are rudimentary when born, and each newborn grasps a nipple with its mouth and remains attached during the early part of its life. Many marsupials have pouches that retain the young. Marsupials pursue a wide variety of styles of life and utilize many types of food. They occupy Australia and some nearby islands, South America, and North America northward through much of the United States into the southern parts of Canada. Infraclass Eutheria. The Insectivora, the third largest mammalian order (8 families and 406 species), comprises shrews, moles, and hedgehogs. These are generally small mammals with sharp-cusped teeth adapted to an insect diet, and, in most cases, long-snouted skulls and five-toed feet. Insectivores prefer moist habitats and occur in Europe, Asia, Africa, North America, and part of South America. The Dermoptera (2 families and 2 species) contains the colugos, or flying lemurs. These are restricted to tropical forests in southeast Asia and the Philippine Islands. A membrane between the forelimbs and hind limbs enables these animals to glide. The Chiroptera, the second largest order of mammals (18 families and about 850 species), contains the bats. In all bats, wings are formed by membranes stretched between the four fingers of the hand and between the last finger and the body. The thumb remains free. The teeth are usually sharp-cusped and adapted for masticating insects. Most bats use echolocation in addition to vision for perceiving their environment. Bats occur virtually everywhere except in the polar regions and exploit a great array of foods including insects, fish, small lizards, other bats, nectar, pollen, fruit, and blood. The Primates (11 families and about 166 species) include the lemurs, lorises, tarsiers, monkeys, apes, and humans. Primates usually have dextrous hands, with nails instead of claws, large eyes that face forward, and large brains. Most are arboreal and occupy forests, but some live in grasslands or deserts. Primates are mostly herbivorous, and their range (exclusive of that of humans) includes tropical parts of both the Eastern and Western hemispheres. The Edentata (3 families and 31 species) includes the sloths, anteaters, and armadillos. All members lack front teeth (incisors and canines); some have no teeth; and when teeth are present they lack enamel. Armadillos have a jointed, bony shell. Edentates occur from the central United States southward through most of South America; most species occupy tropical areas. Sloths eat leaves; armadillos and anteaters eat mostly insects. The Pholidota (1 family and 8 species) comprises the pangolins. Found in Africa and southeast Asia, pangolins are covered with scales, lack teeth, and gather termites with their enormously elongate tongues. The Tubulidentata (1 family and 1 species) comprises the aardvarks. These burrowing African mammals have a compact piglike body, a long snout, and reduced teeth. The aardvark uses its stout claws to dig for termites, which it gathers up, anteaterlike, with its long tongue. The Lagomorpha (2 families and 63 species) contains the hares, rabbits, and pikas. Lagomorph dentition is similar to that of a rodent, but in back of each of the two prominent upper front teeth (incisors) is a second, peglike tooth. The rabbits typically have long ears, and many have long, slim limbs adapted for running. Pikas have small ears and short legs. These mammals occupy environments ranging from Arctic to desert and are nearly worldwide in distribution. The Rodentia is the largest mammalian order (34 families and more than 1,700 species). Due to introductions by human beings, rodents are nearly worldwide in distribution. The skull of a rodent has four prominent front teeth, two above and two below, that are used for gnawing, a broad space with no teeth (canines are lacking), and a series of back teeth used for grinding food. In size, rodents range from more than 45 kg (100 lb) to about 5 g (0.18 oz). Rodents occupy almost every terrestrial habitat from the Arctic to the tropics and utilize a tremendous variety of foods. The Mysticeti (3 families and 10 species) contains the baleen whales. These whales lack teeth, having instead fringed plates of horny material (baleen) that grow downward from the upper jaw. Water rich with small marine organisms is brought into the mouth and forced out through the baleen by the enormous tongue; organisms are strained out and swallowed. Hind limbs are absent, and the front limbs are modified into steering surfaces (flippers). Propulsion is accomplished by the horizontally oriented tail (flukes). Length ranges from about 6 m (20 ft) to more than 30 m (100 ft); weights reach over 100 tons. The Odontoceti (7 families and 74 species) are the toothed whales, which include porpoises and dolphins. Odontocete jaws typically bear large numbers of simple, conical teeth. These social animals have a highly developed ability to echolocate. The limbs and tail are like those of baleen whales. Odontocetes range from about 1.5 to more than 15 m (5 to 50 ft) in length. They occupy all oceans and many large rivers and lakes; they feed on many kinds of prey, including squid, bony fish, sharks, seals, and whales. The Carnivora (10 families and 284 species) comprises dogs, cats, civets, seals, walruses, and others. Terrestrial carnivores (fissipeds) have large canine teeth, and most have one upper and one lower "carnassial" tooth on each side of the jaw. The carnassial teeth function to shear, or slice, meat in most carnivores but are flattened in omnivores, such as bears and raccoons. Terrestrial carnivores have clawed feet and are often swift runners or adroit climbers. Seals, sea lions, and walruses (pinnipeds--often considered a separate order, Pinnipedia) usually have simple conical teeth, and both the fore and hind limbs are flipperlike. The upper canines of walruses are enlarged into huge tusks. Some weasels weigh less than 60 g (2 oz), whereas the elephant seal reaches almost 4 tons in weight. Carnivores eat a wide assortment of animals and some plant material and are found nearly worldwide. The Proboscidea (1 family and 2 species) contains the elephants. Elephants are the largest living terrestrial mammals, exceeding weights of 5.5 metric tons (6 tons). The snout is extended into a trunk, and one upper incisor on each side develops into a huge tusk. The rear grinding teeth emerge in sequence; only one on each side of each jaw is functional at a time. The last grinding teeth come into place when the elephant is some 23 years old and may be functional until an elephant is 60 or more years old. The limbs, which must bear great weight, are built like pillars. Elephants eat vegetation and live in forested or partly forested parts of Africa and Southeast Asia. The Hyracoidea (1 family and 11 species) comprises the hyraxes. These rabbit-size animals resemble rodents but are more closely related to elephants. Like elephants, hyraxes have an incisor tooth on each side of the upper jaw that forms a tusk (in this case a short one) and feet with nails instead of claws. These almost tailless herbivores are nimble climbers, and the bare soles of their feet provide remarkable traction. The Sirenia (2 families and 5 species) includes the dugongs and manatees. These fully aquatic creatures reach weights of more than 450 kg (1,000 lb), and their skin is nearly hairless. Like whales and porpoises, the forelimbs of sirenians are flippers and the tail is a horizonal fluke. Dugongs and manatees graze on aquatic plants in coastal waters, lakes, or rivers, along many tropical coastlines of the world. Sirenians are in danger over much of their range because of persistent hunting by humans. The Perissodactyla (3 families and 16 species) encompasses horses, rhinoceroses, and tapirs. These animals all have hooves or broad nails. The front feet have three or four digits, and the hind feet have three digits in most species; horses retain only one functional toe (the third) on each foot. The skull is long, and the back teeth are adapted to grinding vegetation. Today, perissodactyls are native to parts of Africa, central and southern Asia, and Central and South America, but in the past they were far more diverse and widely distributed. The order Artiodactyla (8 families and 171 species) contains the cloven-hoofed mammals, such as camels, giraffes, sheep, and cattle. There is a good deal of variation in foot structure and in the head. Many species, such as deer and antelope, have antlers or horns. Hippopotamuses, pigs, and javelinas lack these weapons but have tusklike canine teeth. Artiodactyles are nearly worldwide in distribution but are absent from Antarctica, the Australian region, and remote islands. Exploitation Human beings have a long history of exploiting their fellow mammals, and their impact has been devastating. Today more than 100 species of mammals are threatened with extinction, and within the last 400 years some 36 species have become extinct, mostly because of the destructive acts of human beings. Because clothing designers and others in Europe and the United States have considered spotted furs as symbols of high fashion, leopards have been killed in large numbers over much of Africa and bobcats are being trapped heavily in the western United States. Marine mammals have been ruthlessly decimated. Whaling has reduced populations of some of the large whales to the verge of possible extinction, and many thousands of dolphins have been killed and discarded during commercial netting operations for tuna. Many mammals cannot continue to resist this intense pressure, and unless attitudes and practices are altered the mammalian fauna of the world will continue to be impoverished at an ever-increasing rate. Terry Vaughan Bibliography: Booth, E. S., How to Know the Mammals, 4th ed. (1982); Corbett, G. B., and Hill, J. E., A World List of Mammalian Species, 2d ed. (1986); Grzimek, Bernhard, ed., Grzimek's Encyclopedia of Mammals, 5 vols., 2d ed., (1989); Kemp, T. S., ed., Mammal-Like Reptiles and the Origin of Mammals (1982); Lever, Christopher, Naturalized Mammals of the World (1985); Parsons, Alexandra, Amazing Mammals (1990); Savage, R. J., Mammalian Evolution (1986); Voelker, William, The Natural History of Living Animals (1986); Walker, E. P., et al., Mammals of the World, 4th ed. (1983).