Mammals are the dominant life forms on Earth today and have been for many millions of years, even though the number of living species (somewhat more than 4,000) is small when compared with the 750,000 species of insects. Over much of the world the character of the landscape has been transformed by the activities of humans and the other mammals that they have introduced into new environments.
The class Mammalia includes about 19 living orders of mammals and an equivalent number of extinct ones. The individual species 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 mammalÑindeed, the largest animalÑthat has ever lived.
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 (see chordate) 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; see ear).
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 (see excretory system). 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 with living reptiles and with amphibians, mammals are unusually active, and the cost in energy of maintaining an even body temperature is high. This cost is partly met by several features that increase efficiency of circulation and respiration (see circulatory system; respiratory system). 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 joint 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 chainÑconsisting of the malleus, or hammer (articular); the incus, or anvil (quadrate); and the stapes, or stirrupÑoccurs 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 develop a true language and to think abstractlyÑabilities unmatched by the great apes or any other animals.
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 (see hibernation).
A remarkably interesting aspect of the biology of mammals is their complex social behavior (see animal 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 hugh 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 (see animal communication).
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 it 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 to 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 (see animal courtship and mating).
Mammals are an ancient group that appeared more than 190 million years ago in the Triassic Period, and their ancestry goes even further back. Mammal-like 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 mammal-like ability to maintain a constant body temperature. Their limbs developed a mammal-like 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 (see Cynognathus). 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), Symmetrodonta, 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 Era.
For more than 100 million yearsÑthe vast sweep of time when dinosaurs dominated the EarthÑmammals 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, such as 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 lifestyles 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 in