MAMMAL

any member of the class Mammalia, a group of vertebrate animals in which the young are nourished with milk from special secreting glands (mammae) of the mother. In addition to these characteristic milk glands, mammals are distinguished by several other unique features. Hair is a typical mammalian feature, although in many whales it has secondarily disappeared except in the fetal stage. The mammalian lower jaw is hinged directly to the skull, instead of through a separate bone (the quadrate) as in all other vertebrates. A chain of three tiny bones transmits sound waves across the middle ear. A muscular diaphragm separates the heart and the lungs from the abdominal cavity. Only the left aortic arch of the primitive fourth pair persists (in birds the right arch persists; in reptiles, amphibians, and fishes both arches are retained). Mature red blood cells in all mammals lack a nucleus; all other vertebrates have nucleated red blood cells. Except for the monotremes (echidnas and duck-billed platypuses), which lay eggs, all mammals are viviparous (i.e., bear live young). In the placental mammals (including man) the young are carried within the mother's womb, reaching a relatively advanced stage of development before being born. In the marsupials (kangaroos, opossums, and allies) the newborn, incompletely developed at birth, continue to develop outside the womb, attaching themselves to the female's body in the area of her mammary glands. Some marsupials have a pouchlike structure or fold, the marsupium, that shelters the suckling young. The class Mammalia is worldwide in distribution. It has been said that mammals have a wider distribution and are more adaptable than any other single class of animals, with the exception of certain less complex forms such as the arachnids and insects. This versatility in exploiting the Earth is attributed in large part to the ability of mammals to regulate their body temperatures and internal environment both in excessive heat and aridity and in severe cold. The Editors of the Encyclopdia Britannica a member of the class Mammalia, a group of backboned animals in which the young are nourished with milk secreted by special glands (mammae) of the mother. Another unique mammalian feature is hair; all mammals possess hair at some point in life, though it is found only in the fetal stage of certain whales. Mammals are also warm-blooded and four-limbed (except in some aquatic varieties), but these features are not unique to the class. Mammals are an extremely diverse group, ranging in size from the largest animal that has ever lived, the 150-ton blue whale, to shrews that weigh only a few grams. The primary developmental advance represented by mammals is the ability of their young to learn from the experience of their elders. The dependence of the young mammal on its mother for nourishment makes possible a period of training, and this in turn has brought about a degree of behavioral adaptability unknown in any other group of organisms. Mammals first appeared in the Triassic Period (180,000,000 to 220,000,000 years ago). Their immediate ancestors were members of the reptilian order Therapsida. Therapsids were small, active carnivores equipped with several specialized types of teeth and with limbs that functioned close to the plane of the trunk (i.e., the limbs were positioned more directly under the trunk as opposed to sprawling out to the sides). Both of these characteristics are prominent features of mammals. For the past 70,000,000 years mammals have been the dominant animals in terrestrial ecosystems. Mammals have a decisive advantage over their reptilian ancestors because they are endotherms (warm-blooded), meaning that by physiological means they maintain a relatively constant body temperature independent of that of the environment. Endotherms are capable of longer terms of sustained activity than are cold-blooded organisms and can exploit environments that would be inhospitable to cold-blooded animals. Endothermy requires a high rate of metabolism, and many attributes of mammals are related to maintaining high metabolic levels. For example, the four-chambered heart and its resultant dual circulatory systems, red blood cells that lack nuclei, and the secondary palate (which permits breathing while chewing or sucking) are all features that assist in delivering large amounts of oxygen to the body tissues, where it is used to metabolize food. The mammalian hallmark of hair is also directly related to endothermy, as insulation is one of its chief functions. However, hair also serves for defense and in tactile sensation. Today there are two surviving subclasses of mammals, the Prototheria (monotremes) and the Theria (marsupials and placentals). Another subclass, the Allotheria (multituberculates), is known only from the fossil record. Although primitive in some respects, the multituberculates early evolved into specialized herbivores with gnawing incisors and grinding cheek teeth. They were probably displaced from their ecological niche and driven to extinction by early forms of the rodent order. The last surviving monotremes are the spiny anteaters (Tachyglossus aculeatus and Zaglossus bruijni) and the duckbilled platypus (Ornithorhyncus anatinus). Like the multituberculates, they are an ancient offshoot of the original mammal stock. Much about the monotremes seems out of character for mammals: they lay shelled eggs, which are incubated by the mother; they have no organized nipple (milk is secreted through ducts that open onto the fur covering the abdomen, where it is lapped up by the young); and they lack teeth altogether, substituting for them a horny beak. Their survival reflects the ancient geological isolation of Australia and New Guinea, which protected them from competition with later mammal forms. The therians are the dominant mammalian group, and its members have invaded virtually every habitat on the planet, including the oceans, freshwater lakes and streams, the air, the trees, and even under the Earth. They are represented by approximately 4,000 species. Therians are classed according to their reproductive processes as either marsupials (literally meaning pouched) or placentals. Marsupials take their name from the teat-lined pouch in which their unweaned young are nurtured. Marsupial young are born in a near embryonic state; they must complete their development attached to the mother's teats. Marsupials were once a fairly widespread group, but they are now found only in the Americas and in Australia and nearby islands. It was in Australia that marsupials reached the apogee of their range, filling many of the ecological niches elsewhere occupied by such placental mammals as cats, wolves, moles, anteaters, and rats. The best known marsupials are kangaroos, koalas, wallabies, and opossums. Reproductive patterns among placentals vary, but they all involve the preparation of the female's uterine wall by the hormone progesterone for the reception of a fertilized egg. The resulting complex of fetal and maternal tissues is a true placenta, and by means of it the young gestate within the mother's uterus for periods ranging from two weeks for the domestic hamster to 22 months for the African elephant. Among the 18 orders of placental mammals are the artiodactyls (pigs, cattle, giraffes, sheep, camels, hippopotamuses), carnivores (dogs, cats, foxes, bears, skunks, sea lions, seals), chiropterans (bats), edentates (armadillos, anteaters, sloths), insectivores (shrews, moles, hedgehogs), lagomorphs (rabbits, hares), perissodactyls (horses, zebras, rhinoceroses), primates (monkeys, apes, man), proboscids (elephants), rodents (mice, rats, squirrels, hamsters, beavers, porcupines), sirenians (sea cows), and cetaceans (whales). Man has been dependent on other mammals for a large portion of his food and clothing throughout his cultural evolution. Domesticated mammals have provided transportation, heavy labour, and companionship as well. Many large mammals that competed directly with man for food or space have been rendered extinct or exist now only in captivity. Other types are now endangered by hunting or from the destruction of natural habitats. Additional reading The literature on the biology of mammals is vast. The following list includes only a sample of available sources, with emphasis on those in the English language. Standard general references on the mammals are: F.E. Beddard, Mammalia (1902); W.H. Flower and R. Lydekker, An Introduction to the Study of Mammals, Living and Extinct (1891, reprinted 1978); and J.Z. Young, The Life of Mammals (1957, reprinted 1970).Less technical accounts include: F. Bourlire, The Natural History of Mammals, 3rd rev. ed. (1964; originally published in French, 1951); and L.H. Matthews, The Life of Mammals, 2 vol. (196971). Widely used textbooks in mammalogy are: E.L. Cockrum, Introduction to Mammalogy (1962); and D.E. Davis and F.B. Golley, Principles in Mammalogy (1963). A thorough treatment in English of the families of living mammals is S. Anderson and J.K. Jones, Jr. (eds.), Recent Mammals of the World: A Synopsis of Families (1967). E.P. Walker et al., Mammals of the World, 4th ed. (1983), is a semitechnical work including illustrations of representatives of most living genera; the third volume is a classified bibliography. Perhaps the most comprehensive reference on the biology of mammals is P.P. Grasse (ed.), Trait de zoologie, vol. 1617 (196768, 1955).The morphology and classification of early mammals is reviewed by J.A. Hopson and A.W. Crompton, Origin of Mammals, Evolutionary Biol., 3:1572 (1969); and J.A. Hopson, The Classification of Nontherian Mammals, J. Mammal., 51:19 (1970). A standard reference on the classification of mammals is G.G. Simpson, The Principles of Classification and a Classification of the Mammals, Bull. Am. Mus. Nat. Hist., 85:1350 (1945); the evolution of major mammalian groups is treated by A.S. Romer in Vertebrate Paleontology, 3rd ed. (1966), and Notes and Comments on Vertebrate Paleontology (1968). See also Jason A. Lillegraven, et al. (eds.), Mesozoic Mammals: The First Two-Thirds of Mammalian History (1979).Summaries of specialized topics in mammalogy include: H.T. Andersen (ed.), The Biology of Marine Mammals (1969); S.A. Asdell, Patterns of Mammalian Reproduction, 2nd ed. (1964); L.S. Crandall, The Management of Wild Mammals in Captivity (1964, reissued 1971); R.F. Ewer, Ethology of Mammals (1968, reissued 1973); C.P. Lyman and A.R. Dawe (eds.), Mammalian Hibernation (1960); W.V. Mayer and R.G. Van Gelder (eds.), Physiological Mammalogy, 2 vol. (196364); A.G. Searle, Comparative Genetics of Coat Colour in Mammals (1968); George G. Simpson, Splendid Isolation (1980), a study of South American mammals.Some widely known journals that publish papers dealing exclusively with mammals are: Journal of Mammalogy, Mammalia, and Sugetierkundliche Mitteilungen (all issued quarterly), and Zeitschrift fr Sugetierkunde (6/yr.). Additional technical literature is published by university and public museums and government agencies. Semitechnical and popular accounts of mammals appear in a wide variety of publications of state game departments, museums, and zoological gardens. J. Knox Jones, Jr. David M. Armstrong Evolution and classification The evolution of the mammalian condition Mammals were derived in the Triassic Period from members of the reptilian order Therapsida. The therapsids, members of the subclass Synapsida (sometimes called the mammal-like reptiles), generally were unimpressive in relation to other reptiles of their time. Synapsids were present in the Carboniferous Period (about 280,000,000 to 345,000,000 years ago) and are one of the earliest known reptilian groups. They were the dominant reptiles of Permian times (about 225,000,000 to 280,000,000 years ago), and although they were primarily predacious in habit, the adaptive radiation included herbivorous species as well. In the Mesozoic (about 225,000,000 to 65,000,000 years ago), the importance of the synapsids was generally assumed by the archosaurs or ruling reptiles, the therapsids, in general, being small, active carnivores. Therapsids tended to evolve a specialized heterodont dentition and to improve the mechanics of locomotion by bringing the plane of action of the limbs close to the trunk. A secondary palate was developed and the temporal musculature was expanded. The several features that separate modern reptiles from modern mammals doubtless evolved at different rates. Many attributes of mammals are correlated with their highly active habit; for example, efficient double circulation with a completely four-chambered heart, anucleate and biconcave erythrocytes, the diaphragm, and the secondary palate (which separates passages for food and air and allows breathing during mastication or suckling). Hair for insulation is a correlate of endothermy, the physiological maintenance of individual temperature independent of environmental temperature, and endothermy allows high levels of sustained activity. The unique characteristics of mammals thus would seem to have evolved as a complex interrelated system. Because the characteristics that separate reptiles and mammals evolved at different rates and in response to a variety of interrelated conditions, at any point in the period of transition from reptiles to mammals there were forms that combined various characteristics of both groups. Such a pattern of evolution is termed mosaic and is a common phenomenon in those transitions marking the origin of major new adaptive types. To simplify definitions and to allow the strict delimitation of the Mammalia, some authors have suggested basing the boundary on a single character, the articulation of the jaw between the dentary and squamosal bones and the attendant movement of accessory jaw bones to the middle ear as auditory ossicles. The use of a single osteological character allows the placement in a logical classification of numerous fossil species, other mammalian characters of which, such as the degree of endothermy and nursing of young and the condition of the internal organs, probably never will be evaluated. It must be recognized, however, that were the advanced therapsids alive, taxonomists would be hard-put to decide which to place in the Reptilia and which in the Mammalia. Classification Distinguishing taxonomic features The higher classification of the Mammalia is based on consideration of a broad array of characters. Traditionally, evidence from comparative anatomy was of predominant importance, but more recently information from such disciplines as physiology, serology, and genetics has proved useful in considering relationships. Comparative study of living organisms is supplemented by the findings of paleontology. Study of the fossil record adds a historical dimension to knowledge of mammalian relationships. In some cases, the horses for example, the fossil record has been adequate to allow lineages to be traced in great detail. Relative to that of other major vertebrate groups, the fossil record of mammals is good. Fossilization depends upon a great many factors, most important among which are the structure of the organism, its habitat, and conditions at the time of death. The most common remains of mammals are teeth and the associated bones of the jaw and skull. Enamel covering the typical mammalian tooth is composed of prismatic rods of crystalline apatite and is the hardest tissue in the mammalian body. It is highly resistant to chemical and physical weathering. Because of the abundance of teeth in deposits of fossil mammals, dental characteristics have been stressed in the interpretation of mammalian phylogeny and relationships. Dental features are particularly well suited for this important role in classification because they reflect the broad radiation of mammalian feeding specializations from the primitive predaceous habit.