MARSUPIAL

any member of Marsupialia, a mammalian order (superorder according to some authorities) characterized by premature birth and continued development of the newborn while attached to the nipples on the lower belly of the mother. The pouch, or marsupium, from which the group takes its name, is a flap of skin covering the nipples. Although prominent in many species, it is not a universal feature among marsupials; in some species, for example, the nipples are in a well-defined area but are fully exposed or are bounded by mere remnants of a pouch. The young remain firmly attached to the milk-giving teats for a period corresponding roughly to the latter part of development of the fetus in the womb of a eutherian, or placental, mammal. Gray kangaroo (Macropus canguru) The largest and most varied assortment of marsupialsmore than 100 speciesis found in Australia alone: kangaroos (see photograph), wallabies, wombats, the koala, and a bewildering assemblage of smaller rodent-like forms. About 70 more species are distributed more widely, in Australia (including Tasmania), New Guinea, and a cluster of nearby islands. The wide array of Australian marsupials is reflected in the extensive popular vocabulary of names, many of which are derived from descriptive Aboriginal words. Only two families of marsupialstotalling more than 70 speciesare found in the Americas, vestiges of a larger group that originated there as long ago as the Cretaceous Period (from 136,000,000 to 65,000,000 years ago). The family Didelphidae comprises about 65 species of South and Central American opossums, one of which ranges as far north as southern Canada. The family Caenolestidae consists of seven species of ratlike marsupials confined to South America. a member of the mammalian superorder (or orderauthorities differ) Marsupialia, which includes a diversity of primitive mammals that undergo premature birth and complete their development outside the mother's body while attached to her nipples. Most are also characterized by a pouch, or marsupium, a flap of skin that lies over the nipples, but this is not always present; in some species there is merely a fold of skin encircling the nipples and in others there may be no such feature at all. Common opossum (Didelphis marsupialis) Gray kangaroo (Macropus canguru) Living marsupials are limited in distribution to the Australasian region, where about 175 species are found primarily in Australia, New Guinea, Timor, and Celebes; and to the New World, where more than 70 species inhabit South and Central America, although a few range into southern North America and one, the common opossum (Didelphis marsupialis; see photograph), extends into southern Canada. They range in size from the great gray kangaroo (Macropus giganteus; see photograph), 3 m (9 feet) long and 2 m (6 feet) tall, down to the planigale (Planigale ingrami), a 12-centimetre (5-inch) marsupial mouse. The earliest-known marsupials are from the Cretaceous Period some 100 million years ago. Many of these early forms were members of the still extant family Didelphidae (the opossums). Marsupials originated in the New World and are presumed to have crossed to Australia by an early land bridge at some time preceding the rise of the placental mammals. No fossil marsupials have been found in Africa or Asia. By the beginning of the Tertiary Period (about 66.4 million years ago) Australia had become isolated from the other continents, and its marsupial fauna was able to diversify widely, free from competition from other mammals. Various members of this superorder developed forms or habits similar to those of many of the orders of placental mammals (a phenomenon known as convergent evolution). Thus the grazing kangaroos (Macropodidae) occupy the ecological niche filled on other continents by various hoofed mammals, the predatory dasyures (Dasyuridae) are equivalent to small cats, and the marsupial mole (Notoryctes) has converged remarkably in both form and habit on the true moles. Other Australian marsupials resemble, in form or habit, bears, squirrels, wolves, and mice. In the New World only two families of marsupials survive, the Didelphidae and the Caenolestidae (rat opossums). These animals are for the most part generalized omnivores, which has aided their survival in competition with placental mammals. More specialized groups, such as the large predatory Borhyaenidae, were unable to compete with the influx of placental competitors. One fact which undoubtedly contributed to the demise of many marsupials is that their brains are relatively much smaller than those of similar-sized placentals. In many marsupials the hind legs are noticeably larger than the forelegs, a feature most apparent in kangaroos but appearing throughout the Marsupialia. Unique to this superorder are the epipubic, or marsupial, bones associated with the pelvic girdle, which were once thought to help support the pouch. Where a pouch occurs, it tends to open anteriorly in upright and climbing forms and posteriorly in quadrupedal, ground-dwelling species. The dentition of marsupials also differs from that of other mammals. The number of teeth ranges from 22 in the honey possum (Tarsipes spenserae) to 52 in the numbat (Myrmecobius fasciatus). Typically there are three premolars and four molars in each jaw, contrasting with the placental four premolars and three molars. As marsupials evolved into diverse forms, they came to occupy various habitats. Several forms are highly arboreal, including the koala (Phascolarctos cinereus) and the tree kangaroos (Dendrolagus). The greater glider (Schoinobates volans) has membranes that stretch between the fore and hind limbs and enable it to glide from tree to tree in the manner of the flying squirrels. Only one marsupial has taken to an aquatic life-style; this is the New World water opossum (Chironectes minimus). Many marsupials are herbivorous. The kangaroos are mainly grazers, the wombats (Vombatidae) live on roots, fungi, and grass, and the honey possum feeds on nectar. The most highly specialized is the koala, which can survive only on the leaves of certain species of eucalyptus trees. The numbat lives on ants and termites, which it ingests with its long tongue. The possums and bandicoots are generally omnivorous, living on a variety of plant matter and invertebrates. Many of the marsupial mice and the smaller native cats (Dasyuridae) live on insects and various small vertebrates such as lizards and mice. Larger carnivores such as the Tasmanian devil (Sarcophilus harrisii) prey on birds and other mammals. The most remarkable characteristic of marsupials is their mode of reproduction. Gestation is relatively short, ranging from 8 to 40 days, and the young are born blind and in a very embryonic condition. In spite of this they must travel unassisted over the mother's body from the birth canal to the pouch or teat. The young are equipped for this purpose with relatively large forelimbs, with which they grasp the mother's fur as they squirm and wriggle their way to the pouch. Once inside the pouch each young clamps onto a nipple, which becomes engorged within its mouth. The young marsupial remains in the pouch and continues to develop for weeks to months. After several weeks, the growing young are able to leave the pouch for short intervals, but they will remain with the mother for some time thereafter, often clinging to her fur. Marsupials are typically solitary animals, exhibiting minimal sociability outside of breeding season, although kangaroos often travel and graze in groups known as mobs; these lack, however, the leadership and cohesion found in herds of placental mammals. Marsupials seldom vocalize, a trait that, like their unsocial nature, may derive from their relatively low intelligence. Additional reading Among general works on marsupials are the following: Charles L. Barrett, Wild Life of Australia and New Guinea (1954); Charles W. Brazenor, The Mammals of Victoria and the Dental Characteristics of Monotremes and Australian Marsupials (1950); Albert S. Le Souef and Harry Burrell, The Wild Animals of Australasia, Embracing the Mammals of New Guinea and the Nearer Pacific Islands (1926); Basil J. Marlow, Marsupials of Australia (1962); Ellis Troughton, Furred Animals of Australia, 8th ed. rev. (1966); Ernest P. Walker et al., Mammals of the World, 3 vol. (1964; 2nd ed., vol. 1, 1968); F. Wood Jones, The Mammals of South Australia, 3 vol. (192325, reprinted 1968).Journal and magazine articles include: J. Pearson, Some Problems of Marsupial Phylogeny, Rep. Meet. Aust. N.Z. Ass. Advmt. Sci., 25:71102 (1947); H.C. Reynolds, Studies on Reproduction in the Opossum (Didelphus virginiana virginiana), Univ. Calif. Publs. Zool., 52:223284 (1952); G.G. Simpson, The Affinities of the Borhyaenidae, Am. Mus. Novit., no. 1118 (1941); and The Beginning of the Age of Mammals in South America, Bull. Am. Mus. Nat. Hist., vol. 91, art. 1 (1948); G.H.H. Tate, On the Anatomy and Classification of the Dasyuridae (Marsupialia), Bull. Am. Mus. Nat. Hist., vol. 88, art. 3 (1947); Studies on the Anatomy and Phylogeny of the Macropodidae (Marsupialia), vol. 91, art. 2 (1948); and Studies in the Peramelidae (Marsupialia), vol. 92, art. 6 (1948); D. Fleay, Strange Animals of Australia, Natn. Geogr. Mag., 124:388411 (1963); J.H. Calaby, Australia's Threatened Mammals, Wildlife, 1:1518 (1963); H.H. Finlayson, Mitchell's Wombat in South Australia, Trans. R. Soc. S. Aust., 85:207215 (1961); A.G. Lyne, Australian Mammals, Aust. Mus. Mag., 12:121125 (1956); J. McNally, Koala Management in Victoria, Wildl. Circ. Vict., no. 4 (1957); W.E. Poole and P.E. Pilton, Reproduction in the Grey Kangaroo, Macropus canguru, in Captivity, C.S.I.R.O. Wildl. Res., 9:218234 (1964); G.B. Sharman, Studies on Marsupial Reproduction. III. Normal and Delayed Pregnancy in Setonix brachyurus, Aust. J. Zool., 3:5670 (1955); G.B. Sharman and J.H. Calaby, Reproductive Behaviour in the Red Kangaroo, Megaleia rufa, in captivity, C.S.I.R.O. Wildl. Res., 9:5885 (1964); E.M.O. Laurie and J.E. Hill, List of Land Mammals of New Guinea, Celebes and Adjacent Islands, 17581952, Br. Mus. Nat. Hist. (1954). Hobart Merritt Van Deusen