By Phil Myers
This marsupial family is restricted to wooded areas of eastern Australia and contains a single living species, the familiar koala ( Phascolarctus cinereus). Koalas are of medium size (7-15 kg). Their tail is short. Their ears are large and rounded, and white on the edge. The pelage is densely wooly. Koalas have a large, bulbous nose. Their marsupium is well developed, opens to the rear, and contains only 2 teats. Their stomachs are unusual, long and folded and with a unique gastric gland. Koalas also have an enormous cecum and substantial cheek pouches.
Like other members of the order Diprotodonta, koalas are 
diprotodont; their dental formula is 3/1, 1/0, 1/1, 4/4 = 30. A large diastema separates incisors and post incisor teeth. All teeth are rooted. The molars appear selenodont and have a highly folded enamel pattern. Also like other diprotodonts, koalas are syndactylous, with digits two and three of the hind feet fused up to (but not including) the claws. On the forelimbs, digits one and two are opposable to three through five.
Koalas are unusual among marsupials in that they briefly form a placenta during the gestation of their embryos. Members of the families Peramelidae , Peroryctinae, and Vombatidae are the only other marsupials with placentae.
Koalas inhabit eucalyptus woodlands where they feed on eucalyptus leaves, stems, flowers, and bark. These trees contain chemicals that are poisonous to most animals. It is likely that the unusually well-developed stomachs and gastric glands equip them to handle the toxins in their diet. This material is also not very nutritious. The large cecum (in which microbial fermentation augments the koala's digestive processes) and cheek pouches, along with dentition highly developed for grinding plant cells, facilitate handling large amounts of forage. Further, koalas have an unusually low metabolic rate, which also reduces their need for food.
Literature and references cited
Feldhamer, G. A., L. C. Drickamer, S. H. Vessey, and J. F. Merritt. 1999. Mammalogy. Adaptation, Diversity, and Ecology. WCB McGraw-Hill, Boston. xii+563pp.
Marshall, L. G. 1984. Monotremes and marsupials. Pp 59-115 in Anderson, S. and J. Knox Jones, eds, Orders and Families of Recent Mammals of the World. John Wiley and Sons, NY. xii+686 pp.
Strahan, R. (ed.). 1995. Mammals of Australia. Smithsonian Institution Press, Washington, D.C. 756 pp.
Vaughan, T. A. 1986. Mammalogy. Third Edition. Saunders College Publishing, Fort Worth. vi+576 pp.
Vaughan, T. A., J. M. Ryan, N. J. Czaplewski. 2000. Mammalogy. Fourth Edition. Saunders College Publishing, Philadelphia. vii+565pp.
Wilson, D. E., and D. M. Reeder. 1993. Mammal Species of the World, A Taxonomic and Geographic Reference. 2nd edition. Smithsonian Institution Press, Washington. xviii+1206 pp.
Contributors
Phil Myers (author), Museum of Zoology, University of Michigan.
