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Koalas

The koala (Phascolarctos cinereus) is a tree-living Australian marsupial, or pouched mammal, which early English settlers in Australia called the native bear. The koala is not a bear, but is the only living species in the family Phascolarctidae, though fossils indicate that there were once were a number of species of koala. The name is derived from an Aboriginal word meaning "animal that does not drink," for koalas get their water from the leaves they chew. Koalas are found primarily in dry forest of eastern Australia, and their closest relatives are wombats. Koalas were once hunted for their fur, and millions were killed, rendering it an endangered species.

Koalas weigh up to 30 lb (13.6 kg), with males being considerably larger than females, and measuring from 23-33 in (60-85 cm) long. The thick fur on their round, compact body is primarily dark gray with white markings. The furry ears have a large white fringe and there is a white "bib" on the chest and white on the bottoms of the arms and legs. The nose is black and leathery and the eyes are black, giving the koala a button-eyed look typical of toy plush animals.

Unlike most climbing marsupials, the koala has only a small tail almost hidden in its fur. Koalas climb by means of their large hands and feet, which are equipped with long, strong, curved claws. The hand has three fingers and two thumbs, while on the hind foot, a big clawless thumb works separately from the fingers, and the first two toes are fused and longer than the others. Koalas use these two fused toes (sometimes called toilet claws) for combing their thick, woolly fur. Koalas do not have a sweet disposition and will use their claws against animals or people who molest them.

Koalas usually sit upright in a tree, as if they were perched on a chair. They leave their tree awkwardly, in order to go to a different one. Koalas depend on the trees A koala (Phascolarctos cinereus). The animal has a highly specialized diet that consists of around 2.5 lb (1.1 kg) daily of about 12 species of eucalyptus leaves. Photograph by Robert J. Huffman. Field Mark Publications. Reproduced by permission. in which they live because they have one of the most specialized diets of any mammal. They eat only the leaves of several dozen species of eucalyptus (or gum) trees, and then only at certain times of the year. They sniff each leaf carefully before consuming it. Scientists are not yet certain what chemicals in the leaves cue the koalas to accept them only some of the time and reject them otherwise. Apparently, certain oils become poisonous as the leaves mature. If their tree becomes unacceptable, a koala climbs down and goes in search of another tree with more appealing leaves. Koalas are found in the largest numbers in forest dominated by manna gum trees (Eucalyptus viminalis).

Most animals cannot digest tough eucalyptus leaves, but koalas have a long sac (called the caecum) at the point where the small intestine meets the large intestine. The caecum contains bacteria that help break down the cellulose in the leaves, releasing organic acids and other useful chemicals.

Their specialized diet is the reason that koalas are so difficult to keep in captivity. The correct species of eucalyptus tree has been planted in southern California, and the San Diego Zoo is the only foreign place to which the Australian government will allow koalas to be exported.

Koalas spend at least two-thirds of the day resting or asleep in their tree. At night, when active, koalas do not move hurriedly, nor do they travel far, perhaps only to the next branch or so, feeding primarily after dusk and before dawn. When a male challenges another for his tree or mate, koalas can move quickly, grabbing the other's arm or biting the elbow. Because of their unwillingness to move quickly, koalas have long served as easily-caught food for aboriginal Australians.

Mating among koalas is timed to insure that food will be most abundant when the young emerge from the pouch. Usually, adult koalas live a solitary life, but during the mating season males will issue loud bellows, which draw females from the nearby area. About 35 days after mating, the female gives birth to one (rarely two) young in late spring or early summer. The young are less than an inch (about 19 mm) long when born, and remain in the backward-facing pouch for six months, by which time it has fur, teeth, and open eyes. Once it is out of her pouch, the mother will carry her young koala on her back for another six months as it learns to eat leaves, nestling on her belly when sleeping. Adolescent females stay near their mother, often producing their own first young at about two years of age. Young males gradually disperse through the forest, not mating until they are older and stronger. Koalas probably live 12-14 years in the wild, though they have reached 16 years in captivity.

The population of koalas dropped drastically during the early part of the twentieth century because they were over-hunted for their fur. In addition, diseases of their reproductive tract limited their fertility, and the destruction of their forest habitat has also played a big role in their population decline. Koalas are now protected and their numbers are increasing again. When the number of koalas in a particular area becomes too large for the local food supply, government naturalists move some animals to habitat elsewhere. This prevents excessive defoliation of the food-trees, and helps to returns the species to its former habitat.


Resources

Books

Arnold, Caroline. Koala. New York: William Morrow & Co., 1987.

Koalas. Zoobooks series. San Diego: Wildlife Education, 1988.

Lavine, Sigmund A. Wonders of Marsupials. New York: Dodd, Mead & Co., 1978.

Lee, Anthony, and Roger Martin. The Koala: A Natural History. Australian Natural History series. Kensington, NSW: New South Wales University Press, 1988.


Jean F. Blashfield

KEY TERMS

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Caecum

—A sac, open at only one end, in the digestive system of the koala. It is apparently used to help digest the tough fibers of eucalyptus leaves.

Defoliation

—Removal of leaves from a tree.

Fertility

—The ability to reproduce.

Sternal gland

—A gland, located on the chest (or sternum) of a male koala, which secretes a smelly fluid used in marking his territory.

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