Koala

BIRD: linking the biodiversity community

Category: Marsupials

The Koala (Phascolarctus cinereus) is an aboreal marsupial herbivore endemic to eastern Australia. Koalas are unmistakable: although broadly similar to a wombat, they have a thicker, softer coat, much larger ears, and longer limbs which are equipped with large, sharp claws for climbing. Weight varies from about 14 kg for a large, southern male, to about 5 kg for a small northern female. They are generally silent, but male Koalas have a very loud advertising call (a nasal snort that human children delight in imitating) which can be heard from almost a kilometre away during the breeding season.

Koalas are found all along the eastern coast from near Adelaide to the base of the Cape York Peninsula, and as far into the hinterland as there is enough rainfall to support suitable forest. The Koalas of South Australia were exterminated during the early part of the 20th Century but the state has since been repopulated with Victorian stock.

Although a member of the large and successful marsupial order Diprotodontia, the Koala is only distantly related to most other diprotodonts: where there are more than 110 species in the Phalangerida branch of the order (including the kangaroos ansd wallabies, the possums, and many others), there are just four species in the sub-order Vombatiformes: the Koala and its closest living relatives, the three wombats.

Although three Koala subspecies were once described, these are arbitary selections from a cline and are not generally accepted as valid. A typical Victorian Koala has longer, thicker fur, is a darker, softer grey, often with chocolate-brown highlights on the back and forearms, and has more a more prominently light-coloured ventral side and fluffy white ear tufts. Typical weights are 12 kg for males and 8.5 kg for females. In tropcal and sub-tropical Queensland, however, Koalas are smaller (at around 6.5 kg for an average male and just over 5 kg for an average female), and have a shorter, thinner coat which is a lighter, often rather scruffy, grey. New South wales individuals are intermediate. The variation from one form to another is continuous, and there are substantial differences between individual Koalas in any given region.

Koalas live almost entirely on Eucalyptus leaves: a mysterious choice of diet requiring a number of specialised adaptations, as eucalypt leaves are low in protein, high in indigestable substances, and contain phenolic terpene compounds that are toxic to most species. Like wombats, Koalas have a very low metabolic rate by mammal standards (which conserves energy), and they rest motionless for about 20 hours a day, sleeping most of that time. They feed at any time, but mostly at night. An average Koala eats a half-kilo of eucalyptus leaves each day, chewing them in powerful jaws to a very fine paste before swallowing. The liver deactivates toxic components ready for excretion, and the hind gut is greatly enlarged to extract the maximum amount of nutrient from the poor quality diet. Much of this is done through bacterial fermentation: when young Koalas are being weaned, the mother passes unusually soft faeces rich in these bacteria to pass these essential digestive aids onto her offspring.

Koalas will eat the leaves of a wide range of gum trees, and even some exotic species, but have firm preferences for particular varieties, which vary from one region to another: in the south Mana Gum, Blue Gum and Swamp Gum are favoured; Grey Gum and Tallowwood are important in the north, and the ubiquitous River Red Gum of the isolated seasonal swamps and watercourses that meander across the dry inland plains allows Koalas to exist in surprisingly arid areas.

Female Koalas are solitary and occupy distinct home ranges that they rarely leave. In the more fertile areas, these ranges overlap; in areas where suitable food trees are scarce they tend to be larger and more exclusive. Males are not territorial, but do not tolerate one another, particularly not during the breeding season: domminant individuals attack subordinate ones, and most adult males carry scars on their face, ears and forearms as a result.

Koalas are almost entirely aboreal. They do not make nests, simply sleep in a tree fork or on a branch. They climb using their powerful claws for grip, usually quite slowly but rapidly at need, and will leap confidently from one tree to another if the branches are reasonably close together. Longer distances are traversed on the ground in a slow but effective waddle. If threatened, Koalas break into a surprisingly athletic gallop, heading for the nearest tree and bounding up it to a safe height; then waiting for the intruder to go away with the endless paitence of a creature that routinely sleeps for 18 hours a day.

Female Koalas reach maturity at two years of age and, if healthy, will produce one young each year for about 12 years. Gestation is 35 days; twins are very rare. At birth, the tiny young crawls into the downwards-facing pouch on the mother's belly (it is closed by a drawstring-like muscle which the mother can tighten at will) and attaches itself to one of the two teats. Young remain hidden in the pouch for about six months, and leave it permanantly after 7 months. They ride on the mother's back for another six months or so, feeding on both milk and gum leaves until weaning is complete at about 12 months of age. Young females disperse to nearby areas at that time; young males often stay in the mother's home range until they are two or three years old.

Although the Koala was hunted almost to extinction in the early 20th Century, largely for its fur, conservation programs since then have succeeded in increasing populations substantially — to the point where large numbers of Koalas in some isolated areas where migration is not possible have needed to be culled or translocated, or else just left to destroy all food trees and then starve. Some colonies have been hard hit by disease, especially chlamydia.

Like the three large kangaroos, Koalas have a disproportionate conservation status: they are considerably less threatened than many lesser-known bird and mammal species, and commonly coexist with humans so long as food supplies are ample. Nevertheless, Koalas occupy a more restricted range than formerly, and depend entirely on the continuing availability of large habitat blocks, and the ability to move through connecting corridors to find new territory and mates. The ever-increasing human population of the continent continues to cut these corridors for agricultural and residential development, forestry and road-building, leaving Koala colonies marooned in decreasing areas of bush.

[edit] See also:

• Australian Koala Foundation
• City of Ballart - Australian Koala Foundation Koala Conservation Project