The koala (Phascolarctos cinereus) is a thickset arboreal marsupial herbivore native to Australia, and the only extant representative of the family Phascolarctidae.
The koala is found in coastal regions of eastern and southern Australia, from near Adelaide to the southern part of Cape York Peninsula. Populations also extend for considerable distances inland in regions with enough moisture to support suitable woodlands. The koalas of South Australia were largely exterminated during the early part of the 20th century, but the state has since been repopulated with Victorian stock. The koala is not found in Tasmania or Western Australia.
Although three subspecies have been described, these are arbitrary selections from a cline and are not generally accepted as valid. Following Bergmann's Rule, southern individuals from the cooler climates are larger.
A typical Victorian koala (formerly P. cinereus victor) has longer, thicker fur, is a darker, softer grey, often with chocolate-brown highlights on the back and forearms, and has a more prominently light-coloured ventral side and fluffy white ear tufts.
Typical and New South Wales koala weights are 12 kg (26 lb) for males and 8.5 kg (19 lb) for females. In tropical and sub-tropical Queensland, however, the koala is smaller (at around 6.5 kg (14 lb) for an average male and just over 5 kg (11 lb) for an average female), a lighter often rather scruffy grey in colour, and has shorter, thinner fur.
In Queensland, the koala was previously classified as the subspecies P. cinereus adustus, and the intermediate forms in New South Wales as P. cinereus cinereus. A fourth variation, though not technically a subspecies, is Phascolarctos cinereus aurum, or in English "golden koala," which has a slight golden tinge to the fur as a result of an absence of the melanin pigment that produces albinism in most other mammalian species. The variation from one form to another is continuous and there are substantial differences between individual koalas in any given region such as hair colour.
The origins of the koala are unclear, although almost certainly they descended from terrestrial wombat-like animals. Koala fossils are quite rare, but some have been found in northern Australia dating to 20 million years ago. During this time, the northern half of Australia was rainforest. The koala did not specialise in a diet of eucalyptus until the climate cooled and eucalypt forests grew in the place of rainforests. The fossil record indicates that before 50,000 years ago, giant koalas inhabited the southern regions of Australia. The koala fills the same ecological role as the sloth of South America.