Koala - Physical Description

Physical Description

The koala is broadly similar in appearance to the wombat (its closest living relative), but has a thicker coat, much larger ears, and longer limbs. The koala has large, sharp claws to assist with climbing tree trunks. Weight varies from about 14 kg (31 lb) for a large southern male, to about 5 kg (11 lb) for a small northern female. The koala's five fingers include two opposable thumbs, providing better gripping ability. The first two fingers are positioned in apposition on the front paws, and the first three fingers for the hind paws. The koala is one of the few mammals (other than primates) that have fingerprints. Koala fingerprints are similar to human fingerprints; even with an electron microscope, it can be quite difficult to distinguish between the two. The teeth of the koala are adapted to their herbivorous diet, and are similar to those of other diprotodont marsupials, such as kangaroos and wombats. They have sharp incisors to clip leaves at the front of the mouth, separated from the grinding cheek teeth by a wide diastema. The dental formula for koalas is

The male koala, like many marsupials, has a bifurcated penis. The female has two lateral vaginas and two separate uteri, which is common to all marsupials.

The brain in the ancestors of the modern koala once filled the whole cranial cavity, but has become drastically reduced in the present species, a degeneration scientists suspect is an adaptation to a diet low in energy. One of the smallest in marsupials with no more than 0.2% of its body weight, about 40% of the cranial cavity is filled with cerebrospinal fluid, while the brain's two cerebral hemispheres are like "a pair of shrivelled walnut halves on top of the brain stem, in contact neither with each other nor the bones of the skull. It is the only animal on Earth with such a strangely reduced brain."

It is generally a silent, nocturnal animal, but males have a very loud advertising call that can be heard from almost a kilometre away during the breeding season. Females glean clues regarding a male's suitability as a mate from these calls, showing a preference for larger males. When under stress, koalas may issue a loud cry, which has been reported as similar to that of a human baby. There is little reliable information about the lifespan of the koala, but in captivity they have been observed to reach the age of 18 years.