The name "koala" means in an extinct Aborigine language "does not drink." Even if called koala bear, this animal has nothing to do with the actual bear; it is not even a placental mammal, but a marsupial, like the kangaroo. Its closest living relatives are the wombats. A koala (Phascolarctos cinererus) is about 60-80 cm (2-2.6 ft) long and weighs 5-14 kg (11-30 pounds). The extremely fine gray fur is the reason why koala has been heavily hunted in the past. The blunt muzzle, long nose, small eyes and large ears give koala a funny look. Koalas are unique amongst mammals in the adaptation of their limbs for arboreal life: they have opposable digits in an arrangement of two against three (like in chameleons).

The koala is a nocturnal mammal; during the day, it sleeps in the canopy of the eucalyptus trees. Like in most marsupials, the koala male has a double-headed penis. Female koalas are notorious for their lesbian "sex sessions," rejecting male suitors (females are slightly larger!) and trying to mate with each other, sometimes up to five at a time. This tendency has been observed till now only in individuals kept in captivity, while in the wild, they seem to remain heterosexual. In captivity, koalas have a ratio of three homosexual mating sessions for each heterosexual one.

The researchers have tried to explain the bizarre behavior: lesbian sex could be used by females to turn on the males; it may also be a hormonal impairment occurring in captivity, or employed to relieve stress.

The female gives birth to an offspring the size of a pea grain. She takes it with her mouth and places it into her marsupium (poach). In 6 months, the young can get out and feed on leaves. At the beginning, the young feed on semi-digested food from the mother. This way, the infant receives the microorganisms required for digesting plant food (imagine that some rodents take them by eating the parents' excrements!). A female koala gives birth once at 2 years, and between mother and young strong bonds establish.

This animal feeds almost exclusively on eucalyptus leaves. The eucalyptus leaves, besides being toxic for most animals, are rich in cellulose and poor in proteins and fats. The adult must detoxify leaves (with energy expense), while cellulose is hard to break down. Because of this poor diet, koalas save energy by sleeping most of the day, while their movements are very slow. One individual consumes about 1 kg (2.2 pounds) of eucalyptus leaves daily and it may never drink water!

Koala is related to two of the most formidable marsupials that have ever existed. One is the extinct Australian marsupial lion (Thylacoleo carnifex), the largest carnivorous marsupial mammal ever. This animal would have killed a modern African lion (Panthera leo) in a fight. Thylacoleo killed its victims very quickly, penetrating with its "bolt-cutter" teeth through the hide and flesh of large animals, causing huge wounds and blood loss. Its massive carnassial cheekteeth effect major trauma and a rapid kill.

The other koala relative is the Diprotodon, an Ice Age species that was the biggest of all marsupials that have ever existed: as large as a hippopotamus! Diprotodon was 2 m (6.5 feet) tall and 3 m (10 feet) long, weighing around 3 tonnes and was part of a pre-historic Australian "mega-fauna" that included giant cow-sized kangaroos and marsupial lions.