Koalas are well-known for their large round head, big furry ears and big black nose. Their fur is usually grey-brown in colour with white fur on the chest, inner arms, ears and bottom. They have no fur on their nose or the palms of their paws. Koalas have particularly hard bottoms, which is similar to their closest relative, the wombat. For the koala, this feature enables them to wedge comfortably in tree forks for long periods of time, whereas the wombat uses its hard bottom as a defence mechanism. Koalas have poor vision and rely heavily on their other senses. They have excellent hearing which helps them detect predators and other koalas. They have an acute sense of smell which also helps them detect other koalas and their favourite food trees. The male uses a scent gland on his chest to mark trees and attract females, by rubbing his chest up and down the trunk. The gland oozes a clear, oily, strong musky smelling liquid.
The size, colour and shape of koalas differs slightly across eastern Australia. Koalas in Queensland are smaller, lighter in colour and have less fur than those in New South Wales, Victoria and South Australia. Queensland's adult female koalas on average weigh between 5 and 6kg and adult males weigh between 6 and 8kg, compared to Victoria’s adult female koalas with an average weight of 8.5kg and adult males 12kg.
Koalas live over a range of open forest and woodland communities but ultimately their habitat is defined by the presence of a select group of food trees. Koalas are found in higher densities where food trees are growing on more fertile soils and along watercourses. They do, however, remain in areas where their habitat has been partially cleared and in urban areas.
Koalas can sleep for up to 20 hours a day, due to their low energy diet, and the intense amount of energy required to break down toxic leaves. Koalas are mostly active at night (nocturnal) and around dawn and dusk. However, they can be seen moving during the day if they are disturbed, get too hot or cold, or need to find a new tree. Koalas are solitary animals living within a network of overlapping home ranges, which allows contact between individuals for mating. Males will try to establish dominance over the home ranges of a number of females during the mating season. These home ranges in southern and central Queensland vary in size from 1km to 135km, depending on the density of the population and the abundance of suitable food trees. In spring, adult males begin to call as a way of advertising their presence to surrounding koalas. Males will seek out a mate and fight with rival males to establish their dominance.
Koalas eat a variety of eucalypt leaves and a few other related tree species, including lophostemon, melaleuca and corymbia species (such as brush box, paperbark and bloodwood trees). They consume around 500 grams of leaves each day, (approximately the size of a small lettuce) and obtain most of their water requirements from the leaves, which can contain up to 50% water. This removes the need for a koala to climb down a tree for a drink of water, except during very hot or dry periods. The leaves are also very low in energy comprising only 5% sugars and starches on average.
Males begin mating at three to four years of age. Females begin mating, and can breed, when they are two years of age, generally giving birth once a year, for the next 10 to 15 years. The gestation period of a female koala is 35 days, after which she gives birth to a single joey. Female koalas are also capable of giving birth to twins, however this is quite rare. Birth usually take place between the months of November and February. The young stays in the pouch for the next six months before emerging for the first time. The joey will then spend between six and 12 months riding on its mother's back. By 12 months of age, the young is weaned and takes up a home range, which overlaps with its mother, for much of the next year. Between the age of two and three years, these young disperse beyond their original home range to establish their own range, usually during the breeding season. On average, koalas live for 10 to 12 years of age in the wild. Although females can continue to breed into their 'teens' and may live as long as 18 years; males are thought to have a slightly shorter lifespan.