Koala

The koala is an arboreal marsupial restricted to the eucalypt forests and woodlands of eastern and south-eastern Australia. Its diet is almost entirely eucalyptus leaves, a food source most animals cannot use: the foliage is fibrous, low in nutrition and loaded with toxic phenolic compounds. Koalas cope through a long specialised caecum housing a microbial community that breaks the material down, and through sleeping eighteen to twenty-two hours a day to conserve energy on such a marginal diet. Young are born after a very short gestation, crawl into the pouch and remain there for roughly six months. Koala populations have been under sustained pressure from land clearing, drought, disease — chlamydial infection is widespread and severe — vehicle strikes and, increasingly, catastrophic bushfire. Populations in Queensland, New South Wales and the Australian Capital Territory carry additional national-level protection.

About the koala

The koala is a tree-dwelling, leaf-eating marsupial native to eastern and southeastern Australia. Despite the nickname "koala bear," it is not a bear at all. It is the only living member of the family Phascolarctidae, and its closest living relatives are the wombats. It lives in eucalyptus woodland, feeding almost entirely on eucalypt leaves.

It is a stout, sturdy animal with a large head, round fluffy ears, a big dark nose and almost no tail. Adults measure 60–90 cm (24–35 in) long and weigh 4–15 kg (8.8–33 lb), which places them among the largest tree-dwelling marsupials. Males are about 50 percent larger than females, have more curved noses and carry a scent gland on the chest that shows as a bald patch.

Fur ranges from silver gray to chocolate brown on the back, with a whitish belly. Koalas in the north are smaller and paler than those in the south, and Victorian koalas can weigh twice as much as Queensland ones, with shaggier brown coats. The back fur is the best insulator of any marsupial's and sheds wind and rain.

The body is built for climbing. Sharp, curved claws grip bark, and each front paw has two digits that oppose the other three, letting it grasp small branches. On the hind feet, two toes are fused together but keep separate claws, which the koala uses as a grooming comb. Its pupils are vertical slits, unusual for a marsupial, and its sense of smell is good enough to sniff out which branches are worth eating.

The koala's brain is small for a mammal, about 19.2 g (0.68 oz), and doesn't completely fill the skull. That may be an adaptation to its low-energy diet. Males also have an unusual voice box: an extra pair of "velar vocal cords" in the soft palate lets them make far deeper sounds than their size would suggest. The genus name Phascolarctos means "pouch bear," and cinereus means "ash-colored."

Behavior and diet

Koalas live life slowly. Their diet provides so little energy that they sleep up to 20 hours a day and are mostly active at night, making them largely nocturnal. They often eat and sleep in the same tree for a day or more and come down to the ground only to move between trees, usually just two or three times a night for about 10 minutes in total. In heat, they rest low on the trunk where the bark is cooler; in cold, wet weather they curl into a ball.

What koalas eat

Eucalyptus leaves make up most of the diet, though koalas also visit trees such as Acacia, Melaleuca and Callitris. Of more than 600 eucalyptus species, they strongly prefer about 30, picking leaves higher in protein and lower in fiber. A koala eats up to 400 g (14 oz) of leaves a day over four to six feeding sessions, and it rarely needs to drink because the leaves supply most of its water.

Eucalyptus is low in nutrients and laced with toxic compounds that put most mammals off. Koalas cope with a liver that breaks down the toxins and a gut built for slow digestion. Their cecum, the pouch where plant material ferments, is about 200 cm (79 in) long, possibly the largest for an animal of their size, and food may stay in the gut for 100 hours. Their metabolism runs at about half the usual mammal rate.

Social life and calls

Koalas are asocial, spending only about 15 minutes a day on social behavior. Dominant resident males hold territories near breeding females, and they mark trees by rubbing their chest gland on the bark. Males advertise themselves with a loud bellow, described as a long series of deep, snoring inhalations and belching exhalations, which carries far through the forest and makes the caller sound larger than he is. Females bellow more softly and also snarl, wail and scream when distressed.

Joeys

Koalas breed seasonally, giving birth from October to May. After a pregnancy of just 33–35 days, a single joey (occasionally twins) is born weighing no more than 0.5 g (0.02 oz) and crawls into the pouch. It stays there for six to seven months, then emerges at 300–500 g (11–18 oz). Around this time the mother feeds it pap, a soft dropping full of gut bacteria that prepares it for a eucalyptus diet. The joey then rides on her back and is weaned at 6 to 12 months. Females mature at about three years and males at about four.

Koalas live 13 to 18 years in the wild. As they age, their teeth wear down, and when the grinding surfaces are gone they can no longer chew enough to survive. Natural predators are few: dingoes, large pythons and some birds of prey.

Range and habitat

Koalas live along the eastern and southern coastal regions of mainland Australia, in Queensland, New South Wales, Victoria and South Australia, covering about 1,000,000 km² (390,000 sq mi) and 30 ecoregions. Magnetic Island marks the northern limit. Fossils show they once lived as far west as southwestern Western Australia.

They live in both tropical and temperate habitats, from dense woodland to open forest, but always need eucalyptus. In semi-arid areas they stick to trees along streams and creeks, which give refuge during drought and extreme heat. Koalas can also survive in towns and suburbs if enough trees remain. South Australia's populations, including those in the Mount Lofty Ranges and on Kangaroo Island, came from koalas moved there from Victoria around 1920.

Alongside kangaroos, koalas are recognized worldwide as symbols of Australia.

Conservation status

The koala is listed as Vulnerable on the IUCN Red List, assessed in 2020. In February 2022, koalas in Queensland, New South Wales and the Australian Capital Territory were listed as Endangered under Australia's federal Environment Protection and Biodiversity Conservation (EPBC) Act.

Hunting for fur devastated koalas in the late 1800s and early 1900s; about 8 million were killed for pelts between 1888 and 1927. Today one of the biggest threats is habitat loss and fragmentation, driven by urban growth near the coast and land clearing for farming inland. Cars and dogs kill about 4,000 koalas a year. Disease is a major problem too, especially chlamydia, which causes eye, urinary and reproductive infections, and koala retrovirus, which can cause an AIDS-like immune deficiency.

Bushfires and climate change add pressure. Koalas are slow, and in a fire they climb higher into flammable eucalyptus trees. On Kangaroo Island the population fell from about 48,000 before the 2020 fires to an estimated 5,000–10,000. Population estimates vary widely, from 92,000 in a 2021 government estimate to a 2025 survey figure of 729,000–918,000, compared with 8 to 10 million at the start of the 20th century. Some places now have too many: koalas are over-abundant in parts of South Australia and Victoria, where they are over-grazing their trees.