Mammals
Koala
Phascolarctos cinereus · also called koala bear
The koala is one of Australia’s most recognisable animals and one of its most specialised. It lives its whole life in eucalyptus trees, eating a food so poor and so toxic that almost nothing else will touch it, and paying for that diet with a life of near-constant sleep.
Overview
Koalas are found only in the eucalyptus woodlands of eastern and southeastern Australia. Each koala has a home range built around a set of specific food trees, and it spends the day wedged in a fork dozing, feeding for a few hours around dusk and dawn. Northern koalas (Queensland) are smaller and lighter-furred; southern koalas (Victoria) are up to twice the weight with thicker coats.
Living on eucalyptus
Eucalyptus leaves are about the worst diet a mammal could choose: fibrous, low in calories and protein, and laced with oils and other compounds that are toxic to most herbivores. The koala has rebuilt its body around making it work:
- A caecum (a pouch off the gut) up to two metres long, packed with bacteria that ferment the fibre and neutralise toxins.
- A liver that filters out the eucalyptus poisons.
- A very low metabolic rate and up to 22 hours of sleep a day to minimise how much energy it has to find.
- Cheek teeth shaped to shear tough leaves into a fine paste.
It gets nearly all its water from the leaves and rarely drinks.
Behaviour and life cycle
Koalas are largely solitary. Males mark trees with a scent gland on the chest and, in the breeding season, produce a loud, deep bellow — startlingly big for the animal — to advertise themselves.
A single “joey,” the size of a jellybean, is born after about 35 days and crawls into the backward-opening pouch, where it stays for roughly six months. It then rides on its mother’s back or belly for another six months, sampling leaves and eating pap to build its gut flora, before becoming independent.
Threats and conservation
The koala’s decline has been steep, and in 2022 it was reclassified as Endangered across Queensland, New South Wales, and the ACT. The pressures stack up:
- Habitat loss. Australia has one of the highest rates of land clearing in the developed world, and koala forest is cut for farming, mining, and housing.
- Bushfire. The 2019–20 “Black Summer” fires burned huge areas of koala habitat and killed or injured tens of thousands of koalas.
- Disease. A chlamydia epidemic causes blindness, infertility, and death in many populations.
- Cars and dogs at the edges of towns, and heat and drought that force koalas down to the ground in search of water.
Koala hospitals, wildlife corridors, disease research, and legal protection of habitat are the front lines, but reversing the trend depends above all on keeping the trees standing.
Koala: common questions
Is a koala a bear?
No. The koala is a marsupial — it carries and nurses its young in a pouch — and its closest relative is the wombat. Early English speakers called it a "koala bear" because of its round, stocky, bear-like look, but it is no more a bear than a kangaroo is.
Why do koalas sleep so much?
Eucalyptus leaves are a terrible food — low in energy, low in protein, and full of toxins and tough fibre. A koala copes by extracting what it can with an enormous specialised caecum full of detoxifying gut bacteria, and by moving as little as possible. Sleeping 18 to 22 hours a day is how it balances the books.
How do koalas eat leaves that would poison other animals?
Their liver breaks down the eucalyptus toxins, and their gut microbes ferment the fibre. Joeys inherit the right microbes by eating a special soft form of their mother's droppings, called pap, during weaning. Individual koalas also learn to prefer particular eucalyptus species and will refuse leaves from the "wrong" tree.
Are koalas endangered?
The koala is listed as Vulnerable nationally and, since 2022, Endangered across its northern range in Queensland, New South Wales, and the ACT. Land clearing, the 2019–20 bushfires (which killed or harmed tens of thousands of koalas), a widespread chlamydia epidemic, drought, dogs, and cars have driven populations down steeply.