Koala

Definitely not a bear

Despite being called a “koala bear” colloquially, koalas are marsupials, not bears, and not even closely related to bears. Their nearest living relative is the wombat — both belong to the order Diprotodontia. The bear association comes from superficial appearance; koalas have round ears, a stubby black nose, and grip with two opposable “thumbs” on each forepaw, making them look bear-like.

Why so much sleeping

Koalas sleep 18–22 hours per day, more than almost any other mammal. The reason is metabolic: their diet of eucalyptus leaves is extraordinarily low in nutrients and high in toxins. Detoxifying the leaves requires significant liver work, and there’s not enough energy left over for active behavior. Koalas have one of the smallest brains relative to body size of any mammal — also linked to the low-energy diet.

A specialized digestive system

Eucalyptus leaves are toxic to most animals. Koalas have evolved several adaptations:

• A massively elongated cecum (an organ analogous to the appendix) — up to 2 m long — where bacteria ferment cellulose and break down toxins.
• Specialized liver enzymes that detoxify eucalyptus oils.
• Selective feeding — koalas can identify and reject leaves with toxin levels too high to handle, sniffing and tasting carefully before eating.
• Microbial inheritance — joeys inherit gut microbes from their mother (via pap, a soft fecal substance she produces specifically for them) before transitioning to leaf-eating.

A vulnerable species

Koala populations have collapsed in many parts of their range. Threats include:

• Habitat loss — clearing of eucalyptus forests for agriculture, roads, and development.
• Bushfires — the 2019–2020 Black Summer fires killed an estimated 60,000+ koalas and destroyed millions of hectares of habitat.
• Vehicle strikes and dog attacks — significant mortality near suburban edges.
• Chlamydia — a bacterial disease causing blindness and infertility, with outbreaks affecting up to 100% of some populations.

Australia officially listed koalas as Endangered in eastern states in 2022, with national protection upgraded.

Drinking — except when they don’t

The name “koala” derives from a Dharug word gula, often translated as “no water” — koalas reportedly meet most of their water needs from the moisture in eucalyptus leaves. They do drink during droughts and heat waves, sometimes from puddles, sometimes (problematically) from suburban swimming pools.