Koalas survive on toxic leaves.

To most animals, including humans, eucalyptus leaves are highly toxic; to koalas, they’re basically the only food they know. The Australian marsupials subsist almost exclusively on them, which from an evolutionary perspective is fairly odd: In addition to their toxicity, the leaves are also low in nutrients and provide little energy, hence why koalas sleep between 18 and 22 hours a day. Their digestive systems have adapted to the eucalyptus by detoxifying the poisonous chemicals found in the leaves.

Koalas aren’t entirely undiscerning in their appetites, however. They often sniff leaves before deciding whether or not to eat them, and they’ve been known to avoid trees that grow in certain soils — the less fertile the soil, the more toxic the leaves. There are around 600 different eucalypt species, many of which koalas avoid entirely. They eat between 200 and 500 grams of leaves per day, which account for most of their water intake as well.