In the eucalyptus forests of Australia, you'll find the world's remaining wild koalas eating leaves and sleeping. That's pretty much all these marsupials do. They sleep for up to 20 hours a day cradled in eucalyptus branches, wake up to eat some leaves and go back to snoozing on a full stomach. It's a lazy life that revolves around a monotonous diet, and koalas are perfectly adapted to it.
Eucalyptus leaves have very little nutritional value, provide almost no energy (in the form of calories), are hard to digest and are poisonous to almost every mammal besides the koala (Phascolarctos cinereus). Needless to say, koalas face little competition for their favorite food source. They consume up to 1.5 pounds (680 grams) of leaves in a single day -- that's about 1,000 leaves [sources: San Diego Zoo, Globio]. They spend pretty much all of the energy they get from the leaves on chewing and digesting them. Eucalyptus leaves are so fibrous that most animals wouldn't go near them even if they were safe to eat.
How can koalas eat eucalyptus? Koalas have cheek teeth that grind up the tough leaves, and their other teeth are spaced specifically to slice the leaves into smaller pieces they can swallow. But the most important part happens inside, when a eucalyptus-friendly digestive system takes over.
A koala's digestive tract is full of different types of bacteria that make eucalyptus both safe and (somewhat) digestible. A koala has an organ called a caecum (humans actually have it, too, but a koala's is much bigger). The caecum contains bacteria that break down the eucalyptus fibers. This makes it so at least some of the leaf (about 25 percent) can be digested -- converted into calories for nutrition and energy [source: AKF]. The other digestive trick is a bacterium that neutralizes the toxins in eucalyptus oil, mostly cineole, the poisonous component that makes the leaves unsafe for most mammals.
Of the roughly 500 types of eucalyptus trees out there, koalas only go for a couple of dozen types. And of those, a particular group of koalas will have a few favorites. Eating the same few types of eucalyptus all of the time probably helps koalas learn the scent of a leaf that has a different toxic substance in it called prussic acid. Prussic acid is toxic even to koalas, so they have to be very careful not to eat those leaves.
Koalas are all about eucalyptus, and eucalyptus is an ingredient in many a cold remedy. But how can a cold remedy contain a poison? On the next page, we'll learn what makes eucalyptus medicinal -- and find out if koalas really smell like cough drops.