The koala (Phascolarctos cinereus) is not a bear: it is a marsupial. Its brain is exceptionally tiny for an animal of its size: just 0.2 percent of its body weight. This is thought to be an adaptation to its low-energy diet of eucalyptus leaves. It has an extremely specialized diet, preferring only five of the 350 species of eucalyptus. Metabolizing bacteria in its stomach help it to cope with the highly toxic leaves, of which it eats 500g per day. The koala was almost exterminated 100 years ago by fur hunters and burning of the forests. Protected since 1927, it is falling victim to the same mystery virus as is killing the Tasmanian Devil and its habitat is still threatened by clearance. The micro-organism Chlamydia psittaci also renders it sterile. The koala, which has no sweat glands, cools itself by licking its front legs and stretching out to rest on a tree.
