Koalas live 13 to 18 years in the wild. In captivity, koalas have lived over 20 years. Females usually outlive males, who sometimes fall from trees while fighting.
Dingos, large pythons and birds of prey sometimes kill koalas, but bushfires, droughts and starvation are bigger threats. Koalas depend on specialized chewing teeth to handle their diet of eucalyptus leaves. These teeth start wearing down when the animals are about 6 years old. The teeth eventually wear to the point that the koala can no longer chew its food. Koalas are also subject to koala retrovirus, which causes immune deficiency and reproductive system infections; the latter can cause sterility.