Which animal can survive eating only eucalyptus leaves?

The koala is an arboreal herbivorous marsupial native to Australia. It is the only extant representative of the family Phascolarctidae and its closest living relatives are the wombats. The koala is found in coastal areas of the mainland's eastern and southern regions, inhabiting Queensland, New South Wales, Victoria, and South Australia. It is easily recognisable by its stout, tailless body and large head with round, fluffy ears and large, spoon-shaped nose.

The vast majority of a koala's diet consists of the leaves from the eucalyptus plant. There are some 600 species of eucalyptus available to the koala, but the koala only eats the leaves of about three dozen varieties. The fibrous leaves, which are low in nutritional value, are also difficult to digest; the koala saves its energy by sleeping or resting up to 22 hours a day.