After a baby has been in the pouch for about six months, its mother begins to produce a special substance called pap. The joey feeds on this in addition to the milk it's already getting. Pap comes from the mother's intestines and contains bacteria that the joey needs to have in its own intestines so that it can digest an adult diet of eucalyptus leaves. At about seven months, the joey leaves the pouch to eat leaves, but returns to it to nurse. By the time the joey is about one year old, it stops nursing and eats just leaves.
Koalas are monotremes, like the platypus and the echidna.