Koalas give birth to tiny and underdeveloped young, with the mothers weighing hundreds of times more than the newborns. The infant animal immediately crawls into the pouch of its mother -- the pouch faces back toward the birth canal -- and attaches to a nipple. It remains in the pouch for around 6 months and is fed milk and pre-digested leaves until it is weaned about a year later.
Koalas are specialists that eat highly toxic eucalyptus leaves, and rely on their internal microorganisms to process the toxins. This is why the young are fed only pre-digested leaves, and why the young, near the end of their time in the pouch, also eat the mothers' dung, thus acquiring the necessary microorganisms themselves. Koalas have very well-developed molars and long digestive tracts to help them process their high-fiber, low-quality diet.
Both male and female koalas become fertile at about two years of age, but males typically do not breed until four years of age. This is because larger, dominant males control access to the females within their home ranges, mating with all of them until successfully challenged. It is not until four years that a male koala can typically present such a challenge.