Koalas are polygynous. During the summer breeding season a dominant male will attempt to mate with any estrous females he encounters in his range. Copulation lasts only a
couple of minutes, with the male mounting the female from behind and holding her against a branch. Females are sexually mature at two years old, but generally do not start to breed until they are older—full physical maturity is reached at about four years old in females, five years old in males.
Females have an estrous cycle of about 30 days, and usually breed once every year, between November and March. Gestation lasts about 35 days before a single young (very rarely twins) is born, weighing less than 0.02 oz (0.5g) and measuring about 2 cm long.
The tiny newborn koala crawls into the mother's large pouch and attaches itself to one of the two teats. By 13 weeks the young joey will have grown to about 2 oz (50 g), and by 22 weeks its eyes open and it begins to poke its head out of the pouch for the first time. Joeys have a pouch life of five to seven months, after which they spend most of their time out of the pouch, clinging to the mother's belly and later sitting on their back. Joeys are weaned at six to 12 months, but towards the end of their pouch life also feed regularly on soft, partially-digested leaf material passed through the mother's digestive tract. This "pap," which contains a high concentration of microorganisms, is believed to be important in intro-
ducing to the young koala's gut the microbes it will need to digest eucalyptus leaves.
A joey will remain with its mother until about one year old, when it weighs around 4.4 lb (2 kg) and can begin to fend for itself. Juveniles disperse to find their own home range at about two years old, searching for another breeding group to join, but becoming nomadic if no area is available.
Koalas can live in excess of 10 years in the wild, and 17 years or more in captivity. Longevity is probably related to stress factors such as habitat pressure, disease, and human interference.
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