Koala
Reproduction and development

Koalas are seasonal breeders, and give birth from October to May. Females in oestrus lean their heads back and shake their bodies. Despite these obvious signals, males try to copulate with any female during this period, mounting them from behind. Because of his much larger size, a male can overpower a female. A female may scream and vigorously fight off her suitors but will accede to one that is dominant or familiar. The commotion can attract other males to the scene, obliging the incumbent to delay mating and fight off the intruders. A female may learn who is more dominant during these fights. Older males typically accumulate scratches, scars, and cuts on the exposed parts of their noses and their eyelids.

Koalas are induced ovulators. The gestation period lasts 33–35 days, and a female gives birth to one joey or occasionally, twins. The young are born tiny and barely formed, weighing no more than 0.5 g (0.018 oz). However, their lips, forelimbs, and shoulders are relatively advanced, and they can breathe, defecate and urinate. The joey crawls into its mother's pouch to continue its development. Female koalas do not clean their pouches, an unusual trait among marsupials.

The joey latches on to one of the female's two teats and suckles it. The female lactates for as long as a year to make up for her low energy production. Unlike in other marsupials, koala milk becomes less fatty as the joey grows. After seven weeks, the joey has a proportionally large head, clear edges around its face, more colouration, and a visible pouch (if female) or scrotum (male). At 13 weeks, the joey weighs around 50 g (1.8 oz) and its head doubles in size. The eyes begin to open and hair begins to appear. At 26 weeks, the fully furred animal resembles an adult and can look outside the pouch.

At six or seven months, the joey weighs 300–500 g (11–18 oz) and fully emerges from the pouch for the first time. It explores its new surroundings cautiously, clutching its mother for support. Around this time, the mother prepares it for a eucalyptus diet by producing a faecal pap that the joey eats from her cloaca. This pap comes from the cecum, is more liquid than regular faeces, and is filled with bacteria. A nine month old joey has its adult coat colour and weighs 1 kg (2.2 lb). Having permanently left the pouch, it rides on its mother's back for transportation, learning to climb by grasping branches. Gradually, it becomes more independent. The mother becomes pregnant again after a year, when the offspring reaches around 2.5 kg (5.5 lb). She permanently severs her bond with her previous offspring and no longer allows it to suckle, but it remains nearby until it is one-and-a-half to two years old.

Females become sexually mature at about three years of age; in comparison, males reach sexual maturity at about age four, although they can experience spermatogenesis as early as two years. Males do not start marking their scent until they reach sexual maturity, though their chest glands become functional much earlier. Koalas can breed every year if environmental conditions are good, though the long dependence of the young usually leads to year-long gaps in births.
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