Koalas, Australia’s marsupials, have a distinctive reproductive cycle. Their young, called joeys, undergo significant growth and development from birth to independence.
The Astonishing Size of a Newborn Koala
A newborn koala joey is tiny, typically measuring around 2 centimeters (16 to 21 millimeters) and weighing less than 1 gram. At this stage, it is entirely hairless, blind, and lacks developed ears, resembling a pink jellybean. While its hind limbs are underdeveloped, its forelimbs are strong and equipped with claws.
This lack of development at birth is characteristic of marsupials, which have a short gestation period, lasting approximately 35 days for koalas. Unlike placental mammals, marsupial development largely occurs outside the womb, within the mother’s pouch. Immediately after birth, the joey embarks on an unassisted climb from the birth canal to its mother’s pouch. This climb, taking only minutes, is guided by smell, touch, and innate direction, using its forelimbs and claws. Once inside the pouch, the joey attaches to one of two teats, which swells to fill its mouth, securing it. The mother’s muscular sphincter at the pouch opening further helps keep the joey enclosed.
Journey of Growth and Development
Once in the pouch, the koala joey spends its initial months drinking only milk. During this period, lasting about six to seven months, the joey grows steadily, developing fur, eyes, and ears. Around five to six months of age, its eyes open, and it may start to peek its head out of the pouch.
As the joey approaches six to seven months, its diet begins to diversify. It starts to consume “pap,” a specialized, soft, runny substance produced by the mother, distinct from her regular faeces. This pap, originating from the mother’s caecum, is an important transitional food. It contains microorganisms essential for inoculating the joey’s gut, preparing it to digest the fibrous eucalyptus leaves of its adult diet. The joey stimulates the mother’s cloaca to prompt pap production.
After this phase, the joey gradually spends more time outside the pouch, often riding on its mother’s back. It continues to consume milk, but increasingly samples eucalyptus leaves, learning to distinguish edible varieties. Koala joeys typically remain with their mothers until about 12 months old, though this can extend to 18-24 months. The mother encourages the joey to become independent, especially when a new joey is on the way, prompting the older offspring to find its own territory.