The Australian wilderness is home to unique creatures whose young are known as joeys. A joey is the term for the young of marsupial species, which give birth to highly underdeveloped offspring. This life stage allows mothers to remain mobile while their young mature, a journey unlike that of almost any other mammal.
The Definition and Species
A joey is defined as a juvenile marsupial, a class of mammals primarily native to Australia and the Americas. While the name is most commonly associated with macropods like kangaroos and wallabies, the term applies to many other Australian species. Koalas, wombats, possums, Tasmanian devils, and bilbies all have young referred to as joeys. Marsupials are distinct from placental mammals due to their short gestation period, resulting in young born in a highly undeveloped state.
Life Inside the Pouch
The birth of a joey is a remarkable event, as the newborn is minuscule, often described as being the size of a jelly bean and weighing less than a gram. This tiny, blind, and hairless creature must immediately climb from the birth canal to the mother’s pouch. It relies on instinct and its well-developed forelimbs and claws, sometimes guided by the mother licking a path in her fur.
Once inside the pouch, the joey latches onto a teat, which swells immediately to secure the attachment for several months of continuous feeding. The pouch itself is a specialized environment, hairless and warm, maintaining a stable temperature slightly higher than the mother’s body. The skin inside the pouch secretes antimicrobial compounds that help maintain a hygienic and protected nursery for the vulnerable young.
One of the most complex biological feats is the mother’s ability to produce different compositions of milk simultaneously from different teats. This allows her to nourish a tiny, newly-attached joey with one type of milk while feeding an older, larger joey outside the pouch with a separate, more nutrient-rich variety. The mother also maintains cleanliness by consuming the joey’s waste, a process that may also help her recycle water.
Growth and Independence
After several months of development, the joey begins the transition to the outside world. Around five to six months of age, the joey starts to poke its head out of the pouch, observing its surroundings. These initial explorations are short, quickly followed by a return to the safety of the pouch.
As the joey grows, the mother encourages it to spend longer periods outside the pouch to graze and exercise, necessary for muscle development. For koala joeys, this transition includes consuming “pap,” a specialized form of predigested eucalyptus from the mother that introduces the gut flora needed to process their adult diet.
The weaning process is gradual, with the joey continuing to suckle milk even after it is too large to fully enter the pouch. For larger macropods like the Eastern Grey Kangaroo, the joey may not fully exit the pouch until around 11 months. It remains close to the mother for guidance and protection until achieving complete independence at over a year old. The mother eventually constricts the pouch opening, making re-entry too difficult and signaling the final separation.