The duration a kangaroo carries its baby, known as a joey, involves a unique three-stage process that sets marsupials apart from placental mammals. This complex reproductive strategy combines brief internal gestation, a long dependency in the pouch, and extended suckling outside the pouch. This allows the female kangaroo to manage the joey’s development based on environmental conditions and the needs of her existing young.
The Extremely Short Gestation Period
The initial phase, internal gestation, is remarkably short, lasting only about 20 to 36 days depending on the species. This brief period results in a birth that resembles an embryonic stage rather than a fully formed neonate. The newborn joey is tiny, measuring around two centimeters long and weighing less than a gram.
The undeveloped joey is hairless, blind, and has premature eyes and ears, yet it possesses a highly developed sense of smell and strong forelimbs. Immediately after birth, the mother licks a pathway in her fur. The joey instinctively uses its front claws to make an unassisted climb from the birth canal to the safety of the pouch.
Development Within the Pouch
The pouch phase is the longest continuous period of carrying, representing the duration of the joey’s physical development. Once inside, the joey attaches firmly to one of the mother’s teats, which swells inside its mouth, holding it securely in place for the first few months. For Red Kangaroos, the joey remains in the pouch for about eight months; for Grey Kangaroos, this period extends to around 11 months.
During this time, the joey completes developmental milestones most mammals achieve in the uterus, such as growing fur and opening its eyes. The mother’s milk composition changes to suit the joey’s evolving nutritional needs. Around six to seven months, the joey begins to poke its head out and make short, exploratory trips outside the pouch to practice walking and grazing. It quickly returns to the pouch for warmth, safety, or milk.
The Mechanism of Embryonic Diapause
The mother kangaroo manages her reproductive timeline through embryonic diapause, a biological mechanism involving the temporary suspension of a fertilized embryo’s development in the uterus. The embryo remains dormant until conditions are right for its growth to resume.
The presence of a suckling joey in the pouch usually triggers this pause. Lactation causes a hormonal signal, mediated by prolactin, that prevents the new embryo from implanting and continuing its growth. Environmental factors, such as drought or lack of food, can also cause the mother to delay the birth until resources are available. This allows a female to simultaneously manage a dormant embryo, a developing joey in the pouch, and an older joey feeding at her side.
Achieving Full Independence
The end of the physical “carrying” period occurs when the joey permanently vacates the pouch. For Red Kangaroos, this is typically around eight months, and for Eastern Grey Kangaroos, it is closer to 11 months. Leaving the pouch does not signal the end of dependence, as the young kangaroo enters a phase of “at-foot” suckling.
The joey continues to rely on its mother’s milk, inserting its head back into the pouch to feed for several more months. The mother is capable of producing milk of two different compositions from two separate teats to cater to the distinct needs of a pouch-dwelling joey and an at-foot joey. Full nutritional independence, or weaning, occurs much later, often around 12 months for Red Kangaroos and extending to 18 months or more for Grey Kangaroos.