Kangaroos, marsupials of Australia, possess a reproductive strategy that sets them apart from most other mammals. Unlike placental mammals, which develop their young extensively inside the womb, kangaroos give birth to offspring at an incredibly early stage of development. This unique approach involves a brief gestation period followed by an extended period of growth and nourishment within a specialized pouch.
The Newborn Kangaroo
A kangaroo joey at birth is remarkably small and underdeveloped. It is the size of a jellybean or a lima bean, measuring 2.5 centimeters (1 inch) in length. The weight of a newborn joey is less than a gram, 0.75 to 2 grams. At this stage, the joey is pink, hairless, and blind. Its limbs are largely undeveloped, except for its forelimbs, which are strong and equipped with tiny claws.
The Journey to the Pouch
Immediately after birth, the tiny joey embarks on an instinctive journey to its mother’s pouch. The mother licks a path through her fur from the birth canal to the pouch, guiding the blind joey. Using its strong forelimbs and sense of smell, the joey crawls unassisted through the mother’s fur. This trek is entirely self-directed. If the joey falls during this journey, it will likely not survive.
Early Development in the Pouch
Once inside the pouch, the joey locates and firmly attaches itself to one of its mother’s teats. The teat swells inside the joey’s mouth, securing it in place and allowing milk to be secreted slowly, as the newborn lacks the muscle strength to suckle effectively. The pouch serves as a secondary womb, providing warmth, nourishment, and protection. The joey remains attached to the teat for several months, undergoing significant development, including growing fur, opening its eyes, and developing its hind limbs.
The mother kangaroo’s lactation process is highly adaptive, allowing her to produce different types of milk simultaneously for joeys at various developmental stages. She can nurse a newborn joey with one type of milk from one teat, while an older joey, even if out of the pouch, receives a different composition of milk from another. This remarkable ability, known as asynchronous concurrent lactation, ensures each joey receives the precise nutrients it needs. Joeys begin to poke their heads out of the pouch around six months of age, making short exploratory trips before fully leaving the pouch between 8 and 11 months, though they may continue to nurse for up to 18 months.