Kangaroos have a unique method of raising young, centered around a specialized pouch. This unique reproductive strategy begins with the birth of a tiny offspring, which then undergoes a significant developmental journey.
The Tiny Newborn: Size and Appearance
A baby kangaroo, known as a joey, is born at an underdeveloped stage. At birth, a joey measures about 1 to 2.5 centimeters (approximately 0.4 to 1 inch) in length. This minuscule size is often compared to familiar objects such as a jellybean, a grape, or a lima bean. Weighing less than a single gram, around 0.75 grams, the newborn joey is blind and hairless.
Its appearance at this stage is embryonic, resembling a pink worm. While its hind limbs are mere stumps, its forelimbs are disproportionately developed and equipped with claws. Its strong forelimbs are essential for the joey’s survival, enabling it to undertake a crucial journey without assistance.
The Unique Marsupial Birth Process
The small size of a newborn joey is linked to the short gestation period characteristic of marsupials. Kangaroo pregnancies are brief, lasting about 28 to 36 days. Unlike placental mammals, marsupial embryos do not form a complex placental connection for prolonged development within the uterus. Instead, the joey is born at an altricial, or underdeveloped, stage.
Immediately following birth, the tiny joey begins an instinctive journey. It crawls from the birth canal, located near the mother’s tail, up through her fur towards the pouch. The mother assists this journey by licking a path in her fur, creating a moist trail for the joey to follow. The joey’s ascent is largely self-directed, relying on its developed forelimbs and keen sense of smell to navigate to its destination. This climb, though short, is a significant undertaking for the newborn.
Development Inside the Pouch
Upon reaching the mother’s pouch, the joey attaches itself to one of the teats. The teat then swells inside the joey’s mouth, securing it in place for several months. This attachment ensures a continuous supply of milk, which is crucial for its growth and development.
The joey remains in the pouch, feeding and developing, for an extended period, typically ranging from six to eleven months, depending on the kangaroo species. During this time, it transforms from its embryonic state into a more recognizable kangaroo. Developmental milestones include the opening of its eyes, the growth of fine fur, and the strengthening of its limbs.
The mother’s milk is highly specialized, changing in composition (fat, protein, and carbohydrates) to meet the joey’s evolving nutritional needs at different stages of development. Eventually, the joey begins to periodically peek its head out of the pouch, typically around six months of age, before making short exploratory trips outside. It will continue to return to the pouch for nourishment and shelter until it is fully independent, which can be up to 18 months for some species.