Kangaroos give birth to tiny, underdeveloped newborns after a remarkably short pregnancy of about 31 days. The newborn, called a joey, is roughly the size of a jellybean, blind, and hairless. It crawls unaided from the birth canal into the mother’s pouch, latches onto a nipple, and continues developing there for months. The entire process looks nothing like birth in most other mammals, and it reflects a reproductive strategy that trades a long pregnancy for an extended period of nursing and pouch life.
A Pregnancy Measured in Weeks, Not Months
Kangaroo pregnancies are strikingly brief. Western gray kangaroos average 31 days of gestation, with a range of 27 to 54 days depending on species and conditions. Red kangaroos carry for roughly 33 days, and eastern grays for about 36. Compare that to a similar-sized placental mammal like a sheep, which gestates for around five months. Kangaroos invest far less energy in pregnancy and far more in the months of pouch rearing that follow.
How the Birth Happens
The mother kangaroo’s reproductive anatomy is different from placental mammals. Female kangaroos have two separate uteri and two lateral vaginal canals, along with a central birth canal that forms (or reopens) specifically for delivery. This structure connects to a shared opening called the cloaca, which also serves the urinary and digestive systems.
Shortly before giving birth, the mother licks and cleans the inside of her pouch, preparing a sterile environment for the joey. She typically sits back or leans against her tail during delivery. The joey emerges from the birth canal looking almost embryonic: pink, translucent, about 2 centimeters long, with closed eyes and fused hind legs. Its forelimbs, however, are relatively well developed with tiny claws.
Using only those forelimbs, the joey drags itself through its mother’s fur and climbs upward into the pouch. The mother does not pick it up or assist the climb. This journey takes only a few minutes, but it’s entirely self-directed. The newborn navigates by instinct, pulling itself hand over hand through the fur until it reaches the pouch opening, tumbles inside, and finds a nipple.
Latching On Inside the Pouch
Once inside the pouch, the joey attaches to one of the mother’s four nipples. What happens next is unusual: the nipple swells inside the joey’s mouth, and the mouth tissue actually fuses around it. For the first several weeks, the joey is permanently attached and cannot let go even if it wanted to. The mother’s body essentially takes over the feeding process, with milk delivered in small, controlled amounts.
During this permanently attached phase, the milk is very dilute, with solids as low as 9% by weight. This watery, low-fat milk suits a newborn whose digestive system is still developing. As the joey grows, the milk composition shifts dramatically. Protein concentration rises steadily throughout the entire lactation period. Fat content increases gradually at first, then jumps sharply in later stages, reaching up to five times the level of early milk. Carbohydrates rise after birth but then drop off around the time the joey starts leaving the pouch. By late lactation, milk solids can reach 54% by weight, producing a thick, energy-dense formula suited for a growing juvenile.
From Pouch to Independence
Joey development inside the pouch follows a predictable timeline, though the exact pace varies by species. For eastern gray kangaroos, the joey’s eyes open and it grows fine fur around seven months. At this stage, the joey starts poking its head out of the pouch, sniffing at the ground and grass, though it generally looks unimpressed by the outside world.
By eight to ten months, the joey takes its first trips out of the pouch, hopping around near its mother before scrambling back in at the first sign of danger. Wallabies hit this stage a bit earlier, around six to seven months. The joey continues returning to the pouch for several more weeks, nursing and sleeping inside, even as it grows almost comically large for the space. Eastern gray joeys are typically weaned by 12 months. Wallabies wean around 10 to 11 months.
The Backup Embryo
Kangaroos have a reproductive trick that most mammals lack: embryonic diapause. A female kangaroo can mate again almost immediately after giving birth and carry a fertilized embryo in a state of suspended development inside her second uterus. This dormant embryo can remain paused for up to 11 months.
The pause is controlled by hormonal signals. In most kangaroo species, the suckling of the joey in the pouch triggers the release of prolactin, which keeps the backup embryo frozen in development. If the pouch joey dies or is weaned, prolactin levels drop and the dormant embryo resumes growing. In some species, particularly those in seasonal environments, the pause is instead controlled by melatonin and day length, so that births are timed to favorable conditions regardless of what’s happening with the current joey.
This system means a female kangaroo can potentially have three offspring in different stages at once: a dormant embryo in the uterus, a developing joey attached to a nipple in the pouch, and an older juvenile that still returns to nurse. Each nipple can even produce milk of a different composition matched to the developmental stage of the joey feeding from it.