Kangaroos and Wallabies

For such a large animal, the gestation period of kangaroos is incredibly short. The longest among the kangaroos is that of the eastern gray (Macropus giganteus), in which the baby is born after only 38 days. However, it is less than an inch long, blind, and hairless like the newborns of all marsupials. It may weigh as little as 0.01 oz (0.3 g).

The kangaroo has virtually no hind legs when born. In fact, the front legs, which are clawed, look as if they are going to be mammoth. These relatively large front paws serve the purpose of pulling the tiny, little-formed creature through its mother's fur and into her pouch. Instinct guides the tiny infant, who moves with no help from its mother. If it moves in the wrong direction, the mother ignores it. If it moves slowly, it may die from exposure. These tiny creatures are born with disproportionately large nostrils, so smell apparently plays a major role in guiding the path to the mother's pouch.

A newborn kangaroo has a longer distance to travel than most marsupials. Most others have a pouch that faces backward, or is an open flap of skin where it is easier for the baby to find the teats. In the kangaroos, the baby must climb up to the top of the pouch, crawl over the edge, and then back down inside to reach a teat.

If successful in reaching the pouch, the baby's tiny mouth clamps onto a teat, which swells into the mouth so that the infant cannot release it. The infant's esophagus expands so it can receive both nourishment and oxygen. The baby, now called a pouch embryo, cannot let go even if it wants to. It will be a month or more before its jaw develops enough to open. Only the teat that the baby is attached to actually produces milk.

After the infant is born and moves into the pouch, a female red kangaroo (Macropus rufus) may mate again. This time, however, the fertilized egg goes into a resting state and does not develop until the female stops nursing the first young. Her body signals that change to the zygote, which then starts developing again. This time lag, called diapause, has great advantages to the species in that if one young dies, another embryo can quickly take its place. Diapause does not occur in the eastern gray kangaroo. The pouch embryo will continue to develop as it would if inside a uterus. In the big kangaroos, it takes 10 months or more before the joey emerges for the first time (often falling out by accident). It gradually stays out for longer and longer periods, remaining by its mother's side until about 18 months old. A male great kangaroo reaches sexual maturity at about two or three years, a female not for several years more.