In the breeding season, usually spring and early summer, rival male kangaroos rear up on their hind legs and box to compete for a female. The joey (baby) is born after a gestation of 29 - 38 days. The grey kangaroo is a marsupial mammal, which means that the baby does not develop attached to a placenta inside the mother's uterus, but is born early and spends most of its development inside a pouch. When ready to give birth, the female leaves her mob, finds somewhere quiet and licks her pouch and birth canal clean. The tiny joey, pink and naked, measuring only 2.5cm in length and weighing 1g, is born headfirst and grasps its mother's fur with the claws on its forefeet. The mother offers no help at all. In about 3 minutes, it has dragged itself up to the pouch, entered it and clamped tightly to one of the four teats, which swells in the mouth.
The joey spends 300 days or more inside the pouch, growing very slowly for the first 3 months. After 15 weeks, faster growth begins when the mother's milk increases its fat and protein content. The joey continues to suckle for another 6 months after leaving the pouch; it can run about and jump easily in and out of the pouch.Read More: Grey Kangaroos and Humans