The eastern grey kangaroo is mainly active at night, resting in shelter during the heat of the day (1) (2) (4) (5). Like other kangaroos, it is able to travel at great speed, using the powerful, enlarged hindquarters for leaping, aided by the long tail, which acts as a balance and rudder (5). Perhaps surprisingly, kangaroos are also good swimmers (4). The diet consists of grasses, herbs, leaves and other low, shrubby vegetation (2) (6). When alarmed, this kangaroo may thump the hind feet (2). The eastern grey kangaroo is one of the most social of the large kangaroos, often gathering in groups, or ‘mobs’, of 20 or more animals of both sexes and all ages where food is abundant (1) (2) (3) (5) (6). Adult males engage in ritualised fights with rival males (2) (5) (9).
Although breeding may occur year-round, the eastern grey kangaroo usually gives birth in summer, between September and March (2) (3) (5) (7) (10). Like other kangaroos, the young is born at an early stage of development, after a gestation period of just 36 days (5) (11). Tiny, naked and blind, the newborn climbs through the female’s fur and into the forward-facing pouch, where it attaches to a teat to undergo the rest of its development (3) (9). The young eastern grey kangaroo develops more slowly than many other kangaroos, first emerging from the pouch after around 283 days, at a time when food is most available, and finally leaving it completely after around 320 days (3) (5) (7) (11). Usually a single young is born, weighing just over 0.8 grams at birth, although twins are sometimes recorded (3) (5). The young is weaned by about 18 months (5) (11). Females reach sexual maturity at around 20 to 22 months and males at 43 months (5), and lifespan may be up to 25 years in captivity (3).
Many kangaroos are able to conceive again soon after giving birth, the new embryo remaining dormant until the first young is ready to leave the pouch or is lost, a process known as embryonic diapause (5) (9). However, this appears to be rare in the eastern grey kangaroo. If conditions are good, the female eastern grey kangaroo may become receptive and mate from 150 days after giving birth, the embryo then entering diapause, but conception does not usually occur until there is time for the previous young to leave the pouch before the birth of the next. The interval between successive births is therefore approximately one year, and dormant embryos are only occasionally found (3) (5) (7) (12).