Theoretically this means that a devil population can double on an annual basis and make the species insulated against high mortality. The pregnancy rate is high; 80% of two-year-old females were observed with newborns in their pouches during the mating season. More recent studies of breeding place the mating season between February and June, as opposed to between February and March. Gestation lasts 21 days, and devils give birth to 20–30 young standing up, each weighing approximately . Embryonic diapause does not occur. At birth, the front limb has well-developed digits with claws; unlike many marsupials, the claws of baby devils are not deciduous. As with most other marsupials, the forelimb is longer () than the rear limb (), the eyes are spots, and the body is pink. There are no external ears or openings. Unusually, the sex can be determined at birth, with an external scrotum present. Tasmanian devil young are variously called "pups", "joeys", or "imps". When the young are born, competition is fierce as they move from the vagina in a sticky flow of mucus to the pouch. Once inside the pouch, they each remain attached to a nipple for the next 100 days. The female Tasmanian devil's pouch, like that of the wombat, opens to the rear, so it is physically difficult for the female to interact with young inside the pouch. Despite the large litter at birth, the female has only four nipples, so there are never more than four babies nursing in the pouch, and the older a female devil gets, the smaller her litters will become. Once the young have made contact with the nipple, it expands, resulting in the oversized nipple being firmly clamped inside the newborn and ensuring that the newborn does not fall out of the pouch. On average, more females survive than males, and up to 60% of young do not survive to maturity. Milk replacements are often used for devils that have been bred in captivity, for orphaned devils or young who are born to diseased mothers. Little is known about the composition of the devil's milk compared to other marsupials. Inside the pouch, the nourished young develop quickly. In the second week, the rhinarium becomes distinctive and heavily pigmented. At 15 days, the external parts of the ear are visible, although these are attached to the head and do not open out until the devil is around