This gives cubs the best possible start in life on a diet that in the best of conditions is of poor nutritional quality. Their poor, low-energy bamboo diet prevents giant pandas from devoting much energy to gestation or lactation. As a result, giant pandas are the smallest newborn of any nonmarsupial mammal and they grow very slowly. Giant panda infants weigh just four to six ounces at birth and young are a year old before they reach 75 pounds, which is about one-third of adult weight. Female giant pandas do not reach sexual maturity until they are five to six years of age, but even young, inexperienced females demonstrate a strong maternal urge and know, without practice, how to care for their young.
Giant pandas start to breed late, and then usually rear just one young every two years.
Because a female is considered old, and possibly post-reproductive, by the age of 20 or 22, a female may rear, at most, about seven young in her lifetime. The giant pandas low reproductive rate makes it very difficult for a giant panda population to recover from a decline in numbers. In 1990 and 1991, two myths about giant panda reproduction were debunked. More than half of all giant panda litters are twins, but it was widely believed females could never raise both of them. Then researcher at Wolong discovered a giant panda den with twin infants, both of which looked fit and thriving.