by Devra G. Kleiman

      Giant pandas exhibit another peculiar evolutionary adaptation known as delayed implantation. A fertilized giant panda egg does not immediately implant on the mother’s uterine wall, but instead "floats" around in her reproductive tract for varying lengths of time. As a result, we do not know precisely the length of the giant panda’s actual gestation period. All we can say is that the time from mating to birth ranges from 95 to 160 days.

      Delayed implantation gives the giant panda more control over when cubs are born because birth dates are not precisely fixed by mating dates. Young may be born in the late summer or in the fall. Overall, however, the general timing of giant panda reproduction is determined by the importance of weaning cubs in the spring, when the newest most protein-filled bamboo shoots are available. 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. And this growth rate is based on giant pandas in zoos, where mothers are fed rich diets, with fruits, vegetables, meat, vitamins, and minerals supplementing their daily ration of bamboo; growth of cubs in the wild is likely to be slower still.

     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. This is important because no breeding season is "wasted" through clumsy parenting - something a female panda cannot afford. 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. Almost simultaneously, a giant panda female named Qing-Qing in the Chengdu Zoo gave birth to twins, and, against all predictions, reared them both, although with some human help.

      When the twins were about a month old, one was found to be gaining less weight than its sib. So zoo staff began a routine of each day removing one cub from Qing-Qing so it could be hand-fed for 24 hours. Cubs were alternated so each received extra nourishment every other day. Clearly, this procedure could be used only with a very calm, nearly tame female, but it was successful. By January 1991, when I had the pleasure of seeing these twins, they were about five-and-one-half months old, weighed 21 pounds each, and were still thriving.

     The second myth was that a giant panda cub could not be human-reared from birth. At the Heptaoping Breeding Center in Wolong, a twin was human-reared, thanks to the combined efforts of staff from Wolong, the Beijing Zoo, Chengdu Zoo, and Susan Mainka, a veterinarian who recently left the Calgary Zoo to over-see Wolong research programs.

      During the past two decades, giant panda conservation has received considerable support from the international community. Nevertheless, the wild giant panda population is steadily declining, and recent successes with captive breeding have not made up for the losses, either in the wild or in zoos. Giant pandas are still being poached, timber is still being extracted from the best habitats, and the local people are still degrading what remains. A conservation action plan developed by the World Wildlife Fund and the Chinese Ministry of Forestry was completed in 1988 but has to be approved by the central government.