The giant panda's reproduction biology is one of the reasons the bears are endangered. Female pandas are only fertile a few days a year and baby pandas are incredibly fragile -- a mother panda is 900 times larger than her newborn cub! Since pandas are endangered, there is international interest in their breeding and approximately 40 panda reserves across China.
Unlike most mammals, pandas have a range for their gestation. A panda's gestation typically lasts from 90 to 160 days. The San Diego Zoo's Dong Dong had a record gestation period of 184 days, but the average gestation period for a panda is 135 days. This unusual range is because fertilized panda eggs don't always develop right away.
The range of a panda's gestation is caused by a unique adaptation. Unlike most mammal eggs, which quickly implant in the wall of the uterus after fertilization, a fertilized panda egg can float in the uterus for several months in a kind of stasis, not developing right away. This could be an adaptation allowing the embryo to wait for optimum conditions to develop. Once the egg implants in the wall of the uterus, it develops like any other mammal.
Panda cubs are born very undeveloped compared to most other mammals. At birth a panda weighs only 5 ounces. He cannot move or even open his eyes. It takes six to eight weeks before his eyes open. It takes up to three months before the cub can move on his own. A cub nurses off his mother for up to nine months, and remains more or less dependent on his mother for almost two years.
Part of why pandas are endangered comes down to the way they breed. Pandas take four to eight years to reach maturity. Once they are mature, the panda breeding season is very short. The female are typically only able to conceive for two to three days a year. Also, the females cannot have young while they are tending to their cubs, and can only care for one cub at a time. This means that under the best circumstances, a female panda can have one cub every two years. Their slow reproduction plus habitat loss threaten the future of the giant panda.