Once successful mating or insemination has occurred, the giant panda will exhibit what is known as delayed implantation. Once the egg has been fertilized, it does not immediately implant on the mother’s uterine wall, but will instead float in her reproductive tract for varying lengths of time. The delayed implantation results in a widely varying gestation period of anywhere between 3 to 6 months. It is believed that delayed implantation gives the giant panda more control over when cubs are born, with implantation taking place when the mother’s nutrition quality is optimal.
Young pandas are typically born in the late summer or early fall (between August and October) in a secluded birth den, usually a tree hollow or a natural rock crevice or cave that the female has lined with bamboo twigs and grasses. The giant panda’s poor, low-energy bamboo diet prevents mothers from devoting much energy to gestation or lactation, Giant pandas are the smallest newborn of any non-marsupial mammal -- 1/900th the size of the mother -- and grow very slowly.
A newborn giant panda weighs an average of four ounces (the size of a stick of butter) at birth, and are pinkish in color with very short, white, sparse hair covering the entire body. Sixty percent of all giant panda births result in twins. The female, however, will deliberately reject or ignore one of the two cubs, which in the wild ensures that at least one cub will survive. In captivity, Chinese breeding centers have become proficient at swapping the cubs back and forth and bottle-feeding the cub that is not nursing, which has allowed numerous sets of twins to survive and thrive.
The giant panda mother holds the tiny newborn constantly in the crook of her arm, or, when moving, carries it in her mouth. The cub vocalizes with a loud, squeaky voice during this time and nurses up to 14 times a day. The female will fast from nine to 25 days after the birth, which may be necessary since the cub cannot stay warm on its own and must suckle frequently. During this time the female is actively engaged in facilitating the cub’s suckling, temperature regulation, grooming, and bowel and bladder stimulation, since the cub cannot eliminate waste on its own. Now that’s a devoted mother!
After eight to ten days, the cub’s skin begins to turn gray in areas where black hair will eventually grow, and in 23 to 35 days, the cub will display its adult black-and-white coloration. The cub’s eyes will begin opening between 40 and 60 days, and by about 72 days, will be on their way to opening fully. At 75 days, the cub’s canine teeth will start to appear, with incisors appearing at 90 days. Cubs begin walking at three to four months.
After approximately 112 days, the mother abandons the birthing dens and places the cub in dense patches of bamboo while the mother feeds nearby. The mother may leave her cub for up to fifty hours to feed herself; older cubs often climb trees and remain there until their mothers return. Within a year, the cub will gain up to 90 pounds (40.8 kg). The cubs may start eating bamboo at five to six months, but typically do not eat much bamboo until after they are one year old or more.
The female giant panda is involved in full-time parenting for at least eight months, and sometimes for up to two years until she is occupied with her next offspring. A female can potentially raise seven to eight cubs during her lifetime. Typically, cubs are fully weaned at one and a half to two years of age (in the spring, when the nutritional content of bamboo is at its highest). Chinese breeding centers, however, wean cubs at six months of age, permitting the females to experience estrus every year instead of every other year like pandas in the wild or in U.S. zoos. The National Zoo’s Tai Shan, who nursed for 19 months, is the longest-nursing cub in captivity. Cubs will go off on their own to lead independent lives between ages one and a half and three.