Baby boom: with scientists' help, pandas are now raising healthy cubs in captivity. These baby bears could keep the species from disappearing

Science World, Jan 24, 2005 by Britt Norlander

Last year, a California celebrity left her sunny home for a new address. The star: a popular panda named Hua Mei (WAH MAY). The bear first grabbed headlines when she was born at the San Diego Zoo in 1999. She was the first Giant Panda, Ailuropoda melanoleuca (AL-yer-uh-POH-dah mel-AN-uh-LOO-kah), in the United States to survive after being born in captivity.

Hua Mei stayed at the zoo until she was old enough to have cubs. Then, last February, she moved to China's Wolong (WOO-long) Giant Panda Breeding and Research Center. There, as part of an international project to breed pandas, scientists introduced Hua Mei to potential mates. And last September, she made headlines again--this time after delivering two cubs. "We still think of her as our baby; now she has twins of her own," says Barbara Durrant, the head reproductive physiologist (scientist who studies reproduction) at the San Diego Zoo's Department of Conservation and Research for Endangered Species.

Hoping to save pandas from extinction (no organisms of the species remain), biologists around the world are working together to try to breed the bears in captivity. As part of the project, China--panda's native land--lends pandas (like Hua Mei's parents) to foreign zoos with the hope that the bears will have cubs. Then, the offspring leave mom and dad behind and move to Chinese breeding centers to start a new family.

"BEARLY" HANGING ON

With fewer than 1,600 wild pandas remaining, scientists think captive-born bears may help rescue wild pandas one day. The bears once roamed over most of China and into other countries to the south. But humans have taken over most of their forest home (see map, right). "Pandas have been pushed to the edges of China that humans do not want to live in," says Colby Loucks, a conservation scientist at the World Wildlife Fund.

Squeezed into small, isolated patches of steep forest, the bears' lives are at risk. For instance, bamboo makes up about 99 percent of a panda bear's diet. In the 1980s, large patches of China's bamboo died off. Surrounded by humans, over 200 wild pandas were unable to move to areas where bamboo still grew and the bears starved to death.

To prevent another disaster, scientists are working to protect panda habitat (region where an animal lives). Then, they may be able to boost the panda population one day by releasing captive-born bears into the wild.

PERFECT TIMING

For now, scientists are trying to learn how to breed pandas in captivity. Their first step with Hua Mei: setting her up on several blind dates. Captive male pandas are very picky about their female partners. The scientists needed to find Hua Mei's perfect match.

The next step--choosing the moment to introduce the bears to each other--was even trickier. That's because a female panda's estrus, the time during which she can become pregnant, occurs only once a year for one to four days.

During this time, the female produces an egg (female sex cell). The egg then needs to be fertilized, or joined, with a sperm (male sex cell). Unfertilized eggs don't survive long, so fertilization needs to happen quickly. "[There is] a very, very small window--about 24 hours [in which fertilization can occur]," says Durrant.

In the wild, solitary female pandas call out to alert nearby males that the time is right. In captivity, scientists need to introduce pandas--normally kept separated--to each other during this period. If fertilization occurs, a single cell called a zygote forms from the joined egg and sperm. Then, a cycle of cell divisions begins to form a panda fetus (unborn young) and eventually a cub (see Nuts & Bolts, p. 11).

CARE BEARS

It turned out that everything was timed right for Hua Mei, and she gave birth to twins--each only about the size of a stick of butter. "The size ratio of a panda mother to her cub is the largest of any bear and one of the largest of any animal," says Durrant. Their extra-small size means the cubs need a lot of care. Like many new morns, Hua Mei initially seemed unsure of her motherly role.

Soon, however, she settled into motherhood. Since all panda cubs are born blind and hairless, moms gently pick them up in their mouths. They keep the tiny cubs cozy against their fur. A panda mother usually won't leave her baby--even to eat or drink--until the cub is about 10 days old. She stays with her cub, teaching it how to survive, until it is about 18 months old. Then the cub leaves to live a mostly solitary life. In captivity, panda cubs are slowly weaned, or removed from their mother's care, after roughly one and a half years.

DOUBLE TROUBLE

It's tough to care for even one growing bear, but--like Hua Mei--pandas give birth to twins about 50 percent of the time. Because the cubs need so much attention, a mother in the wild usually neglects one of her twins. "The cubs are so small and require so much care, she just can't hold [or feed] two of them all the time," says Durrant. Unfed, the neglected cub dies.

Even captive pandas follow these instincts. So in the past, when twins were born in captivity, veterinarians would remove one of the cubs and hand-raise it: They fed it from a bottle and cared for it without any help from morn. But until recently, scientists had never successfully hand-raised a cub to adulthood.