Learning from Tai Shan- page 2 | Science | Smithsonian

Learning from Tai Shan

The giant panda born at Washington, D.C.'s National Zoo has charmed animal lovers. Now he's teaching scientists more than they had expected

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Pandas give birth to twins about half the time. This itself is not unusual—most bears have twins or triplets—but a panda mother usually selects one of her two cubs to raise and lets the other die. Biologists once believed that such an apparently illogical act occurred only in captivity. But in fieldwork conducted at Wolong Nature Reserve in the late 1980s, biologist Pan Wenshi often found a dead cub near a mother that was caring for a healthy one. Scientists speculate that new panda mothers just can’t afford to feed two cubs—another behavior that may be an evolutionary adaptation to the animals’ low-energy diet.

Tai Shan’s story begins in 2000, when his mother, Mei Xiang (may-SHONG), and father, Tian Tian (t-YEN t-YEN), came to the National Zoo on a ten-year loan from China made possible through major sponsorships by Fujifilm and Animal Planet and other donors. (Three other U.S. zoos—in San Diego, Atlanta and Memphis—also host pandas; in exchange, each institution must contribute at least $1 million per year to panda conservation in China.) In 2003, as the bears were just reaching sexual maturity at ages 5 and 6, they mated for the first time, but no pregnancy followed. The next year, after the pandas tried unsuccessfully to mate on their own, Zoo scientists inseminated Mei Xiang with Tian Tian’s sperm, again without conception.

Last spring, as Mei Xiang began to show signs that she was coming into estrus, the scientists prepared to place Tian Tian’s sperm directly into her uterus. Because the procedure would require Mei Xiang to undergo general anesthesia—which always carries a risk—they decided to try the insemination just once, a considerable gamble given how briefly a female is fertile. “In China, we learned just how narrow the window of opportunity is,” says JoGayle Howard, who performed the procedure.

Recent studies have fine-tuned techniques to predict that 24- to 48-hour period. One is to analyze vaginal cells. In exchange for a reward—a biscuit, apple or pear—Mei Xiang has been trained to enter her 5- by 4-foot training cage and submit to all manner of pokes and probes. Zoo technicians examine cells swabbed from her vagina to tell how close to ovulation she is. They also siphon her urine off the enclosure floor. National Zoo endocrinologists Steve Monfort and David Kersey analyze these samples for changes in estrogen levels.

The big day turned out to be March 10, 2005. Earlier that week, Mei Xiang had increased her frequency of “scent marking,” or rubbing a gland near her tail against the ground to deposit a sticky substance with an odor detectable by other pandas. Zoo scientists were monitoring her urine and vaginal cells round-the-clock. When tests showed Mei Xiang was ovulating, they first gave Tian Tian an opportunity to do the job himself. But after 24 hours—during which he “hadn’t achieved the proper alignment,” says assistant curator Lisa Stevens—the scientists took over.

To get Tian Tian’s sperm, the researchers anesthetized him and used an animal breeding technique called electroejaculation, in which a probe inserted into the male’s rectum produces electrical stimulations that cause ejaculation. For the insemination, Howard used a modified laparoscope (a tiny telescope with a fiber optic light often used in human medicine) to guide a catheter through Mei Xiang’s cervix and into her uterus. “We felt the timing was right on,” says Howard. “The procedure couldn’t have gone faster or more smoothly, and that’s what made me nervous.”

Zoo scientists wouldn’t know whether they had succeeded for nearly four months: giant pandas frequently go through “pseudopregnancies” in which non-pregnant females exhibit behavioral and hormonal changes similar to those of pregnant females. (And given the tiny size of a panda fetus, the pregnancy doesn’t show.) “I didn’t relax until that cub was on the ground,” says Howard. That was at 3:41 a.m. on July 9, 2005. Still, Howard wasn’t ready to uncork the champagne. In addition to routinely rejecting a twin, panda moms have been known to ignore single cubs. Says Howard: “They either act like they’re scared of it or look like they’re thinking, ‘I’m not taking care of that thing,’ and walk away.”

But just two minutes after giving birth, Mei Xiang gently picked up tiny Tai Shan and began to cradle and cuddle him. For the following week, she refused to leave their “den”—a darkened 10- by 12-foot room—even to eat or drink. On the 7th day, she left him (for three minutes) to get a drink of water; she did not eat until day 17. “From the beginning, Mei Xiang couldn’t have been a more perfect mother,” says Howard. “And Tai Shan couldn’t be a healthier cub.”

The most important reason for keeping pandas in captivity—beyond public education, research and fundraising—is to prevent their extinction in the wild. Captive populations of endangered animals are insurance, should the species vanish in its native habitat, and a potential source of animals for reintroductions into the wild. But captive populations are prone to inbreeding, a major threat to their survival. U.S. and Chinese scientists now meet before each spring’s breeding season to recommend the best panda pairings to ensure a diverse mix of genetic backgrounds, and most breeding centers move animals or their sperm from one institution to another as needed. Giant pandas have an advantage over other endangered species, such as the black-footed ferret and the California condor, whose numbers fell so low that inbreeding could not be avoided. “We know we’re growing a genetically healthy population of pandas,” says Wildt. According to National Zoo population manager Jonathan Ballou, the next step is to increase the number of captive pandas until the population is self-sustaining. He calculates that the magic number is 297 pandas; today there are 185, an all-time high.

Most scientists say it is not time yet to return captive pandas to the wild. Reintroduction is risky to the captive-born animals and potentially to any wild pandas they might fight with or infect with diseases. And what’s left of the panda’s habitat is not yet secure.

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