SCIENCE -- September 27, 2012 at 5:22 PM ET
Why Pandas Have Trouble Getting Pregnant
On Sept. 16, after five years of trying, a female giant panda at Smithsonian's National Zoo finally gave birth to her second cub. Zookeepers were overjoyed; director Dennis Kelly called it "a great day for pandas."
But a week later, the cub died unexpectedly. It was a heartbreaking story. Mei Xiang, the mother, yelped in distress, alerting keepers that something had gone terribly wrong. That night, after the cub was pronounced dead, Mei Xiang clung to a Kong toy as if holding a newborn.
But the loss was also disheartening due to the exhaustive, five-year effort to impregnate the female panda since her first cub, Tai Shan, was born in 2005. Panda breeding is notoriously tricky. The animals have a scant 36-hour window every year to get pregnant, and the process is fraught with complications.
On Wednesday, we spoke with Pierre Comizzoli, a reproductive physiologist with the Smithsonian Conservation Biology Institute and Mei Xiang's "personal gynecologist," about the challenges of breeding pandas in captivity.
There are about 300 giant pandas in captivity and fewer than 1,600 in the wild, according to the International Union for the Conservation of Nature. While the biggest threat to pandas is habitat loss, their low reproductive rate puts the pressure on zoos and breeding centers to ensure a viable captive population.
Slide show by Ellen Rolfes.
The National Zoo's first pandas, Ling Ling and Hsing Hsing, tried mating on their own for ten years with no success. They eventually had five cubs, but they all died before reaching adulthood.
With such a short fertile window, timing conception has to be exact for a pregnancy to occur. Veterinarians monitor the panda's urine year round, watching closely for a peak in estrogen levels. Once that peak is detected, they open a gate between the male and female cages.
Natural mating must overcome a number of obstacles. The chemistry must be there and the male panda must have the libido and the technique to accomplish the feat.
"If they aren't interested or the positioning is incorrect, they only get one opportunity a year to practice," she said.
Megan Owen, conservation program manager for San Diego Zoo Global, said their first male panda, Shi Shi, had no interest in female Bai Yun, no matter how hard the panda tried to solicit him. But their next male, Gao Gao, had more success with the female; they had their fifth cub together this July.
Often, zoos revert to artificial insemination to increase the bears' chances for conception. And then they wait for months to find out if the bear is pregnant.
Detecting pregnancy is also tricky, since females produce the pregnancy hormone progesterone whether or not they are pregnant. And even with an ultrasound, it's tough to spot the tiny eraser-sized fetus hidden beneath layers of fat, bladder and a stomach full of bamboo in the 230-pound mother. Pregnancies can last anywhere from three to five months because the embryo may not embed in the uterine lining immediately after conception.
And it's not uncommon for fetuses to get reabsorbed -- that happened to Memphis Zoo panda Ya Ya three times. Matt Thompson, director of animal programs at the Memphis Zoo, said the veterinarians even saw a heartbeat on the ultrasound once, but Ya Ya never gave birth.
Still, each attempt advances the body of research on these bears. Comizzoli said the zoo's latest experience taught them, at the very least, that even after years of failed pregnancies, it is still possible for a female to conceive. Since 1996, the number of giant pandas in captivity has grown from 126 to 330, which is a significant improvement, he said. Learning more about what causes pregnancy failures and early death will help veterinarians who care for these animals, and it may extend to other species also struggling to breed in captivity.
"So to some extent the giant panda could be a good model for pregnancy monitoring in other species," he said.