A Primer on the Zoo’s Possibly Pregnant Giant Panda

Science
پنجشنبه 7 تیر 1397 ساعت 16:44
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Smith sonian mag
The announcement was explicitly tentative. For a few days, Mei Xiang had been nesting near her bed, turning away food, spending more time asleep and reacting to loud noises—behaviors typical of an early pregnancy. In response, her caretakers demanded bedrest and began turning the public away: the expecting mother, after all, would need some peace and quiet.
Mei Xiang is, of course, the Smithsonian National Zoo ’s resident female giant panda. On Sunday, June 24, the Zoo publicized her possible pregnancy and closed the Panda House to visitors in anticipation of a potential birth.
Panda pregnancies are notoriously uncertain, however, meaning that even the Zoo's experts don’t know if she’s truly with cub—and may not until a cub is born. In the meantime, they’re performing routine tests and looking for additional behavioral cues to clue them in. Here’s what we do know: On March 1, Mei Xiang was successfully artificially inseminated with the semen of her mate Tian Tian, which puts her smack dab in the window of a potential birth this summer.
But Mei Xiang has led us down this path before. Sometimes residents of Washington, D.C., where the famous panda pair reside, wait in vain, spending days or weeks refreshing the Zoo’s infamous Panda Cams , hoping to be among the fortunate few to witness Mei give birth on camera… only to receive word that she had experienced a disappointing false pregnancy.
If you’re one of the thousands of hopefuls refreshing the Zoo’s camera pages, take a break with this refresher course on the history, biology and mythology of the Smithsonian pandas.
Mei Xiang and Tian Tian’s predecessors, pandering to politics
On his historic trip to Beijing in 1972, President Richard M. Nixon re-established connections between the United States and China, opening up diplomatic communications between two of the world’s most powerful countries for the first time in 25 years. But one of the most memorable—and certainly most publicly visible—outcomes of his meeting with Chairman Mao Zedong was the gifting of pandas Ling-Ling and Hsing-Hsing to the National Zoo.
The male giant panda Hsing-Hsing and female Ling-Ling arrived at the National Zoo on April 16, 1972. (Institutional History Division, Smithsonian Institution Archives)
Ling-Ling and Hsing-Hsing successfully mated several times over the course of their 20-year tenure in the U.S., but none of their five cubs survived infancy. After both Ling-Ling and Hsing-Hsing died in the early 1990s, the nation’s capital was suddenly without its most charismatic megafauna. In 2000, China offered a new pair—this time on a 10-year loan—and Mei Xiang and Tian Tian made a trip halfway around the world to capture the hearts of a new generation of Americans.
In exchange for the opportunity to host Mei Xiang and Tian Tian, the U.S. agreed that any cubs born of the union would return to China at age four , long after weaning from their mother. The terms of the agreement have since been renegotiated several times; Mei Xiang and Tian Tian are expected to remain on U.S. soil until 2020. But two of their cubs, Tai Shan and Bao Bao, have already left the Zoo via FedEx cargo planes , and will soon be followed by their younger brother Bei Bei in 2019. They now reside at the China Conservation and Research Center for the Giant Panda in Sichuan Province.
Desperate times have called for desperate measures
Strangely, some pandas don’t seem terribly aroused by the idea of mating in captivity with pre-selected mates.
But reproductive breeding researchers have made heroic attempts to circumvent the pitfalls of incompatibility. While most rumors of panda porn and panda Viagra are false (at least, within U.S. borders), scientists now have better solutions like playing recordings of sexy vocalizations or wafting in the scents of pandas in estrus. Meanwhile, in China, some breeders have made attempts at early sex ed for cubs: bringing them into adult enclosures during mating sessions.
Even if zoologists do manage to pair up pandas with natural chemistry, the menstrual cycles of these bears makes captive breeding tricky: females only go into estrus for 24 to 72 hours each year , leaving a very narrow opportunity for males to make their move. The window is so small that Mei Xiang’s mate, Tian Tian, has never actually fathered a cub naturally—only through artificial insemination of fresh or frozen sperm.
Giant panda cub Bei Bei celebrated his first birthday at the National Zoo in August 2016. (Smithsonian's National Zoo / flickr)
Giant pandas spend most of the year as solitary creatures, avoiding their mates unless interaction is absolutely necessary. In those cases, encounters often end in violence. But during the March-to-May breeding season, Tian Tian will, like his wild counterparts, pace his enclosure, vocalize to Mei Xiang, and enthusiastically mark the surrounding foliage with the scent of his urine. Males in the wild sometimes accomplish this last feat through “ urine-hopping ,” a delightfully spirited dance that involves hopping on one foot as they pee to signal their readiness to mate.
“Tian Tian tries really hard,” zoologist David Wildt told Smithsonian.com in 2013. But the two can never seem to get the positioning quite right. Mei Xiang will often flop onto her belly when it comes time to mate instead of firmly planting herself on all fours, the stance that would best facilitate copulation. And Tian Tian isn’t much help; even with attempts to enhance his strength training regimen, he has so far failed to pull Mei Xiang into a more amenable position . The pair are like fumbling virgins in a first erotic encounter. Though to be fair, due to the ephemeral nature of panda estrus, they’ve had only had so many opportunities to acquire sexual prowess.
At the same time, this isn’t Mei Xiang’s first rodeo
In their 20 or so years at the Zoo, Mei Xiang and Tian Tian have produced three surviving cubs: male Tai Shan in 2005; female Bao Bao in 2013; and male Bei Bei in 2015. Sadly, three more cubs born to Mei Xiang in 2012 , 2013 , and 2015 (two of which were twin siblings to Bao Bao and Bei Bei) were stillborn or died a few days after birth.
In the wild, giant pandas average five to eight cubs over a lifetime. Within Zoo walls, the “ most fertile couple” title goes to Bai Yun and Gao Gao at the San Diego Zoo, who have together produced five cubs. But as Mei Xiang reaches the end of her reproductive years, the National Zoo is anxious for one last hurrah.
One of the first images of Bao Bao, Mei Xiang and Tian Tian's daughter, who was born in 2013. She now lives at the China Conservation and Research Center for the Giant Panda. (Smithsonian's National Zoo / flickr)
In previous years, Mei Xiang has had at least six pseudopregnancies , exhibiting the habits and hormonal profiles of pregnancy without actually carrying a fetus. Pseudopregnancies are common in giant pandas and difficult to distinguish from the real thing: only ultrasounds can provide definitive evidence. But that works only if zoologists can get an accurate read in the first place. (On the bright side, Mei Xiang has been taught to adeptly apply her own smear of ultrasound gel prior to exams.). When they’re born, panda cubs are 1/900 the size of the mother, making detection in utero quite the challenge. Rogue lumps of partially digested food and feces can block the detection of a fetus. Even if fertilization has indeed occurred, spontaneous abortion, miscarriage and reabsorption of the fetus are not uncommon.
If Mei Xiang is confirmed to be pregnant, it may still be challenging to pinpoint an exact due date. Panda gestation periods can last anywhere from three to six months, due in part to the fact that fertilized eggs will often wander around the womb for a time, taking the scenic route to implantation.
One thing the Zoo’s panda keepers know for certain? If there’s a cub on the way, it will come when it comes. Stay tuned: Mei Xiang and her caretakers may soon have their hands full.
At the Smithsonian's National Zoo , the outdoor panda habitats and viewing areas remain open and the giant pandas Bei Bei and Tian Tian have daily access to that area until 2 p.m. The Zoo says that the best time to see them is between 8 a.m. and 10 a.m.
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