There may be only two thousand pandas in the world, of which a sixth are in captivity. Illustration by Martin Ansin

A female giant panda unambiguously signals her approaching receptivity to mating. She wanders over a wide territory and scent-marks stones, the ground, and other surfaces with a waxy, hormone-rich secretion from a gland under her tail. She walks through water, to spread her scent farther. Her main vocalization changes from a throaty whinny to a high-pitched chirp, which one zookeeper translated for me as “I’m here! I’m here! My time is coming!” She masturbates, and when she encounters an adult male at the critical moment she lumbers toward him, rear end first, and lifts her tail.

Still, things don’t always work out. David Wildt, the head of the Center for Species Survival at the National Zoo, in Washington, D.C., told me, “Some pandas know how to have sex, and some don’t.” The pair at the National Zoo—Tian Tian (male) and Mei Xiang (female)—don’t. They have been together at the zoo since 2000, but until last week they had produced just two cubs, both by artificial insemination, and one of these had died. At the end of last week, Mei Xiang gave birth for a third time, live on one of the zoo’s panda Webcams.

The latest arrival is also the product of artificial insemination. As Wildt described it to me, Tian Tian and Mei Xiang are simply “reproductively incompetent.” A key difficulty is that Mei Xiang places herself in what he called “pancake position”—flat on her stomach, legs outstretched—and Tian Tian isn’t assertive enough to lift her off the ground. Rather than mounting from behind or pulling her toward his lap, he steps onto her back and stands there like a man who has just opened a large box from Ikea and has no idea what to do next.

Female pandas are receptive only once a year, and sometimes are fertile for less than a day—an unusually narrow breeding window. As spring approaches, scientists at the National Zoo monitor Mei Xiang’s behavior and hormone levels, and a Chinese consultant prepares to fly to Washington on short notice, to assist. In China and elsewhere, panda handlers have encouraged unenthusiastic pairs by showing them “panda porn”—footage of other pairs having sex—and giving the males Viagra. Some handlers have claimed success with the videos, but pandas in the wild are solitary creatures, with limited opportunities to observe the behavior of other adults, and many scientists doubt that they engage in social learning.

The National Zoo’s approach has been more mechanistic. Last year, at the suggestion of the Chinese consultant, carpenters built a low wooden platform in the enclosure that had been Mei Xiang’s preferred mating location. “We were thinking that if she would pancake at a higher level Tian Tian might be able to hit the target,” Brandie Smith, the zoo’s panda curator, told me. It didn’t work. This year, two zookeepers placed a plastic cylinder, about a foot in diameter, across the threshold of the enclosure. As Mei Xiang led Tian Tian inside, she fell over the cylinder, and her rump rose into the air—exactly as the zookeepers had hoped. But then Tian Tian lifted her off the cylinder, placed her on the ground, and stepped onto her back.

The reproductive travails of giant pandas are ecologically significant, because the species today may consist of as few as two thousand animals, of which about a sixth are in captivity. (The rest are in wildlife reserves in a mountainous part of central China.) Captive breeding is a way of buying time while the Chinese sort out their feelings about a number of environmental issues, including habitat destruction. Most of the captive animals live in two large research centers, in Wolong and Shandong, but roughly three dozen are in zoos in other countries, and their mating failures and successes have inspired intense public interest. During breeding season in Washington, volunteer Behavior Watchers, fortified by caffeine, sit through the night in a glass-fronted booth, studying video monitors and taking notes on clipboards at four-minute intervals. They fill out “pee maps,” to show keepers where to find urine samples, for hormone analysis. Many more people watch unofficially, from home, by way of the zoo’s Webcams, and, occasionally, offer theories of their own. “One of the things we often hear is that Tian Tian and Mei Xiang don’t mate because they think they’re brother and sister,” Smith told me. “But that’s definitely not the case. We can see very clearly that they know they’re supposed to be doing something that is not brotherly or sisterly.” And yet.

Among the first Americans to see a giant panda were the two eldest sons of Teddy Roosevelt, Theodore, Jr., and Kermit, who encountered one on a scientific expedition to China in 1928. Not surprisingly, given the period and their parentage, they shot it, along with some eight thousand other creatures. (It’s now stuffed, in a diorama at the Field Museum, in Chicago.) In 1936, Ruth Harkness, a recently widowed American socialite, trekked fifteen hundred miles across China and returned with a nine-week-old male, which she named Su-Lin. (She thought he was a girl.) She fed Su-Lin baby formula during the voyage home, and sold him to the Brookfield Zoo, in suburban Chicago. Su-Lin attracted fifty-three thousand visitors during his first day on display. Shirley Temple and Eleanor Roosevelt came to see him, and Time named him Animal of the Year. He died, in 1938, of a throat infection, contracted after swallowing an oak twig, but by then the zoo had acquired a second panda (also from Harkness), and it soon acquired a third. Su-Lin was stuffed and given his own diorama at the Field Museum.

The modern American panda era began in 1972, when China gave the United States a wild-caught pair, Ling-Ling and Hsing-Hsing, to commemorate Richard Nixon’s historic visit. (The United States reciprocated with musk oxen.) They were housed at the National Zoo and eventually produced five cubs, although none lived longer than a few days. Ling-Ling died, of heart disease, in 1992, and Hsing-Hsing was euthanized in 1999. It has been estimated that seventy-five million visitors saw at least one of them. The zoo acquired its current pair a little over a year after Hsing-Hsing’s death.

Long before then, China had stopped giving pandas away. Mei Xiang and Tian Tian, like the forty or so other pandas in non-Chinese zoos around the world, are, in effect, rentals, since China retains ownership and can claim any offspring. (Tai Shan, the surviving adult cub of Mei Xiang and Tian Tian, was shipped to a breeding center in Sichuan in 2010, when he was four and a half years old.) The National Zoo initially paid China a million dollars a year for its pair; the current agreement is for half that amount. Three other American zoos—in San Diego, Memphis, and Atlanta—also have pandas, as do zoos in Australia, Austria, Canada, France, Japan, Mexico, Singapore, Spain, Taiwan, Thailand, and the United Kingdom.

For China, pandas rapidly became a potent global brand. By the nineteen-nineties, though, captive animals there were suffering a variety of serious ailments, including a high level of cub mortality, and few adults were reproducing, so that the Chinese were able to maintain their numbers only by trapping new specimens. In 1996, seeking help, they convened an international conference, in Chengdu, after which a team of scientists from China and the United States undertook a three-year biomedical survey of more than sixty pandas in China. Some of the main findings concerned diet. Giant pandas are classified taxonomically as carnivores—and they have the claws to back that up—but in the wild they live almost entirely on a small selection of the thousand-odd species of bamboo. They favor different types and different plant parts at different times of the year, and because bamboo is low in nutrients they consume huge quantities—as much as fifty pounds per animal per day. Yet the Chinese had been unable to acquire a steady supply, and had been feeding their animals milk, eggs, and a high-protein, low-fibre paste they called “panda bread.” They had also been careless about hygiene and record-keeping, and hadn’t been vaccinating for canine distemper virus, which makes pandas sick, too. The study led to many changes, as well as to an increased appreciation, among the participating scientists, for international coöperation. The pandas rapidly became healthier, and, among other things, began having more sex.