The Lives of Pandas

On a tight energy budget, newborns no bigger than chipmunks grow into roly-poly superstars

The nation’s capital is abuzz with gossip of its new celebrity couple: Who is being invited to the welcome parties? Which reporters get exclusives? Does the duo still spend all night in wrestling bouts that scatter dirt and uproot plants around their new home?

Washington’s new celebrity couple. National Zoological Park

Less than a month old, San Diego Zoo’s Hua Mei has a long way to grow. San Diego Zoo

Schematic view of bones in a panda hand as it grasps. Nature

Mei Xiang and Tian Tian, like the newcomer pair on a different avenue, are settling into a historic federal location in town. However, nobody had to go to court or recount votes to decide that the young female and male pandas on a 10-year loan from China are welcome inside the beltway.

Their public debut Jan. 10 at the National Zoo boosted the U.S. contingent of Ailuropoda melanaleuca to seven—with two in Atlanta and three in San Diego. As admiring fans crowd the space in front of the exhibits, researchers hover behind the scenes, dipping syringes into urine puddles, charting hormone levels, recording every move and bleat, and categorizing play bouts.

Despite their artificial conditions, zoos offer scientists an irresistible chance to study an elusive, solitary species up close. Observations possible only in captivity are helping to make sense of rare sightings in the field.

Panda research, both in captivity and in the wild, has an urgent edge these days. The remaining tatters of China’s bamboo forests support only about 1,000 wild pandas. Although prospects for captive pandas have turned rosier in the past few years, the zoo population of 100 or so animals worldwide has yet to yield enough new cubs to sustain itself. Science alone can’t save a species, but a dedicated core of researchers argues that perhaps it can help.

A tight energy budget

The picture emerging from up-close research reveals the giant panda as a bear on a tight energy budget. For years, debate smoldered over whether the giant panda belonged to the bear lineage at all.

Raccoons and their relatives might be closer kin, some researchers argued. A third option moves the giant panda into the family of another bamboo eater, the roughly fox-size red panda. However, molecular tests in the mid-1990s convinced many researchers that giant pandas lie closer to bears than to either the red panda or representatives of the raccoon family.

Zoo pregnancies, such as the happy event in San Diego in 1999, offer insight into panda gestation. Females become fertile for only a few days each year. What happens, or doesn’t, during those vital hours has proved one of the most popular romantic dramas of recent years.

Two facilities in China, Chengdu and the captive enclosures in Woolong Preserve, hold the record for the greatest successes in panda pairings. In 1999, for example, eight out of eight reproductive-age females at Woolong mated without artificial aid.

In the Western Hemisphere, the Mexico City Zoo reported the first successful panda breeding in 1980, although the infant died after only 8 days. Washington, D.C.’s National Zoo hasn’t had any better luck so far. All four offspring born to the panda pair previously in residence died in infancy.

Matters may be improving, though. In August 1999, San Diego’s young female gave birth, and the youngster seems to be thriving. Zoo officials had rigged a security camera in the mother’s maternity den, so they were able to follow the infant’s development in detail.

Birth of a panda

Pandas don’t show obvious pregnancies, and it’s not even clear how long gestation typically lasts. After fertilization, the egg drifts around the female’s reproductive tract in a state of suspended animation, not implanting in her uterine wall for months. Once the embryo implants, the female gives birth in what seems to be a matter of only a few months.

A typical 200-pound mother produces a youngster weighing barely a quarter of a pound. That’s less than one-fortieth the size predicted by the general pattern of mammalian mother and baby weights, points out Don Lindburg, head of the giant panda research team in San Diego. Other bears tend toward the skimpy side in baby weights, Lindburg says, yet “pandas are the furthest off the norm.”

Newborn kittens, puppies, and most other mammals have to find their own way to that all-important nipple. However, both in zoos and in the wild, a mother panda scoops up the newborn and cradles it against her body so it can snuggle into her warm fur and feed. In the way she holds the infant, “she’s almost apelike,” Lindburg marvels.

The new panda mother fasts at first, devoting all her time to the infant. “At birth, maternal investment shoots up,” Lindburg says. The panda mother at the San Diego Zoo stayed in her den for days as Lindburg and other zoo personnel watched grainy images on a security camera. “Everyone was getting nervous,” he recalls.

The staff had planned to avoid any intrusions into the den. As the mother’s fast dragged on, however, the team began debating an emergency mission to move provisions within easy reach. On day 5, however, the mother darted out of the den for a drink of water, returning in less than 2 minutes. On day 9, she began eating again.

The panda’s short pregnancy amounts to no more than “a trifling cost” for the female, Lindburg says. It’s after birth that motherhood strains her endurance and resources.

Lindburg presents information on the fate of panda twins to illustrate this distinction. So far, pandas have given birth in captivity on 133 occasions, producing twins roughly half the time. “The mother routinely selects one, and the other dies in a few days,” Lindburg says.

He doesn’t attribute the high twinning rate to some quirk of captivity. Instead, he argues that only in captivity could people routinely discover the phenomenon. In the wild, Wenshi Pan and his team at Woolong have on a few occasions found a dead infant, presumably a discarded twin, near a mother with a live infant.

Bears other than pandas often give birth to multiple offspring, and several scientists including Lindburg speculate that there hasn’t been much evolutionary pressure for pandas, with their tiny fetuses, to counter the tendency. Hence, he suggests, the mother makes the cut right after birth because she can’t care for more than one of the fragile infants.

Lindburg compares the panda’s postpartum fast to the months of hibernation of other bear species. Black and brown bears give birth while hibernating. With their varied diet, they pack on enough fat before hibernation to get them through the weeks without feeding and to bear young more developed than panda newborns. The panda mother, in contrast, doesn’t gain noticeable weight before or during pregnancy.

A bamboo diet

The panda’s bamboo diet makes it difficult to build up a store of fat. This specialization on bamboo may drive the middle phase of panda motherhood, when she leaves the baby alone in the den for hours at a time while she feeds.

Various species of bamboo make up at least 98 percent of the panda’s diet in the wild. A panda’s digestive tract, a modified version of a carnivore’s basic equipment, extracts only about 17 percent of the nutrients in those leaves and stalks. So, just to keep the basic machinery of life going, pandas need to feed most of the day.

Providing milk for a baby adds to that demand, and unlike most mammals, pandas nurse their offspring for more than a year. Between days 40 and 50 in the wild, when the baby is just beginning to open its eyes, its mother spends about half her time crunching bamboo away from her den. Zoo mothers with easier access to food spend more time with their infants.

Baby pandas, starting out so tiny, need a long time to grow. They’re almost 5 months old before they can stand and take their first wobbly steps.

Their forelegs strengthen before their hind legs. Lindburg remembers worrying unnecessarily that the San Diego Zoo’s youngster had weak hind legs from some developmental deficit. However, the baby’s hind limbs eventually caught up with her forelegs, and she’s grown into a normal climbing, chasing, wrestling 1-year-old.

In the wild, mothers hide their infants in shrubbery before going off to feed. When the baby’s hind legs finally build up the strength for climbing, the mother sends it into a lofty hiding place while she’s away. Lindburg reports that the most common place to spot infant pandas in the wild is high in trees or bamboo.

After roughly a year and a half of nursing, the youngster begins feeding on bamboo. Then, it can trot along with Mom when she goes off to feed.

Zoos typically have separated mothers and infants before this milestone, sometimes as early as 4 months after birth, says Rebecca Snyder, a behavioral researcher at Zoo Atlanta. Such a swift weaning improved the chances that the mother would cycle into estrous again the following year, a big deal in the iffy endeavor of breeding pandas.

Now, Snyder’s studying the effects of early separation on the youngsters. Among the young pandas she’s following, Atlanta’s two plus a dozen at Chengdu, Snyder so far finds slight differences between the early and late weaners. Those separated early seem a bit more likely to show worrisome behaviors such as pacing their enclosures or sucking on their paw or a buddy’s ear. Lin Lin, the young female in Atlanta, for example, occasionally sucks the fur on her chest.

Also, those who stayed with their mothers longer spend more time playing, according to preliminary results, Snyder says. One of the surprises of her work: Young pandas raised just with each other spend less time playing than a lone infant and its mother.

Panda mothers don’t just indulge a little one that wants to play, Snyder says. Unlike a mother in most other species that Snyder has considered, a female panda often starts the game herself. “She’ll go over to her infant and wake it up to start to play,” Snyder says. The two will gently nip and gnaw on each other, wrestle, swat each other with their paws, and play chase.

A solitary life

When a panda cub outgrows its mother’s company, it takes up a solitary life, wandering in search of bamboo. The scent equivalent of a community bulletin board announces such matters as who’s interested in company. Much like dogs, both male and female pandas leave informative scents with spurts of urine or smears from a gland under the base of the tail.

Unlike dogs, though, panda males also do some of their scent-marking upside down. With considerable agility for such a pudgy, short-legged animal, a male backs up to a tree or other inviting vertical surface, puts his front paws on the ground, and works his hind paws upward. When he’s achieved a creditable handstand, he rubs his scent gland on the surface or releases a little urine. Such acrobatics, Lindburg explains, enable a male to show just how high he can mark, an accomplishment that may affect his social standing.

Lundburg, his zoo colleague Ron Swaisgood, and their coinvestigators are working with 20 captive pandas at Woolong. The researchers offered them fir boards scented by other pandas, either from urine or from the gland. Pandas sniffing the boards distinguished unfamiliar and familiar individuals, the researchers have reported. Pandas spent roughly 15 minutes investigating a board with the same scent as one previously in their enclosure. A board scented by a different animal received about 45 minutes of attention.

In a second test, the researchers let the pandas sniff around another animal’s empty enclosure. Male pandas’ reactions suggest they can pick out females of particular interest, the team reports in the August 2000 Animal Behaviour. Males made more noise when checking out the enclosure of a female in estrus compared with the home of a female not at a fertile stage.

Many types of animals are thought to be able to make such a distinction, but Swaisgood and his colleagues note rather tartly that “relatively few species have been studied.” Among other carnivores, they say, only cats and dogs have been tested.

This result suggests that zoos may not need Viagra if animals get a chance to sniff interesting urine. Swaisgood points out that the San Diego Wild Animal Park’s efforts to breed cheetahs floundered until zoo managers arranged for males and females to check each other’s scents before close encounters. He and his colleagues partly attribute the low reproductive success of a variety of captive animals to insufficient opportunities for chemical communication.

Washington’s new residents still have a few years to go before they communicate much more than “Tag, you’re it.” Despite the pair’s youth, however, locals are already speculating on family prospects, and The Washington Post has published an analysis of panda sex. This is one cause, at least, that’s enjoying full bipartisan support.

Inside story on the panda’s thumb

The panda’s grasp is “one of the most extraordinary manipulation systems in mammalian evolution,” according to Hideki Endo of the National Science Museum in Tokyo and his colleagues. They’ve examined the panda’s opposable digit with magnetic resonance imaging and other advanced medical-imaging techniques.

Pandas handle their food with considerable dexterity, in part because one of their paw bones has evolved a padded spur that works like a thumb. Their five regular, clawed digits can pinch an object against the spur, as a person’s four fingers press an object against the thumb. But while a human thumb is, in essence, a finger jutting out at an odd angle, evolution has jerry-rigged the panda’s spur from bones unrelated to our thumbs. As Stephen Jay Gould described it in The Panda’s Thumb (1992, W.W. Norton and Co.), the panda’s so-called pseudo-thumb represents an outgrowth of the sesamoid, analogous to one of the bones below the fingers in our hands.

The pseudo-thumb can’t move by itself, Endo’s team learned more recently. Yet when the other bones in the hand curl into a grasping position, they reposition the pseudo-thumb. In its new position, it pokes out at just the right place and angle to provide a brace for a pincer grip.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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