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Kleiman, 1992

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http://nationalzoo.si.edu/Publications/ZooGoer/1992/2/giantpandasbamboobears.cfm

Giant Pandas: Bamboo Bears

by Devra G. Kleiman

[1]Ling-Ling and Hsing-Hsing’s arrival in Washington in 1972 marked the official beginning of my long years of studying giant pandas at the National Zoo. (I have to admit to informal and somewhat surreptitious observations of London’s Chi-Chi and Moscow’s An-An during their ill-fated breeding encounters in London in 1968.) Since 1978, I have also visited China several times. But I have always watched giant pandas in zoos or breeding centers, even though one breeding center is actually within the Wolong Natural Reserve - site of one of the longest ongoing studies of giant pandas in the wild.

For all of these years, however, I’ve had another vision. I dream that someday I will be able to study giant pandas in the wild. I imagine following a female giant panda to her maternity den and, over a series of months, watching her raise twin young to maturity. Then I imagine returning years later to find her now-mature offspring successfully breeding, and I stay to watch their progress. The dream unfolds in a belt of temperate conifer forest with an understory of never-ending bamboo along the slopes of the Min Mountains in Sichuan—a giant panda Garden of Eden.

Can my dream be realized? It’s hard to say. [2]Adequate habitat for this species is becoming rarer and rarer in China. Giant pandas are starving, not only for bamboo—their staple food—but also for enough land for long term survival.

Now confined to a few mountain ranges in western Sichuan, giant pandas once ranged widely throughout China. Fossils have been found almost as far north as Beijing, within several hundred miles of China’s east coast, and as far south as Guangzhou, as well as in Burma and Vietnam.

The dramatic shrinking of the giant panda’s distribution is a result of both natural and human-induced changes in the Chinese landscape. During the Pleistocene glaciations that began nearly a million years ago, vast areas of China alternately warmed and cooled. [3]Giant panda habitat likely expanded and contracted with these climatic changes. During the last warming period, perhaps 10,000 to 20,000 years ago giant pandas probably became restricted to the mountain ranges bordering the Tibetan Plateau and a few other high-altitude or cool habitats capable of supporting the bamboo to which they are so superbly adapted. [4]Many of the sites they once occupied are now subtropical in climate and could not possibly sustain giant pandas.

The second, but ultimately more serious change, was the vast increase in human activity that began even in ancient China. An expanding Chinese population decimated low-lying bamboo forests for agricultural purposes, pushing the giant panda up into higher and higher altitudes. Historical records show that the giant panda’s range is now less than 10 percent of what it was 2,000 years ago, and it continues relentlessly to get smaller still. [5]With ever-increasing pressure from the one billion inhabitants of China the likelihood that giant pandas will survive the next 50 years remains iffy at best.

Giant pandas are only the most dramatic-looking of a unique fauna and flora that survive precariously along the edge of the Tibetan Plateau. (See China's Wildlife Wonders) Golden monkeys, takin, red pandas, and an array of colorful pheasants live in the same mountains as the giant pandas. All are now vulnerable to extinction as people continue to destroy the ecological integrity of the Min, Qinling, Liang, and Qionglai Mountains.

These slopes are also home to an amazing radiation of bamboo species, upon which giant pandas depend for 99 percent of their diet. At least two species of bamboos usually grow within good giant panda habitat. The life history of bamboos can dramatically affect giant pandas. These particular bamboos tend to flower, set seed, and die at species-specific intervals of 30 to 80 years. Plants of any particular species do this in synchrony, leading to a massive die-off of that species throughout its range, followed by slow regeneration. (See Bamboo)

Studying the aftermath of recent bamboo flowerings in giant panda habitats, [6]Alan Taylor of Pennsylvania State University learned that bamboo regenerates more rapidly if it receives some cover from the forest canopy. Where the forest has been clear-cut, as it has in much of the giant panda’s potential habitat, bamboos regenerate very slowly. Thus, bamboo regrowth - and in the best of situations it takes 15 to 20 years for bamboos to regain their full height - depends on forest integrity.

During the long regeneration period, giant pandas have traditionally had two options. They could switch to eating a second, less preferred bamboo species, or they could move, usually to an area where the local bamboo had not flowered and died. But now, because their habitat is so fragmented, the migration option is becoming less and less viable for giant pandas faced with severe food shortages. With the lower mountain slopes being used for agriculture, the distances between habitable areas have become too great for giant pandas to traverse. Only secure "corridors" of natural habitat linking larger areas of panda habitat will alleviate this problem.

Diet and Destiny

Giant pandas are a study in the evolutionary process. Their most recent ancestor was a bearlike creature. Like other bears, giant pandas are technically carnivores—members of the order Carnivora. But, in fact, most bears are omnivores - they will eat almost anything, including meat, fruit, seeds and insects. But at one stage in their evolutionary history, giant pandas abandoned omnivory in favor of strict herbivory—eating green vegetation. Indeed, going a step farther than most herbivores, giant pandas even limited themselves to a single sort of grass—bamboo.

Their adaptations for feeding on bamboo make them morphologically unique. [7]Giant pandas possess huge molar teeth and powerful masticatory muscles for breaking the woody portion of bamboo and chewing the stem, culms, and leaves. In addition, their forepaws are equipped with a sixth "digit" that acts as a thumb for holding bamboo. This "thumb" is derived from the radial sesamoid bone of the wrist.

[8]Surprising as it may seem to observers of giant pandas in the Zoo, black-and-white coloring probably acts as camouflage in the wild. In patches of dense bamboo, an immobile giant panda is nearly invisible, and virtually disappears among snow covered rocky outcrops on a mountain slope. The combined effect of foreshortened snout, round face, black eye rings, small rounded black ears, short squat tail, and distinctive body shape and markings gives the giant panda its characteristic infantile appearance, which is further enhanced by its habit of sitting upright, holding objects in its flexible forepaws, and walking pigeon-toed.

For most of the year, individual giant pandas use fairly small home ranges: In good habitat they can find all the bamboo they need without moving very far. Males and females roam over areas varying in size from about one to 10 square miles, with little difference between the sexes. Some females studied by George Schaller in Wolong even occupied home ranges as small as 75 to 100 acres—less than the size of the Zoo. Seasonal as well as long-term changes in the availability of preferred foods, and the location of potential mates, sometimes results in pandas taking very long hikes outside of their normal territory. But more often they migrate vertically, up and down the mountainside, to find a preferred bamboo species or preferred parts of bamboos. Giant pandas normally exist at altitudes of 3,600 to 10,500 feet, depending upon prevailing local conditions; for example, in Wolong Reserve, the giant pandas occupy habitat from 7,550 to 10,500 feet, while in Shaanxi’s Foping Nature Reserve, the giant pandas remain at 3,600 to 9,500 feet.

Panda Society

Studies by George Schaller, Hu Jinchu, and others suggested that the giant panda is essentially solitary in nature. They found that males and females rarely interact except during the breeding season. At this time, males may also meet in the neighborhood of a female in heat, when they show a dominance hierarchy.

But, based on a study of radio-collared giant pandas in the Qinling Mountains, Pan Wenshi and Lu Zhi, of the Beijing University, recently suggested a somewhat different picture of panda social life. [9]They found that seven to 15 individuals may form a social community of pandas within the local population. These individuals occupy a "group" territory, within which male home ranges overlap almost completely while female home ranges overlap far less. Members of different "groups" generally avoid socializing with each other.

These exciting new results suggest that giant pandas are far more social than we previously believed, and also raises important questions about whether mating occurs exclusively between members of these small groups. If so, and if pandas in other mountain ranges have similar social groups, we will need to reevaluate our interpretation of the giant panda’s genetic example, a mating system that is restricted to "group" members would likely result in giant pandas being genetically inbred in local populations. Yet, genetic analysis of blood from a small number of giant pandas, performed by Steven O’Brien of the National Cancer Institute, showed that the species retains considerable genetic diversity. Clearly, this is a question that requires further study, because the answers could have a significant impact on conservation strategies.

The Pan and Lu findings are similar to some behavioral observations made on giant pandas in zoos and reserves. First, males can be maintained in small groups without aggression if the males have been very carefully introduced to one another. Usually the males develop a clear dominance hierarchy. Second, female giant pandas seem to prefer mating with specific males, usually males that they have known well for a long time. They seek out the preferred male or males during estrus, using a series of very elaborate vocalizations and behaviors that to a human seem almost coy.

The behavior patterns that giant pandas use to communicate with each other reflect the dense habitat in which interactions occur, and the generally low level of sociality. Visual signals—body language, facial expressions, ear and mouth movements—are exceedingly rare because the dense bamboo understory restricts vision and visibility. In addition, massive cheek teeth and masseter muscles make subtle mouth, ear, and eye movements virtually impossible.

In contrast, giant pandas use vocalizations and sounds extensively in social interactions. Indeed, the subtleties of expression that can be seen in the faces of many primates can be heard in the gradations of sounds and the combinations and recombinations of vocalizations of giant pandas. Listening to the complexity of communication between Ling-Ling and Hsing-Hsing during social and sexual encounters always awes me. [10]Giant pandas also use scent to communicate. They rub the large gland that covers their anogenital region on objects, often at the edge of their territory, where a thick, waxy, strong-smelling deposit soon builds up. Both males and females may also urinate at marking sites, adding additional odor information. Most scent marks are placed at nose level or higher. Giant pandas use complex scent-marking postures to deposit urine and secretions at different heights; they even perform hand stands to raise their posteriors to make elevated scent marks.

At minimum, based on odor alone, a giant panda can probably identify the sex, age and individual identity of the scent marker, as well as interpret the marker’s mood and reproductive status at the time it left the scent mark. Because the scent is not ephemeral, as sounds are, giant pandas may be able to tell not only who has visited a particular site and how he or she was feeling, but when the visit occurred! Thus, giant pandas accomplish with scents and sounds what we accomplish with visual and auditory signals.

Reproduction

Giant panda reproduction is unique among mammals. Mating styles vary considerably among different pairs. In some pairs, male and females mate just once and the female becomes pregnant; other pairs may copulate several times a day for two or three days. Regardless of mating frequency, female giant pandas conceive only during their two- to three-day estrous period, which occurs just once a year in the spring, and they show no interest in mating at any other time. Males are sexually active for a longer period in the spring, allowing them to attempt to inseminate more than one female.

Giant pandas exhibit another peculiar evolutionary adaptation known as delayed implantation. A fertilized giant panda egg does not immediately implant on the mother’s uterine wall, but instead "floats" around in her reproductive tract for varying lengths of time. As a result, we do not know precisely the length of the giant panda’s actual gestation period. All we can say is that the time from mating to birth ranges from 95 to 160 days.

[11]Delayed implantation gives the giant panda more control over when cubs are born because birth dates are not precisely fixed by mating dates. Young may be born in the late summer or in the fall. Overall, however, the general timing of giant panda reproduction is determined by the importance of weaning cubs in the spring, when the newest most protein-filled bamboo shoots are available. This gives cubs the best possible start in life on a diet that in the best of conditions is of poor nutritional quality.

Their poor, low-energy bamboo diet prevents giant pandas from devoting much energy to gestation or lactation. As a result, giant pandas are the smallest newborn of any nonmarsupial mammal and they grow very slowly. Giant panda infants weigh just four to six ounces at birth and young are a year old before they reach 75 pounds, which is about one-third of adult weight. [12]And this growth rate is based on giant pandas in zoos, where mothers are fed rich diets, with fruits, vegetables, meat, vitamins, and minerals supplementing their daily ration of bamboo; growth of cubs in the wild is likely to be slower still.

Female giant pandas do not reach sexual maturity until they are five to six years of age, but even young, inexperienced females demonstrate a strong maternal urge and know, without practice, how to care for their young. This is important because no breeding season is "wasted" through clumsy parenting—something a female panda cannot afford. Giant pandas start to breed late, and then usually rear just one young every two years. Because a female is considered old, and possibly post-reproductive, by the age of 20 or 22, a female may rear, at most, about seven young in her lifetime. The giant pandas low reproductive rate makes it very difficult for a giant panda population to recover from a decline in numbers.

In 1990 and 1991, two myths about giant panda reproduction were debunked. More than half of all giant panda litters are twins, but it was widely believed females could never raise both of them. Then researcher at Wolong discovered a giant panda den with twin infants, both of which looked fit and thriving. Almost simultaneously, a giant panda female named Qing-Qing in the Chengdu Zoo gave birth to twins, and, against all predictions, reared them both, although with some human help.

When the twins were about a month old, one was found to be gaining less weight than its sib. So zoo staff began a routine of each day removing one cub from Qing-Qing so it could be hand-fed for 24 hours. Cubs were alternated so each received extra nourishment every other day. Clearly, this procedure could be used only with a very calm, nearly tame female, but it was successful. By January 1991, when I had the pleasure of seeing these twins, they were about five-and-one-half months old, weighed 21 pounds each, and were still thriving.

The second myth was that a giant panda cub could not be human-reared from birth. At the Heptaoping Breeding Center in Wolong, a twin was human-reared, thanks to the combined efforts of staff from Wolong, the Beijing Zoo, Chengdu Zoo, and Susan Mainka, a veterinarian who recently left the Calgary Zoo to over-see Wolong research programs.

During the past two decades, giant panda conservation has received considerable support from the international community. Nevertheless, the wild giant panda population is steadily declining, and recent successes with captive breeding have not made up for the losses, either in the wild or in zoos. Giant pandas are still being poached, timber is still being extracted from the best habitats, and the local people are still degrading what remains. A conservation action plan developed by the World Wildlife Fund and the Chinese Ministry of Forestry was completed in 1988 but has to be approved by the central government. [Editor's Note: Since this article was first published in 1992, a version of this plan has been approved by the Chinese government.]

So I’m not sure I’ll ever have the chance to follow a wild giant panda female and her descendants in a pristine bamboo-covered Garden of Eden, as in my dream. And I wonder whether our grandchildren will ever see a giant panda at all.

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