The Panda is a Panda
An excerpt from
The Last Panda by George B. Schaller

Is the giant panda a bear? Are the red and giant pandas closely related? These two questions have been debated for over a century. Anatomists, behaviorists, paleontologists, and molecular biologists have led the fascinating inquiry into the evolutionary relationships of these species with ingenuity and persistence, yet they continue to derive different conclusions on the basis of different evidence, and they still pursue the elusive answers. Their quest well illustrates the logic and method of scientific inquiry, the search for a resolution that satisfies the intellect. I have no emotional investment in the outcome, such research being peripheral to my interests, but I find the unraveling of the riddle intriguing.

Red pandas and giant pandas are inextricably linked. They share not only a name but also many physical similarities. Skull, teeth, and forepaws in each are similar, evolved to process bamboo. They even grip bamboo in much the same manner, except that the red panda lacks the functional sixth digit or pseudothumb so useful to the giant panda for manipulating stems. No one questions that the two pandas resemble each other. But are they actually related? This has been a subject of scientific controversy since Père David discovered the giant panda in 1869. He gave it the generic name Ursus on the assumption that the animal was a bear. The following year, Alphonse Milne-Edwards looked at the same skeletal material and decided that the giant panda was not a bear but allied to the raccoons. Since then the two pandas have been bounced around in an ever more esoteric and technical manner from one taxonomic home to another.

The issue is basically simple. Some biologists looked at the two pandas and decided they were not closely related, that their physical similarities had evolved because of the same food habits and life-style. They placed the red panda into the raccoon family, the Procyonidae, and the giant panda into the bear family, the Ursidae. Other biologists looked at the same evidence and came away convinced that the two were relatives, belonging to the same branch of the evolutionary tree. They placed the pandas either into a separate family or tucked them in with the raccoons. Each school of thought could point to specific features to reinforce its claims. The bear school, for example, stressed the giant panda's bearlike body proportions, but the raccoon school countered that the giant panda's skeleton was unusually heavy-boned, giving "the impression of being the skeleton of a 'fake' bear," as Ramona and Desmond Morris wrote. The giant panda certainly looks like a bear, but is it a bear pure and simple? Or is it a raccoonlike animal that resembles a bear because it has grown large, with a heavy body that requires special modifications such as stout legs to support it?

In some ways, the long-running issue is trivial, an illustration of a scientific discomfort with uncertainty and a penchant for putting everything tidily in place. If the giant panda is a bear, it is a highly aberrant one. If the giant panda is a raccoon, it can look around at a peculiar hodgepodge of family members: the long-nosed coati, the prehensile-tailed kinkajou, the conservative ringtail, a living fossil little changed from its Oligocene ancestor thirty million years ago, and the nimble-fingered raccoon.

However, the controversy also poses a fundamental scientific problem--what features are important and significant when classifying an animal? Classification was difficult enough when only physical characteristics were considered, but with the advent of molecular biology more, and often conflicting, lines of evidence had to be evaluated. Surprisingly, the puzzle of the panda's origin has seeped into public awareness, and the problem is viewed with some of the same fascination as the disappearance of dinosaurs. When giving a lecture, I am often asked at the end whether the giant panda is a bear or raccoon. To keep my reply brief, I usually answer, "The panda is a panda."

This view places me in a distinct minority at a time when the bear proponents are the most vocal. Take a scientific review on the subject published in 1986 in the journal Nature. The author, clearly impatient with the whole controversy, noted that some still view the two pandas as "each other's closest relatives. It would seem that this suggestion is clearly refuted by the near-unanimous and highly diverse evidence . . . ," and he hoped that the "overwhelming evidence for the bear relationship of the giant panda would end the argument. . . . Only some of the behaviour students were not yet persuaded." When "overwhelming" evidence is invoked, beware. Is the article a polemic promoting a dogma rather than a careful examination of the facts? In science, an issue that has been truly resolved is absorbed and disseminated quietly without need to make the "near-unanimous" unanimous, to prod heretics into becoming true believers.

Am I reluctant to become a bear proponent because of mental inertia, am I unable to divest myself of an outdated notion, or is there reasonable doubt about the majority view? I am admittedly delighted that in spite of its being dissected, observed, measured, and subjected to a host of advanced molecular techniques, the giant panda still cannot be neatly categorized. Just as I hope that there is a yeti but that it will never be found, so I would like the panda to retain this minor mystery. Still, there is intellectual pleasure in trying to solve such a puzzle.

Molecular studies have in recent years provided important insights into the classification of pandas. Since protein molecules are an integral part of DNA, they closely reflect the hereditary and therefore the evolutionary history of an animal. Proteins are composed of different amino acids. The number of differences between the amino acids of any two proteins should, it was reasoned, be proportional to the time elapsed since they diverged from a common ancestor. With molecular evolution supposedly freed from environmental influences, it might be regular enough to be used as a molecular clock, one that provides precise information about the amount of genetic change that has occurred over the past few million years. In one such study, published in 1976, Vincent Sarich used immunological techniques to compare two blood proteins from giant pandas, red pandas, bears, and raccoons. In this method the proteins are injected into rabbits, producing antibodies, which then react strongly against the proteins for which they were prepared and progressively more weakly against proteins in species more distantly related. He concluded that "the association of the Giant Panda and other bears is clear and unequivocal. . . . The one rather unexpected result there is the fact that the Lesser Panda, Ailurus, does not group with the other procyonids," that it seems to have started a separate lineage before the giant panda and bears.

In 1985, Stephen O'Brien and his coworkers published an article in Nature entitled: "A molecular solution to the riddle of the giant panda's phylogeny." Using gel electrophoresis, a technique by which proteins can be sorted by electric charge and size, proteins from pandas, bears, and members of the raccoon family were compared. To check their results, the investigators conducted DNA hybridization tests. This method compares the DNA of each species, the actual hereditary material, rather than just the proteins. Radioactively tagged DNA from one species is hybridized with the DNA of another and the stability of the union measured. The study concluded that "the lesser panda diverged from New World procyonids at approximately the same time as their departure from ursids, while ancestors of the giant panda split from the ursid lineage much later, just before the radiation which led to modern bears." This split is said to have occurred as long as 15 to 25 million years ago. In other words, the giant panda is a bear, the red panda a raccoon. Yet within a year after this study, another investigation into a blood protein showed that the two pandas are more alike than are either giant pandas and bears or red pandas and raccoons. Which blood protein should be used as a taxonomic character: albumin, which makes the giant panda a bear, or hemoglobin, which does not? The molecular clock may at times not be as precise as suggested. Different proteins in an organism may mutate at different rates, and natural selection may affect some proteins, such as hemoglobin, more than others.

David Goldman, Rathna Giri, and Stephen O'Brien returned to the fray in 1989 with a molecular study of all seven bear species, the giant pandas, the red pandas, and the raccoon. Published in the journal Evolution, the study was "based on the extent of electrophoretic variation of 289 radiolabeled fibroblast proteins resolved by two-dimensional gel electrophoresis and among 44 isozyme loci resolved by one-dimensional electrophoresis." I am too ignorant of molecular biology to understand such studies, much less to comment critically on techniques and analyses. The results generally confirmed Stephen O'Brien's 1985 report. The progenitors of raccoons and bears split during the Oligocene, and within 10 million years the red panda lineage diverged from the raccoon lineage. In the Miocene, there were three major radiations among the bears, the earliest line leading to the giant panda, the second to the South American spectacled bear, and the last to the six other bear species. If the authors expected to settle the panda debate with this study, it must have been a forlorn hope. Indeed, given the history of molecular studies, the next one would most likely contradict their findings. And so it did.

In 1991, Zhang Yaping and Shi Liming published in Nature the results of a detailed analysis of mitochondrial DNA from the two pandas, Asiatic black bear, and sun bear. Their conclusion: the giant panda is more closely related to the red panda than to the bears. And they noted that, "there are 1,000-10,000 copies of mitochondrial DNA in every cell; the selection pressure on this DNA is very low. So the similarities between the two pandas . . . may not be the result of convergent evolution."

The contradictory results from molecular studies teach an important scientific lesson. Slick and ultramodern techniques may not always be enough to elucidate the incomprehensible. Morphology, paleontology, and natural history continue to have an important role in unraveling the evolution of the two pandas.

Fossils often provide insights into the past of a species, but there are giant gaps in the fossil record, especially of pandas. The raccoon family, an early branch of the dog family, the Canidae, evolved in North America and spread across Asia to Europe where, during the Miocene, 20 million years ago, the raccoonlike Sivanasua was found. The bears, also a branch of the Canidae, appeared in the early Miocene in the form of Ursavus, a bearlike creature as large as a medium-sized dog. The first definite panda, Parailurus, a small animal resembling the red panda, occurred in the early Pliocene about 12 million years ago in southern Europe and North America; it persisted into Europe's last ice age, where it was quite common in temperate forests. Some researchers think that a small, bearlike animal of the Ursavus lineage named Agriarctos, dating from the mid-Miocene, was the ancestor of the giant panda. Support for this supposition came in 1989 when Qiu Zhanxiang and his colleagues described a new fossil from the late Miocene in Yunnan with teeth that resemble those of the giant panda but also share characters with the ancestral forms of bears. The animal, given the name Ailurarctos lufengesis, was less than half the size of today's giant panda.

The giant panda itself appeared suddenly during the late Pliocene or early Pleistocene, perhaps no more than two to three million years ago. Panda fossils have been found in Burma, Vietnam, and particularly in eastern China, as far north as Beijing, where they appear so often with the Pleistocene elephant Stegadon that the two species are used to designate a distinct fossil fauna. The pandas of the early Pleistocene were about half the size of today's giant panda and are considered a separate species, Ailuropoda microta.

New species originate mainly when a small segment of an ancestral population becomes isolated, and the animals change form and behavior through natural selection until a population with new characteristics is well established. This process may be so rapid that no recognizable intermediate forms or missing links are found in the fossil record. Animals can apparently evolve quickly, through major chromosomal rearrangements, as well as more slowly through mutations of single genes. Then, having settled in, the new species may remain unaltered, except for slight modifications, for millions of years. The giant panda is known only in its existing form, apparently not an ancient relic, as is often claimed, but a relative newcomer. Its previous incarnations still remain uncertain.

Can the behavior of the giant panda provide clues to the animal's evolutionary relationships? This approach has problems. Species living in similar habitats may evolve similar societies and similar physical appearances, which in turn may result in similar behavior without there being a close relationship. When comparing species, one first has to decide which aspects of behavior have been strongly influenced by ecological conditions. For instance, the amount, quality, and distribution of food effects an animal's movements, activity cycles, and social structure. Consequently two separate populations, even of the same species, may behave differently. However, certain kinds of behavior such as scent marking and vocalizing can function well under a wide variety of conditions, and therefore they may be less influenced by ecological pressures.

The giant panda produces a surprising mix of sounds, some of which it shares with bears, some with the red panda, and some with both. For instance, the giant panda's chomping, in which the animal clacks its teeth and smacks its lips when anxious, is found in bears and, in a modified form, also in red panda and even coati. The giant panda's plaintive honk, which denotes light distress, is similar to the grunts made by bears and several procyonids, but in these animals the calling has a different function, that of a contact call between mother and young. More exclusive is the moan, a highly variable call ranging from hoots and whiney groans to long-drawn-out moans. Only giant panda and bear share this warning signal. Particularly noteworthy is the giant panda's goat-like bleat, a friendly call that provides animals with reassurance on meeting. As Gustav Peters of the Alexander Koenig Museum in Bonn has noted, this bleat has its equivalent in the twitters and chitters of red panda and all the procyonids, but nothing similar occurs among the bears. One would not expect such a high-pitched vocalization from an animal the size of a giant panda; in fact, several of the giant panda's calls are surprisingly high-pitched for a large carnivore. Nursing bear cubs produce a most peculiar call, a continuous keckering, harsh and rapid almost like a loud purr. Although the function of this vocalization is unknown, it obviously conveys something important to the mother, possibly signaling her to lie still and release milk. The giant panda lacks this call. If the giant panda is merely a bear, I find it difficult to understand why the tiny young would not have such an emphatic vocalization.

Marking behavior naturally depends on the type of gland available to a species and on the location of the gland. The giant panda claws trees, urinates, and rubs its glandular anal area on objects. Bears lack a specific gland, although they stand on their hindlegs and rub shoulders, neck, and head against tree trunks, and they bite and claw bark, leaving behind their general body odor. The red panda straddles stumps and other protuberances and deposits scent from its anal glands with circular rubbing motions, using actions closely resembling those of the giant panda; male red pandas also squirt urine. Miles Roberts of the National Zoo in Washington, D.C., found that red pandas also have several small pores on their palms that secrete a clear fluid, a means of leaving special information behind when walking. Most procyonids scent mark with urine or anal glands or with both, behavior similar to that of pandas though the details differ.

Giant pandas and bears also resemble one another in the extremely small size of their newborns. While black bear mothers are 250 times heavier than newborns, a giant panda mother is 900 times heavier. (By contrast, a raccoon mother is only 55 times heavier.) Why is the giant panda newborn so much tinier than the others? Reproduction is influenced by ecological conditions, especially by the amount of high-quality food available to mother and young at various seasons. Bears, like giant pandas, have delayed implantation. In temperate climates bears mate and young are conceived in about June but the fertilized egg does not attach itself to the uterine wall and growth of the fetus does not begin until about 60 days before cubs are born in January or February, while the mother is in hibernation. If cubs were large and vigorous at birth, their milk demands on the lethargic mother might be excessive, too great an energy drain on the fat deposits that must maintain her until the end of hibernation in spring. Since the giant panda does not hibernate, different selection pressures must have produced the tiny newborn. The fact that bears and giant pandas both have small young at birth does not imply that they are kin. I wondered, then, why a giant panda newborn was so extraordinarily small.

Looking at red pandas, I found some clues. The red panda also has delayed implantation (something not found in procyonids), and its gestation period is an average of one hundred and thirty-one days, about the same as that of a giant panda. The daily weight gain of a giant panda fetus is less than half that of bears and more like that of the red panda as well as raccoons. A red panda mother usually has one or two young and their birth weight is about four ounces, the same as the birth weight of the giant panda. The red panda female keeps her young hidden in a tree cavity where they develop slowly, emerging only at the age of three months, a denning period longer than that of raccoons and coatis. All these facts show that reproduction in a giant panda is more typical of a small mammal like a red panda than of a bear. There are of course some size adjustments. For example, the long period of care required by the young giant panda extends the mother's lactation period into the next mating season, preventing her from reproducing annually. By contrast, red panda young become independent at eight instead of eighteen or more months, and the female can reproduce each year. I deduce from all this that the giant panda has retained many reproductive features of a small pandalike ancestor, merely adapting some of the historical vestiges to meet current circumstances.

Scattered pieces of evidence point to a definite relationship between giant panda and red panda: the specialized structure of skull, teeth, and forepaws; various aspects of reproduction; certain vocalizations; and scent-marking behavior. If the two species are unrelated, we must accept a remarkable amount of convergence, more than is justified by the evidence. When did their paths diverge? Most likely the giant pandas and red pandas had a common ancestor in the Miocene. Where should the two pandas then be placed, with the bear or with the raccoon family? Even though the giant panda is most closely related to the bears, I think that it is not just a bear. Even a small amount of genetic difference between two species may have a profound influence on appearance and behavior. Chimpanzees and humans may share as much as 99 percent of their genetic material. We should no doubt embrace the chimpanzee as a family member. But is the chimpanzee human? The giant panda and red panda offer several choices. Should the giant panda be with the bears or in a separate family but the red panda with the raccoons? Should each panda have a separate family? Or should the two pandas share a family, the Ailuridae? I favor the last alternative. Science will overcome this paradox and perversity of evolution and ultimately assign each panda a final taxonomic home.

But as yet this game of taxonomic Ping-Pong has no winners. The search for a more complete answer continues "with the bear proponents and the raccoon adherents and the middle-of-the-road group advancing their several arguments with the clearest logic, while meantime the giant panda lives serenely in the mountains of Szechuan with never a thought about the zoological controversies he is causing by just being himself." Edwin Colbert wrote these words in 1938. The giant panda still pseudothumbs his nose at us.

Copyright notice: ©1993 Excerpted from pages 261-67 of The Last Panda by George B. Schaller published by the University of Chicago Press. ©1993 by the University of Chicago. All rights reserved. This text may be used and shared in accordance with the fair-use provisions of U.S. copyright law, and it may be archived and redistributed in electronic form, provided that this entire notice, including copyright information, is carried and provided that the University of Chicago Press is notified and no fee is charged for access. Archiving, redistribution, or republication of this text on other terms, in any medium, requires the consent of University of Chicago Press.

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George B. Schaller
The Last Panda
With a new Preface
©1993, 320 pages, 9 maps, 27 color plates
Cloth $24.95 ISBN: 0-226-73628-8
Paper $15.00 ISBN: 0-226-73629-6

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