Introduction to The Endangered Cheetah
A plaintive meow rises from the deep grass inside a large enclosure at the San Diego Wild Animal Park. Suddenly, the grass rustles and a black-spotted golden cat, the size of a large dog, stalks into view. This sleek, powerful animal with amber eyes and the voice of a tiny tomcat is Punchow, a magnificent male cheetah.
Punchow and his kind are the fastest sprinters on Earth, and human beings have admired, captured, and even hunted with them for thousands of years. But now, as humanity carves up the cat's native habitat in Africa, the survival of the cheetah is at stake. So for decades, researchers at zoos and cheetah breeding centers have been trying to boost the cheetah population by encouraging captive cats like Punchow to breed.
Scientists' limited understanding of cheetah biology has made this task extremely difficult, and Punchow himself symbolizes the puzzle biologists face as they work to save these animals from extinction. Punchow may be genetically almost identical to all other members of his species, a possibility that some experts believe accounts for the health problems they have observed in captive cheetahs—for example, poor breeding success and a high susceptibility to disease. Indeed, a researcher armed only with Punchow's lab tests might conclude that the cat is sterile. What is puzzling is that Punchow managed to father nine cubs by two different females the first two times he was bred.
Punchow's reproductive success—and that of other cheetahs in the 1990's—has sparked a debate over how the cheetah's genetic makeup affects the species' chances for survival. Researchers looking at the cats in the wild point out that free-living cheetahs are endangered primarily because their habitat is shrinking and because cheetah behavior prevents the animals from thriving in game reserves. For cheetahs in captivity, changing the way zoos manage the cats appears to boost the success of breeding programs. These developments have led many researchers to look anew at the cheetah—and to conclude that, at least for captive cheetahs, the outlook may not be as gloomy as they had thought.
Portrait of A Sprinter
Cheetahs have long been famed as the fastest, and among the most fascinating, short-distance runners in existence. Scientists believe cheetahs can accelerate from walking speed to about 65 kilometers (40 miles) an hour in less than two seconds and briefly explode to top speeds of 110 kilometers (70 miles) an hour. The cheetah arches and bends its flexible spine as it gathers its long front and hind legs beneath its body, then leaps as far as 7 meters (23 feet)—about five times the length of its body—in a single stride. The cat can sustain its full-speed pursuit for only 200 to 300 meters (220 to 330 yards), a distance equal to about two or three football fields. But that can be far enough to allow the cheetah to overtake swift prey, typically a Thomson's gazelle or other small antelope. The cheetah then knocks the animal to the ground with a forepaw and strangles it with a crushing bite to the throat.
For thousands of years, cheetahs have been prized in royal menageries for their spectacular hunting style, as well as their regal grace and soulful faces, marked by distinctive black lines that curve like a trail of tears from the eyes to the mouth. The structure of cheetahs' windpipes prevents these gentle-looking cats from roaring like lions. Instead, they make sounds described as chirps, stutters, moans, and purrs. Cheetahs can be tamed, and beginning at least 3,000 years ago with the ancient Egyptians, aristocrats commonly trained the cats to hunt just as they trained falcons. A royal chronicle tells how Akbar the Great, who ruled India's Mogul Empire in the A.D. 1500's, captured at least 9,000 cheetahs during his 49-year reign to aid him in hunting deer.
How Many Cheetahs Remain In the Wild?
Cheetah conservationists would give a great deal to have Akbar's 9,000 cheetahs back today. Conservationists estimate that around the year 1900, hundreds of thousands of cheetahs lived throughout most of Africa, western Asia, and India. That is no longer the case. The cheetah disappeared from India in the 1940's, and by the 1970's, fewer than 200 survived in Iran, probably the cat's last Asian stronghold. Today, only scattered pockets of cheetahs remain, chiefly in southern and eastern Africa and in the semiarid Sahel region south of the Sahara. Estimates of cheetah numbers are imprecise, because cheetahs, like nearly all wild cats, are shy, stealthy, solitary, and therefore difficult to count. Estimates of the current African cheetah population range from a low of 5,000 to highs of 15,000 to 25,000. Another 1,000 cheetahs live in captivity around the world, some 300 of them in North America.
Wild Cheetahs: A Struggle to Survive
The cheetah is endangered today largely because human beings have taken over much of the cat's habitat and killed off the small antelope the cheetah hunts for food. People have also killed many cheetahs directly. By the early 1970's, the fur trade had become a major threat to large cats such as tigers and cheetahs. At the time, the United States alone was importing 25,000 large-cat skins each year for fur coats, rugs, and other fashion items. Passage of the United States Endangered Species Act in 1973 and the Convention on International Trade in Endangered Species, an international treaty administered by the United Nations, in 1975 gave the cats protected status and reduced the trade in cheetah fur. Nevertheless, cheetahs are still routinely shot by African farmers and ranchers who view the cats as a threat to livestock, just as American ranchers view wolves and mountain lions.
Africa's animal parks and game reserves protect many animals from human beings. However, cheetahs are poorly suited to life in these reserves. Within the borders of a typical game reserve, herd animals such as zebras, wildebeest, and antelope are protected from human hunters and so thrive in numbers not normally seen on the open savanna (grassland with scattered trees). As a result, populations of the large or powerful predators that feed on herd animals—predators such as lions, hyenas, and leopards—also are high. But middle-sized predators such as cheetahs and wild dogs suffer when they are forced to compete in this crowded and confined landscape, according to Timothy M. Caro of the University of California at Davis, who has studied wild cheetahs in Tanzania's Serengeti Plain since 1980.
Cheetah Handicaps In Competition With Other Species
Cheetahs are handicapped, for example, by the very fact that they can perform spectacular high-speed chases. To catch its breath after making a kill, a cheetah may need to lie panting for up to 30 minutes before it can even begin to eat. During this resting period, hyenas, lions, leopards, and even flocks of vultures may steal the winded cat's kill. According to Caro, 1 in 10 cheetah kills is lost this way. Moreover, unlike most of their competitors, cheetahs will not eat carrion (rotting meat). If they lose a kill, they must hunt again to get fresh meat.
Because the cheetah is built for speed and not for fighting, the animal has little chance of fending off lions or hyenas even when it is rested. Its bones are light and its body is thin and elongated, making the cat a poor match for a heavier adversary. And cheetahs are the only cats whose claws are always bared, like those of a dog, rather than being pulled back into protective sheaths. This feature gives cheetahs extra traction for running, but it also dulls the claws and makes them relatively useless for fighting. In addition, the cat's unusually broad nasal passages, which help the cheetah take in a large supply of oxygen while running, leave less room in the skull for the roots of long canines (tearing teeth), which are characteristic of lions and other wild cats. As a result, the cheetah's fangs are too short to take on fierce competitors.
Cheetahs face another threat from their larger relatives, Caro discovered. Beginning in 1987, Caro and his student Karen Laurenson attached radio collars to the necks of 20 free-ranging female cheetahs. For the next few years they tracked the cheetahs' movements, and whenever a cheetah had a litter, the researchers periodically examined the cubs. Caro and Laurenson concluded that 95 percent of cheetah cubs born in the Serengeti die before adulthood, most of them while still helpless in their dens, and that 75 percent of these cubs were killed by marauding lions. In protected areas across Africa, the researchers found, a high lion population correlates with a low cheetah population.
Captive Cheetahs: A Puzzling Failure to Breed
As cheetah populations have dwindled in the wild, biologists have felt mounting pressure to try to breed cheetahs in captivity. Their task has seemed monumental. Indeed, Akbar the Great unwittingly set a record that was to last 400 years when one of his cheetahs gave birth to a litter of three cubs in the 1500's. Even with thousands of the cats to work with, Akbar's cheetah keepers never recorded producing a second litter. And no other captive births are on record until 1956, when a female in the Philadelphia Zoo also produced a litter of three, all of which died in infancy. A handful of other cheetah births were reported during the 1960's at zoos in Europe and the United States, but not until the early 1970's did a captive-born cub survive to maturity and give birth to a second generation of captive-born cubs. Zoos still had to purchase wild—caught animals to restock their exhibits.
During the 1970's, zoos began to realize the urgency of establishing a self-sustaining population of captive cheetahs, both to avoid taking any more animals from the wild and to build up a pool of cheetahs that might one day be used to restock Africa's remaining savannas. The zoos redoubled their efforts, and the number of captive births rose steadily. Most of these births, however, were at only a few institutions, including the San Diego Wild Animal Park, the Columbus Zoo in Ohio, and Wildlife Safari in Winston, Ore. By the end of the decade, researchers at other zoos began to wonder whether their efforts were being foiled by biological problems in the cheetahs themselves.
A Surprising Discovery About Cheetah Biology
In the late 1970's, the National Zoo in Washington, D.C., asked David E. Wildt, then a biomedical researcher working with domestic cats, for help with the cheetah. Wildt collected sperm samples from captive cheetahs belonging to several American zoos. By looking at the samples under a microscope, he found to his surprise that about 70 percent of the sperm were abnormal. The abnormal sperm appeared damaged, with strangely bent or coiled tails or other deformities that would prevent them from penetrating and fertilizing a female's egg.
At first, Wildt thought the abnormalities in the cheetahs' sperm must be a result of the stressful conditions under which most zoo cheetahs were kept. Then, in 1981, Wildt went to South Africa to perform similar studies on male cheetahs at the DeWildt Cheetah Breeding and Research Center, near Pretoria. The South African cats, most of them bred in captivity but including some caught in the wild, showed the same level of sperm abnormalities as cheetahs in U.S. zoos.
To help determine why the cheetah had such high rates of abnormal sperm, Wildt sent blood samples from each male cat to Stephen J. O'Brien, a researcher at the U.S. National Cancer Institute in Bethesda, Md. O'Brien wondered whether the cheetah's reproductive problems were due to a genetic problem, possibly a lack of genetic variation among individual cats. Genes are components of living cells that contain the blueprint for the organism's characteristics and functions. In human beings, for example, genes control the color of a person's skin, eyes, and hair, his or her blood type, and all other inherited characteristics. The genes for our species come in many versions, and that genetic variety explains why human beings are not all as alike as identical twins
Why Genetic Variation Matters
Scientists think genetic variation may be a natural safeguard for a species, because such variation increases the likelihood that individuals will respond differently to most threats that arise. For example, a disease may be deadly to most members of a population, but if a few individuals are genetically different enough to survive it, the species will live on.
Genetic variation may also help guard against the disorders caused by inheriting flawed genes. Generally speaking, organisms inherit two copies of each gene, one from each parent. In human beings and other animal species, most inherited diseases occur only if an individual inherits a flawed version of the same gene from both parents. In a population with a great deal of genetic variation among its members, the odds of both parents having the same flawed gene are low. But in a population without much variation, it may be highly likely that both parents will have the same versions of genes. In such a group, many offspring may be born with inherited disorders.
Genetic Variation In the Cheetah
To find out if cheetahs showed a lack of genetic variation, O'Brien tested blood samples from 55 of the cats, comparing 52 different proteins found in the blood of each animal. Genes carry the instructions for building proteins, and any slight variation in the makeup of a protein reflects a corresponding variation in a gene. O'Brien found that for nearly all the proteins he tested, the cheetahs in his study had virtually the same genetic makeup. His tests showed that cheetahs may be as similar genetically as mice that scientists have deliberately inbred—by mating cousins, siblings, and other close relatives for many generations—to create uniform strains of mice for use in laboratory experiments.
But most of the cheetahs in O'Brien's study had been out of the wild only a generation or two at most, so their genetic uniformity was not the result of inbreeding by zoos. How, then, did cheetahs become so similar?
O'Brien speculated that in the distant past, the cheetah population experienced one or more bottlenecks—drastic population crashes in which a large proportion of the species died, leaving, perhaps, only a handful of survivors. Using the cheetah's current degree of variability as a guide, O'Brien calculated that the cheetah probably fell to the brink of extinction beginning 12,000 to 10,000 years ago, at the end of the Pleistocene Ice Age, a period in which many other mammals, such as mammoths and saber-toothed cats, became extinct. According to this view, each of the cheetah's population bottlenecks was followed by extreme inbreeding among the few survivors, which in a drastic case may have numbered only a few litters. This inbreeding, continued over many generations, would have eventually produced a population in which nearly all individuals shared the same limited pool of genes.
Controversy About Genetic Variation
Not all experts accept the bottleneck theory. A more likely scenario, some say, is that the cheetah population remained abundant but was fragmented into numerous small, isolated subpopulations. Such fragmentation could have taken place toward the end of the Ice Age, as climate fluctuations caused the advance and retreat of ecosystems such as forest and savanna. As a forest extended into a savanna, cutting off one portion from another, the animals on either side of the forest could have been separated and remained so for thousands of years. According to this theory, when one such group died out because of disease, overhunting, or some other reason, its territory was eventually recolonized by a few individuals from a nearby subpopulation. Repeated over and over, this process would eventually lead to a loss of genetic variation.
Population geneticist Philip W. Hedrick of Arizona State University in Tempe is one expert who agrees that the cheetah could have achieved its low genetic variation without its population dropping to the verge of extinction. But Hedrick also points out that several animal species—including beavers in Sweden and northern elephant seals living off the California coast—have low genetic variation, yet their large populations appear to suffer no health or reproductive problems as a result. "We don't really know how much variation is required for a species to be fit and healthy," Hedrick says.
Cheetah Genetics: the Debate Over Disease
Thus, a more important question for the cheetah than how it got its low genetic diversity is whether the cat's survival has been compromised by it. Scientists ask, for instance, whether the cheetah's genetic uniformity has indeed made it overly susceptible to disease. To answer that question, they consider a disaster that began in May 1982, when a female cheetah that had been recently brought to Oregon's Wildlife Safari became ill and died from feline infectious peritonitis (FIP), a viral disease that strikes domestic cats but is rarely fatal to them. Within the next year, 18 of the 42 cheetahs at the park died of FIP-related illnesses.
O'Brien suggested that the "catastrophic sensitivity" the cheetahs displayed to the virus was a consequence of their lack of genetic variation. In other words, the genes that code for certain defenses in the cheetah's disease-fighting immune system were so similar among the cats that if FIP could evade the defenses of one cat, the same virus could hit all the cheetahs equally hard.
In research published in 1985, O'Brien and his colleagues tested the amount of genetic variation in the cheetah's immune system by grafting patches of skin between pairs of unrelated cheetahs. Normally, when skin or other tissue is transplanted from one individual to another, the recipient's immune system rejects the foreign tissue, attacking and trying to destroy it. Strong drugs are necessary to prevent this rejection response in human transplant patients, and biologists have found that a house cat usually rejects a skin graft from an unrelated cat within 14 days. In contrast, of the 14 cheetahs that received skin grafts in O'Brien's study, only 3 appeared to reject the grafts, and these rejections took 40 days or more. Thus, the investigators concluded, the cheetahs' immune systems must have been genetically very similar.
Nevertheless, other scientists question whether the cheetahs' vulnerability to FIP resulted from genetic uniformity or simply from the fact that the cats had never before been exposed to the virus. Now that cheetahs have been exposed to FIP, these researchers note, the cats seem to be developing resistance to the disease. Only three captive cheetahs died of FIP between 1987 and 1991, and by 1991 two-thirds of the captive cats in the United States carried antibodies (protective proteins) to FIP in their blood, showing that their immune systems had learned to ward off the virus.
A New Look At Breeding Problems
By the early 1990's, scientists were also questioning the link, first suggested by O'Brien and Wildt, between the cheetah's lack of genetic diversity and the cat's poor breeding history in zoos. Caro and his team, for instance, pointed out in 1992 that cheetahs in the wild have the same sperm defects as captive cheetahs, yet they seem to have no breeding problems. Then, in January 1993, researchers working with the Cheetah Species Survival Plan (Cheetah SSP) of the American Zoo and Aquarium Association, based in Bethesda, Md., published the results of nearly five years of exhaustive studies on the North American captive cheetah population. The reports included a detailed study by Wildt of reproduction among 128 cheetahs in American zoos. The males showed the same high levels of abnormal sperm Wildt had seen before. But surprisingly, these abnormalities seemed to bear no relation to the animals' fertility. Successful sires like Punchow had lab reports that looked just as bad as those of males that had failed to father cubs.
Other SSP researchers pointed out that even as scientists debated the genetics-fertility link, the tide of opinion in the zoo community was turning away from genetic factors as the chief cause of the cheetah's breeding problems. These researchers noted that as zoos changed how they managed cheetahs, the number of captive births in North America had risen to 201 cubs in 58 litters during the five years between 1987 and 1991. That was nearly half the number of cubs born in these institutions during the previous 30 years. These results convinced most cheetah specialists that the main reason for captive cheetahs' failure to breed lay not with the cheetahs' genes, but with how the cats' keepers have managed them.
Strategies of Successful Breeding Programs
Unfortunately, no simple lab tests can help identify just what successful zoos are doing right, though a few strategies are suggested by studies of cheetahs in the wild. For instance, cheetahs seem to breed more readily when they are not housed with other wild cats, where they are intimidated by lions in nearby cages. Another strategy entails keeping female cheetahs alone except for their cubs to mimic their solitary existence on the savanna.
To find out what else goes on in successful breeding programs, SSP researcher Nadja Wielebnowski, another of Caro's students, has spent much time observing the cats in zoos, rather than on the savanna. One thing some successful programs do, she found, is keep the cats in relatively large areas generally secluded from visitors, rather than on exhibit. A second strategy involves keeping a large number of cheetahs on hand so that animals can be introduced to several possible mates. Researchers at the San Diego Wild Animal Park, as well as at other zoos, also have noticed that some females seem to prefer certain males and tend to reject advances by others.
Some of the most successful breeding facilities have set up two separate living areas so that cheetahs can be moved around periodically and be stimulated by seeing and, especially, smelling unfamiliar cheetah neighbors. Zoologists note that because cheetahs are solitary, wide-ranging mammals, their sense of smell is an important means of communication and "courtship" between potential mates. Wielebnowski agrees, adding that a female in the Serengeti may range over 800 square kilometers (300 square miles). Wielebnowski also notes that cheetahs, like other intelligent animals, "seem to get bored and lose interest in their surroundings" if they are deprived of stimulation for prolonged periods. Since 1990, several zoos have tried to combat cheetah boredom by setting up coursing tracks, such as those used for training greyhounds, to encourage cheetahs to do what they do best: run.
Looking Toward the Future
Despite the evidence that behavioral explanations for the cheetah's low reproductive rates are at least as important as genetic ones, few scientists are complacent about the cat's potential for future genetic problems. For this reason, researchers working with the Cheetah SSP are seeking to preserve whatever genetic variation the cheetah does have. Their plan is to avoid inbreeding by reducing zoos' reliance on cheetahs that have already produced many young and attempting to get offspring from cheetahs that have never bred. In November 1993, the Cheetah SSP management group met to pore over the cheetah studbook, a record of all the cheetahs in North America, which includes family trees for the captive-born animals. The group set out a master plan for the coming year's breeding efforts, choosing 20 male and 20 female cheetahs that are not closely related to most of the cats in the North American population. Because a number of cats chosen for the program are past the normal breeding age, the group plans to use techniques such as artificial insemination to help some of them reproduce.
Zoo breeding successes are extremely heartening, according to zoologist Jack Grisham of the Oklahoma City Zoo, who directs the Cheetah SSP. Grisham has even begun to believe that if zoos don't manage the size of the captive population, they'll eventually run out of room for cheetahs. Still, the captive breeding is just "icing on the cake" for the cheetah species, according to Caro. In his view, boosting populations in this way provides only a temporary stopgap to the loss of cheetahs in the wild.
Modest Conservation Efforts On Behalf of Free-living Cheetahs
Unfortunately, few conservation efforts are directed at free-living cheetahs. The most well-known was launched in 1990 by a husband-wife team, conservationists Daniel Kraus and Laurie Marker-Kraus of the National Zoo, who now live in Namibia. This nation on the southwest coast of Africa hosts the world's largest concentration of surviving cheetahs, perhaps as many as 2,500. However, 95 percent of the cats live outside Namibia's reserves and thus come into direct conflict with human beings. The Krauses try to persuade skeptical farmers to ward off cheetahs rather than shoot them when the cats attack their livestock. The conservationists suggest, for instance, that farmers keep cows with vulnerable calves in corrals, or that they use aggressive animals such as horned steers, hard-kicking donkeys, guard dogs, or even baboons to guard their herds against the easily frightened cheetah.
These efforts may buy the cheetah a little more time. It is too early, though, to tell whether biologists will win for wild cheetahs the kind of security that captive cheetahs seem finally to have grasped.