THE cheetah, the world鈥檚 swiftest sprinter, is a symbol of superior biological design. This lean cat reaches its top speed of 112 kilometres an hour within seconds from a standing start, propelled by powerful legs and fuelled by enlarged lungs, heart and adrenals. An image of a cheetah, sitting alert atop a knoll, surveying the savanna for prey, captures what it means to be wild and free.
Yet the cheetah鈥檚 future survival is precarious at best, and at worst, a lost cause. Its natural habitat is rapidly vanishing in the wake of the human appetite for agricultural land. And the cheetah鈥檚 predicament has stripped its genetic heritage to a minimum, leaving it susceptible to disease, incapable of reproducing successfully either in the wild or in captivity, and lacking the evolutionary potential to adapt to different environments.
鈥淚t鈥檚 true, the cheetah鈥檚 plight, particularly its genetic impoverishment, came to symbolise our perception of endangered species, and where our concern about them should lie,鈥� says Stephen O鈥橞rien, a geneticist at the US National Cancer Institute, Maryland. Since the early 1980s, O鈥橞rien and many others have published a stream of scientific papers reporting the alarming fact that cheetahs living thousands of kilometres apart 鈥� in southern and East Africa -are about as genetically similar as inbred laboratory mice.
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Top priority
Biologists have long believed that genetic diversity is fundamental to a species鈥� biological fitness and healthy survival. The absence of diversity was assumed to spell disaster for the health of an impoverished population. As a result, genetic diversity has come to be a top priority in conservation biology.
Recently, however, the intellectual foundation of O鈥橞rien鈥檚 theories have come under attack. What was once accepted as gospel has been described in separate critiques as 鈥渁 loosely strung chain of poorly established findings鈥� and as 鈥渘ot meeting the minimum standards of evidence鈥�.
Even O鈥橞rien鈥檚 former collaborators and coauthors have joined the fray. Tim Caro, an ecologist at the University of California at Davis, described some of O鈥橞rien鈥檚 analyses as 鈥渟everely flawed鈥�.
O鈥橞rien portrays the criticisms as 鈥渟purious and fallacious arguments鈥�. He wrote of a prominent paper in the journal Conservation Biology (vol 8, p 961) that it was 鈥渁 rambling, self-contradictory polemic that has so many misstatements, misinterpretations, disciplinary prejudices, and errors of omission as to be misleading (at best) and a disservice to the readers鈥�. The paper, by Michelle Merola, an ecologist at the University of New Mexico, is 鈥渁n embarrassment鈥� and 鈥渟hould never have been published鈥�, fumes O鈥橞rien.
When researchers set about each other in this way, there is more at stake than the objective assessment of data and technical standards. In this case, there is a turf war going on, with geneticists pitted against ecologists over the priorities in conservation. In the bad old days, the ecologists鈥� story goes, if you were interested in saving endangered species you had to focus exclusively on their genetic health. Now, they urge, you should forget genetics and simply concentrate on the real ecological problems 鈥� that is, preserving habitat.
Beyond the rhetoric, there are some real issues. First, cheetahs are facing the prospect of extinction in the wild, so conservationists need to know the cause of their decline if they are to stop it. Secondly, the captive breeding of cheetahs has been a miserable failure: the zoo population is unable to sustain itself. What is the cause of this failure? How can it be reversed?
O鈥橞rien first became involved with cheetahs in the early 1980s, when the De Wildt Cheetah Breeding and Research Centre in South Africa, invited him and several colleagues to try to find out why only one in ten cheetahs that had been caught in the wild would bred in captivity, and why more than a third of the offspring never reached maturity. They quickly found that the cheetahs鈥� sperm count was just a tenth of that in domestic cats and of the viable sperm, almost 80 per cent were abnormal, compared with the usual 29 per cent in domestic cats. These facts alone seemed to explain the cheetah鈥檚 poor reproduction. They also pointed to an underlying cause. Sperm abnormalities like these are often seen in highly inbred animals.
So O鈥橞rien and his colleagues ran tests to find out whether cheetahs are inbred, with very low genetic variation. In one test, they examined the structure of more than 200 different proteins from cheetahs in zoos in South Africa, Europe and the US. In typical mammals, as many as a third of the proteins would be expected to show some variety in structure within the population. In the cheetahs, there was virtually no variation. Inbreeding also often causes asymmetry in structures such as the head, with, say, the right eye socket being different from the left. Robert Wayne, one of O鈥橞rien鈥檚 colleagues, found just such unevenness in a large sample of cheetah skulls in museum collections.
The most startling discovery, however, was the fact that cheetahs accepted skin grafts from unrelated individuals. In animals in which the immune defences are genetically diverse, such grafts are usually rejected within 10 to 12 days. The grafts survived at least 20 days in cheetahs, some much longer.
Within a few years of embarking on the project, O鈥橞rien and his colleagues seemed to have the answers: cheetahs are genetically impoverished, and this hampers their ability to reproduce and survive both in captivity and in the wild.
Population bottleneck
The cause of the cheetah鈥檚 current plight, O鈥橞rien and his colleagues speculated, was probably a series of recent population crashes. During these crashes, which are also called population bottlenecks, a large proportion of the population dies off. Although the numbers may recover, the passage through the bottleneck robs the population of some of its genetic variability. 鈥淚t鈥檚 like a poker game,鈥� says O鈥橞rien. 鈥淲ith just a few cards in your hand, there鈥檚 a lot less variability than in the whole pack. You may be lucky and be dealt a good hand, but usually you鈥檙e not.鈥�
Ten thousand years ago, cheetahs roamed across large parts of the world, but most of them perished after the end of the Ice Age. More recently, the species has suffered from hunting and loss of its habitat. So O鈥橞rien鈥檚 hypothesis of a series of crippling bottlenecks looked extremely plausible. The geneticist鈥檚 version of the cheetah鈥檚 story entered conservation lore and biology textbooks.
There matters remained until a couple of years ago, when independent researchers tore into O鈥橞rien. Merola and Australian ecologist Graeme Caughley both surveyed the bulk of the genetic work and said the cheetah is not especially impoverished 鈥� adding 鈥渆ven if it is, it doesn鈥檛 imperil the species, either in the wild or in captivity鈥�. There was no evidence that genetic deficiencies contributed to poor reproduction or high infant mortality, they concluded. Neither author had carried out any direct research, but they had examined O鈥橞rien鈥檚 reports and found them wanting. 鈥淢y main concern,鈥� says Merola, 鈥渋s that the preoccupation with genetics diverts attention from the real threat to the cheetah鈥檚 future, which is loss of its habitat.鈥�
Meanwhile, in October last year Caro, and his colleague Karen Laurensen from the University of Stirling, who have spent years studying cheetah behaviour in the Tanzanian Serengeti, also concluded that O鈥橞rien has got it wrong (Conservation Biology, vol 9, p 1329). Instead, they say, 鈥減redation is the key source of mortality鈥�. Only about 5 per cent of cheetah cubs live a year, say Caro and Laurensen. The great majority of them die in the jaws of lions and hyenas 鈥� deaths which cannot be attributed to defective genes.
O鈥橞rien says that when the researchers first reported their data, half the deaths could not be definitively accounted for. He also suggested that the ecologists鈥� presence at cheetahs鈥� lairs probably alerted lions to the location of a potential meal, thus artificially increasing the level of predation. Caro and Laurensen vigorously deny this.
In zoos, the spotlight has also shifted away from genetic factors as the cause of the cheetah鈥檚 problems. Efforts to make the breeding situation more natural have begun to boost success rates, sometimes dramatically. For instance, in December last year, Donald Lindburg of the San Diego Zoo, said that 鈥渢here is nothing inherently wrong with them, only with the way they are handled鈥�. The remark was based on the demonstration at the zoo that even with low sperm counts and high rates of sperm abnormality, males can sire offspring easily. O鈥橞rien is sceptical, and says that it would be a miracle if the low sperm counts and high levels of abnormality did not affect reproduction.
In March last year, the eminent Oxford ecologist Robert May 鈥� now the government鈥檚 chief scientific adviser 鈥� said 鈥淥鈥橞rien鈥檚 case is persuasive鈥�. Nevertheless, he said, its impact on captive breeding is probably less important than better management. The bottom line, in May鈥檚 opinion, is that low genetic diversity does compromise the future of the species, especially since it leaves them very susceptible to disease. Noting that the maintenance of habitat is the prime concern of conservationists, he concluded that genetic diversity 鈥渞emains an important consideration for many conservation programmes, and particularly for cheetahs鈥�.
An important shift in priorities is taking place. Genetic factors are giving way to ecology and management practices in conservation. O鈥橞rien says that he never claimed that genetics held the entire answer, and agrees that habitat loss 鈥渋s the primary concern for the species鈥� future鈥�.