The Flight of the Iguana

by David Quammen

  The cheetah evolved independently of the other big cats and arrived at its modern form much earlier. Today it stands separate from all other living representatives of the cat family, a lonesome anomaly that in some ways shares more in common with dogs. Besides being faster, it is far more delicate, more slender, and less imposingly armed than any lion or leopard. Its teeth are shorter. Its jaws are rather weak. Other cats can voluntarily retract their claws into claw sheaths, thereby preserving the sharp points for piercing and slashing; the claws of the cheetah, in contrast, are not fully retractile and grow dull from being walked on. A cheetah’s footprint, consequently, looks more like the print of a wolf than like the great soft ominous pug of a tiger. Though it may travel in small social groups (a mother with kits, a mixed trio of adults, even a pair of bachelor males), the cheetah seems most often to perform the act of killing solitarily. As slight of build as it is, as poorly equipped with lethal weapons, it would have little chance of winning a meal at all, if not for speed.

  Speed it has, of course—unequaled speed—as well as a nicely matched set of anatomical adaptations that make that speed possible. The femur bone of the cheetah’s leg is elongated, unusual among cats. The spine is long and flexible, and it bows dramatically with each stride, giving still greater reach to the legs. Those short, blunted claws are good for traction and quick turns. The nasal openings are especially large, as are the bronchi and the lungs, invaluable for an animal that needs huge volumes of oxygen for burning huge amounts of energy in very brief stretches of time. The heart is also large. The tail is long and held straight out, for balance, while the cheetah screams along at seventy miles per hour. Even the arteries are exceptionally muscular. A. jubatus is born to run.

  More specifically, born to chase. The cheetah shows a few curious behavioral patterns that become comprehensible only in light of its anatomical assets and limitations, and one among these is perhaps the most intriguing: According to reliable observation, a cheetah will almost never attack a potential prey animal that does not bolt and run.

  Its favored prey species are modest-sized grazers like the impala and the Thomson’s gazelle, generally taken at weights less than the cheetah’s own. But let an impala stand its ground (either from stupid daring or because it’s paralyzed by fear) and the cheetah will pass it right by, focusing instead on one of the other herd members that has taken flight. For two or three hundred yards the cheetah will pursue at top speed, until (on a successful chase) it has pulled up beside the chosen impala’s rear flank. Then it will do what is, to my mind, a charmingly roguish thing: It will swing out a paw (in mid-stride now, remember, at seventy miles per hour) and trip the impala.

  The impala goes head-over-teakettle. The cheetah slams on its own brakes, and pounces. The actual killing is then accomplished with a throat bite—which must be held as long as it takes for the impala to strangle, since the cheetah’s jaw muscles and teeth are too meager for chomping through the spine or ripping out the jugular. But what about that other brazen impala, the one left standing back at the start? Why did the cheetah choose to ignore it? No one can be sure, but the most plausible answer is that, without a high-velocity chase, without a well-timed trip, the speedy-but-weak cheetah simply has no means of knocking an impala off its feet.

  The cheetah’s hunting technique, with that long stealthy stalk followed by that sudden heart-shocking sprint, seems specifically designed to induce—rather than to preempt—a panicky bolt by the prey. That induced panic makes a gazelle or an impala vulnerable to the cheetah’s modest killing tools in a way neither animal would otherwise be. The successive stages (spook-chase-overtake-trip-pounce-strangle) are all linked together with fine economy. The method is highly dramatic and highly successful.

  It is so successful that the cheetah, lacking defensive weaponry in a fiercely competitive habitat, can afford to be (and is) frequently robbed of its own fresh-killed prey by lions and leopards and hyenas; yet the cheetah still survives quite well on the portion of kills left unstolen. And it is so dramatic that, for almost five thousand years, human potentates on three continents kept tamed cheetahs for sport hunting.

  • • •

  The possession of coursing cheetahs has been a self-flattering perquisite of royalty, in fact, for almost as long as the possession of gold. The Sumerian rulers used cheetahs on their hunts around 3000 B.C. The Pharaohs had captive cheetahs. So did the Assyrian kings. In the Caucasus a burial mound dated to 2300 B.C. has yielded a silver vase decorated with the figure of a cheetah—and this cheetah is wearing a collar. In Italy cheetahs were prized and collected from the fifth century onward. Russian princes hunted with them in the eleventh and twelfth centuries. Charlemagne enjoyed running cheetahs after game, as did William the Conqueror, and also Emperor Leopold I of Austria, who used his for deer hunting in the Vienna Woods.

  Marco Polo reported that Kublai Khan had a stable of cheetahs, possibly as many as a thousand, at his summer palace in Karakorum. Those animals were kept hooded and subdued like falcons until the moment of being released for the chase; when a victim was taken away from them by their human handlers and they were rewarded with the viscera, they seem to have accepted that bad bargain with the same equanimity as if they had lost the whole meal to a pack of hyenas. The later Mogul emperor Akbar also had a thousand captive cheetahs, according to an account left by his son. What Akbar’s son especially recalled was that, one time, a male cheetah slipped its collar and found a female to mate with, which union produced three kits who survived to adulthood. It was remarkable, said the son, because it never happened again.

  Indeed: That accidental litter in Akbar’s stables seems to be the only such case of captive breeding recorded from Sumerian times up to 1960. Though cheetahs can be chased to the point of exhaustion by horsemen and then lassoed, though they tame rather easily, they almost never (until recently) have been persuaded to reproduce in captivity.

  All those thousands and thousands of regal pets, all those hunting cheetahs, had been taken straight from wild breeding populations. And it was a one-way trip. Their genes came out of the reproductively active gene pool, and we can safely assume that very few ever went back.

  • • •

  O’Brien’s study reveals that those fifty-five members of the South African cheetah population are just too similar to each other, biochemically, for their own good. One startling bit of evidence was that their bodies failed to reject skin grafts traded surgically between different cheetahs—even their own immune systems couldn’t distinguish among them. Another form of evidence came from electrophoresis, a technique whereby genetic differences can be deduced through the measuring of small electrical differences among enzymes and other proteins. Again, by this standard, the South African population was found to contain “10 to 100 times less genetic variation than other mammalian species.” In addition, the male cheetahs had a drastically low sperm count and a drastically high proportion of abnormally shaped sperm in what they did have, two symptoms common among inbred livestock and inbred populations of lab mice. They also showed a disastrous vulnerability to disease (for instance, when eighteen cheetahs at a wildlife park in Oregon died suddenly from a virus that seldom threatens other cats). O’Brien and his colleagues concluded that “the catastrophic sensitivity of this genetically uniform species does provide a graphic natural example of the protection afforded to biological species by genetic variability.” Without that protection, the same sort of epidemic die-off that happened in Oregon could also strike the wild African populations.

  And still there’s the unanswered question: Where did those missing genes go?

  Possibly, alas, to the entertainment of seigneurial humans, back in the days when coursing cheetahs were such a vogue.

  The O’Brien group hypothesize what they call a population bottleneck in the cheetah’s recent (two hundred years) or less recent (two thousand years) past. This bottleneck could have been any situation where for one reason or another the number of breeding cheetahs w
as dramatically reduced—to a low point from which the population level subsequently recovered, but from which the gene pool so far has not. The American bison went through that kind of bottleneck during its near brush with human-caused extinction a century ago. So did the northern elephant seal at about the same time. The elephant seal survived in only a tiny population on one remote island off Baja, then rebounded prolifically when humankind stopped killing it, but today the seal still shows a severe shortage of genetic diversity. And if the California condor survives at all, squeezing through its present perilous bottleneck, it can look forward at best to the same sort of lingering genetic deficiency for a thousand years.

  The next big question in cheetah research, meanwhile, is whether the East African population (of the Serengeti and the high Kenyan plains) shares the genetic depauperacy of the South African group.

  If the East African population is genetically robust, then it will seem that the South African cheetah’s bottleneck was a recent and localized situation, possibly attributable to killing by skin hunters and ranchers of the Boer period. But if the East African cheetah is similarly impoverished, then the problem is likely much broader and much older.

  It might be as old as Xanadu and Karakorum, as old as the pyramids, as old as that silver vase from a burial mound in the Caucasus. And it might be equally the consequence of a certain implacable, greedy human impulse: the impulse, not only to admire the embodiment of beauty, but to capture and possess it.

  PROVIDE, PROVIDE

  The Gaia Hypothesis and Global Evolution

  An Englishman named J. E. Lovelock believes that he may have discovered the largest of all living creatures. This thing he has found is so incomprehensibly huge that almost nobody until now had even noticed it. Bigger and more advanced than the biggest dinosaur that ever slogged through Cretaceous marshes. Bigger than the biggest whale that ever came up for a gasp of air. It is not an extinct species, furthermore, but an organism that thrives today, a survivor throughout three and a half billion years. Lovelock calls this creature Gaia. He is talking about the Earth itself.

  He suggests that our planet, floating sublimely in space like the Star Child from Kubrick’s 2001, is a single great animate being.

  Lovelock has outlined the idea in various scientific papers (some of them coauthored with the distinguished biologist Lynn Margulis) published over the past dozen years, and developed it at length in a book titled Gaia: A New Look at Life on Earth. The name Gaia is borrowed from the ancient Greeks, who applied it to that goddess they knew also as Mother Earth. What Lovelock proposes is that “the Earth’s living matter, air, oceans, and land surface form a complex system which can be seen as a single organism and which has the capacity to keep our planet a fit place for life.” Of course any cold-eyed reader will note that “can be seen as a single organism” is a slippery formulation, and Lovelock never makes quite clear whether he means that to be a literal suggestion or just a useful metaphor. Elsewhere he calls Gaia “a vast being who in her entirety has the power to maintain our planet as a fit and comfortable habitat for life.” Being is another vague term that never receives precise definition, but never mind. The more solid and persuasive part of Lovelock’s idea—also the more significant part—is the second half. According to his Gaia hypothesis, earthly life has created its own required environment.

  The biosphere has to some extent dictated the physical and chemical conditions on Earth, rather than merely vice versa, he argues. Ever since life began, it has taken active measures to keep the planet livable. The ratio of gases in the atmosphere, the degree of salinity in the oceans, the acidity level of water and soil—all of these, says Lovelock, are selfishly controlled by the collective body of living creatures. Nothing is left to chance. Fluctuations are not suffered passively. Gaia provides for her own.

  If true, this is news indeed. One of the implications, as Lovelock points out, would be that the Earth has the ability to cure itself of catastrophic environmental disruptions—ozone depletion, the greenhouse effect, acid rain, even the poisonous aftermath of a nuclear war—as easily and routinely as the human body cures itself of a cold.

  Some people will find great satisfaction in that idea. Others will judge it a terrifyingly dangerous bit of optimism.

  • • •

  The atmosphere as we know it is highly improbable. It is not the one our planet should have. Based on the overall chemical composition of Earth, certain atmospheric gases that are rare should be far more common. Certain common gases should be far more rare. The law of entropy seems to be in abeyance. The only explanation for these anomalies, says Lovelock, is that life itself has taken a guiding hand.

  Carbon dioxide is one familiar example. Besides being a by-product of respiration by animals and plants, and of the burning of fossil fuels, this gas occurs naturally as a result of various nonbiological processes. By admitting and then trapping solar radiation, it plays an important role in helping the Earth hold heat. We have all heard the dire scenarios about how the increased level of carbon dioxide from all our fossil-fuel burning is raising the Earth’s temperature; a program of human restraint (or, alternatively, nuclear power) seems the only way we can avoid melting the polar ice caps. What we don’t usually hear about is the steadying influence of Gaia, against which our worst excesses and our noblest restraints might both be moot. Currently the concentration of CO2 in the atmosphere is roughly three hundredths of one percent; before humankind began burning coal, it might have been two hundredths of one percent. On the very same planet without Gaia, without any life, the expected CO2 concentration would be 98 percent.

  Without Gaia, the atmosphere would consist almost completely of carbon dioxide and steam. And the temperature of that planetary greenhouse would be about 600 degrees F. Gaia prevents such an unlivable situation, says Lovelock, by extracting CO2 from the atmosphere through biological processes. Plants and marine animals take it up in vast quantity, and a large portion of the planet’s supply gets buried away harmlessly as limestone and chalk.

  Oxygen concentration is also under Gaia’s control, in this case a rare gas being made artificially common. On an Earth without Gaia, according to Lovelock, free oxygen would account for less than one percent of the atmosphere. That’s probably how things stood three and a half billion years ago, at the dawn of organic evolution, when life seems to have first taken hold in the form of simple anaerobic creatures suited specifically to an oxygen-poor environment. But then about two billion years ago there came one of evolution’s most drastic transitions, the changeover to an oxygen-burning metabolic economy, which has provided greater supplies of chemical energy for a greater range of biological possibilities. Since that time, Gaia has worked to keep the atmosphere pumped up richly with oxygen. She does it (at least partly) by splitting CO2 in half during photosynthesis and burying the carbon away in forms such as peat, coal, and oil. In other words, through the life and death of plants.

  But too much oxygen would be worse than too little. If the level stood just a few percent higher than it does now, even the wettest rainforests would constantly be bursting into flame—because free oxygen is precisely what makes fire possible. Without some counterbalance to oxygen production (as it proceeds at the current rate), such a flash point would have been reached in just 50,000 years. Many millennia ago, that is, the biosphere would have burned itself to a cinder.

  Fortunately for life, there is a counterbalance. Free oxygen is constantly removed from the atmosphere by reaction with methane gas.

  And the methane gas is another product of Gaia. It is created at the rate of about a billion tons each year, exactly enough to hold oxygen at the optimal level. Just where does the methane come from? Thirty years ago one distinguished ecologist postulated that it came mainly from the farts of large animals. Well, yes, undeniably that is methane, produced by anaerobic bacteria performing their routine digestive function in the animals’ guts—bacteria such as the Escherichia coli we carry in ours. But a billion tons annua
lly? No, more likely we large animals account for only a small (if noisy) fraction. The major part, Lovelock explains, must come from free-living anaerobic bacteria at work on plant decay in the world’s seabeds and peat bogs and lakes and marshes. Methane, remember, is the same stuff we call “swamp gas.”

  • • •

  The Gaia hypothesis is a very complicated idea. It would have to be: It merely seeks to encompass all of ecology, biochemistry, chemical oceanography, tectonic geology, and atmospheric physics. Applying bits of information from each of these disciplines, Lovelock goes on to explain how soil and water acidity, ocean salinity, trace-element distribution, and a number of other crucial conditions are maintained within life-fostering ranges through the active intervention of Gaia. He even suggests that the spawning migrations of eels and salmon, from salt water to fresh, might be Gaia’s way of recycling washed-away phosphorus.

  Most evolutionary biologists would roll their eyes at that one. And it makes me wonder what other points in Lovelock’s grand hypothesis might set the biochemists, the geologists, and the physicists to rolling their eyes. Lovelock himself admits that his presentation of the idea—if not the idea itself—is inescapably contaminated with anthropomorphism and teleology. Personally, I can forgive anthropomorphism and, after checking a dictionary, I can even forgive teleology. But I have a different reservation about Gaia.

  Lovelock’s central tenet is that the Earth possesses a potent, cybernetic, and vastly underrated capacity to keep itself healthy. To heal itself, when its environment has been injured. To clean and restore itself, just as a human’s kidneys and liver clean and restore the blood. He is emphatically reassuring on this point. The worry over ozone reduction by aerosols is ridiculous, says Lovelock. The concern over greenhouse-effect warming is unwarranted, he says. All right, possibly that much is true. Possibly. The worst industrial pollution belched forth from smokestacks, puked forth into rivers, spilled forth onto seas is no more than a minor discomfort to Gaia, Lovelock tells us. The most egregiously toxic dumping amounts to nothing, he claims. This, I think, is rather more dubious. Lovelock even goes so far as to state that “a nuclear war of major proportions, although no less horrific for the participants and their allies, would not be the global devastation so often portrayed. Certainly it would not much disturb Gaia.”