The Flight of the Iguana

by David Quammen

  Fine for her.

  Evidently this Gaia of Lovelock’s is as cold and as Olympian a bitch as any goddess that any man ever dreamed into being. She will endure as she has endured, she will provide as she has provided, she will cure herself of whatever damage humanity may inflict. Which is good, I suppose. It’s grounds for a certain stoic and hyperopic sort of satisfaction. Okay, well, whoopee. Life on Earth will continue, according to J. E. Lovelock’s hypothesis, whether humankind cooperates, or the contrary.

  But the question that nags me is this. When humanity’s earthly misbehavior has progressed to the point where even our farts can’t redeem us, won’t Gaia simply cure herself of Homo sapiens?

  THE FLIGHT OF THE IGUANA

  Evolution and Extinction in the Galápagos and Beyond

  Imagine a moment in the history of ideas: A young man stands on the coast of a tropical island, many miles and many years from his home, throwing an oversized lizard into the sea.

  The lizard swims back to shore. The young man follows this animal, corners it, catches it by its long muscular tail, and throws it again into the sea. Again the lizard swims back to shore. Still another repetition. Always the lizard swims straight back to that same stretch of rocky shoreline where the young man waits to catch it again, throw it again. The lizard is a strong swimmer but seems stubbornly disinclined to try to escape through the water. The young man takes careful note of that fact and, despite his homesickness, wonders why.

  “Perhaps this singular piece of apparent stupidity may be accounted for by the circumstance,” the young man eventually writes, “that this reptile has no enemy whatever on shore, whereas at sea it must often fall a prey to the numerous sharks. Hence, probably, urged by a fixed and hereditary instinct that the shore is its place of safety, whatever the emergency may be, it there takes refuge.” The young man of course is Charles Darwin, and the lizard is Amblyrhynchus cristatus, better known as the Galápagos marine iguana.

  The incident of the tossed iguana occurred in autumn of 1835, during Darwin’s brief stopover in the Galápagos Islands toward the end of his five-year trip on board the surveying ship Beagle. It was recounted in his popular book The Voyage of the Beagle, published a decade later—which was still fourteen years before The Origin of Species suddenly made Darwin the most famous and controversial biologist in the world. Today we all know about The Origin; we all know about Darwin’s great idea, that evolution has been produced by a process he called natural selection; we all know about how this idea was made vivid to him by the Galápagos finches, their different species having evolved to fit different niches on the various individual islands of the group. So we all know, or at least think we know, the biological significance of the Galápagos archipelago. These islands, volcanic nubs pushed up above the ocean surface five hundred miles west of Ecuador, constitute one of the shrines of modern science. We can tune in public television any night of the month, it seems, and find them gorgeously photographed, reverently explained. Giant tortoises, blue-footed boobies, finches with a spectrum of different beak shapes, exotic fauna and flora showing all manner of unique adaptations, Darwin’s visit, presto, theory of evolution—the syllogism has been polished by repetition. The incident of the tossed iguana, though, generally passes unmentioned.

  This is too bad, because that incident happens to be rather eloquent. It not only tells us about Charles Darwin the person, as he was in 1835—boundlessly curious, unsentimental about nature, doggedly systematic, groping, and yet in some measure still just a wealthy young remittance-man off on a round-the-world lark, riding horseback with the gauchos and throwing helpless lizards out to sea on a dull afternoon. It also hints toward a fuller understanding of the Galápagos Islands themselves.

  A century and a half after Darwin, in the course of my own modest pilgrimage, I find myself seated on a rocky Galápagos shoreline. This particular island is Santa Cruz, one of the largest and ecologically most rich. The stretch of coast where I sit, in a jumble of sun-heated lava boulders and salt-tolerant vegetation, is not far from the Charles Darwin Research Station. Across the bay, prickly pear cactuses transmogrified into tree form, tall and thick as oaks, stand in weird silhouette above black lava cliffs. At close range, I am surrounded by a dozen marine iguanas.

  One of these animals is a dominant male, looking resplendent with his crest of dorsal spines, his strong stubby face, his black skin mottled with orange and olive; there are also three or four adult females, and the rest juveniles. When I walked up and sat down among them, an hour ago, they paid me little attention. Four feet of distance seems to be all they require. Like most other animals on the Galápagos, the marine iguanas show an indifference toward human proximity that gets them labeled, perhaps misleadingly, with the word tame. Now we are all of us sunning. And I am wondering what might happen if I picked up the big male by his tail, swung him around carefully, and tossed him as far as I could into the surf.

  I have just reread Chapter XVII of The Voyage of the Beagle, and the question intrigues me. The most probable answer is this: I would be arrested.

  • • •

  Today the Galápagos Islands are an Ecuadorean national park, with strict regulations protecting the native wildlife and vegetation from being tampered with by the likes of you or me. Even Darwin himself would now need a research permit, before he heaved his first marine iguana into the brine or (another of his lighthearted experiments during that visit) climbed up to ride on the back of a giant tortoise. Some tourists are prone to the self-indulgent delusion that those regulations can reasonably be bent—surely there’s no harm in petting a “tame” sea lion or feeding bread crusts to a finch?—but it isn’t so. The Galápagos require extraordinary, uncompromised protection from human impact. They need that extraordinary protection for four reasons: 1) because they have already, in the past three hundred years, been pillaged almost beyond rescue; 2) because they hold precious significance in both our natural and our intellectual heritage; 3) because they now endure heavy traffic as a tourist destination; and 4) simply because they are islands.

  The first three of those reasons are, admittedly, tendentious and self-evident. The fourth is an intricate, fascinating matter of science.

  Islands are different. Evolutionary biology as manifested on islands is an exaggerated and specialized subcategory. The same general principles apply—there’s a competitive struggle for reproductive success, in the course of which those organisms best adapted to their environment are “selected” to perpetuate their genes—but, on an island, the application is so stark and unbuffered as to seem almost qualitatively distinct. For one thing, islands are generally poorer in species diversity than any equal area of similar habitat on a mainland. They have more than their share of unusual species, but fewer than their share of species overall. A corollary to that low diversity is that they are more fragile than the mainlands. Complexity translates to stability, for almost any ecosystem, and islands because of their isolation show biologically simplified communities.

  Being islands, therefore, the Galápagos are especially vulnerable. Being islands, they are also especially instructive. Being what they are, the Galápagos are both drastically unique and at the same time quite similar to most of the planet’s other islands.

  That paradox is part of the insular condition: All islands tend to harbor ecosystems that are full of bizarre features (a shared pattern), but each island or group of islands is bizarre in its own eccentric way.

  In the case of the Galápagos, so celebrated, so familiar, that paradox is commonly overlooked. Even a fairly eminent biographer of Darwin has declared: “The fame of the [Galápagos] islands was founded upon one thing; they were infinitely strange, unlike any other islands in the world.” The statement is true, almost tautological, and it’s also very misleading. The fuller truth is that most islands are infinitely strange, and unlike any others in the world.

  The Galápagos are broadly representative for the very fact of being so strange, so unique—and that
representativeness is what made them useful to Darwin. He might just as well have based his great insight upon a stop in Hawaii, or the Seychelles, or the Malay Archipelago, or Madagascar. And we might now talk about “Darwin’s lemurs” or “Darwin’s honeycreepers” instead of Darwin’s finches. He might even have recognized natural selection from a study of South American rodents—though an island experience made his task easier, because those simplified island ecosystems display the evolutionary process in a boldface, cartoonish version. He might, he might have, he might—but as it happened, he didn’t. Darwin didn’t go to Hawaii, he didn’t go to Madagascar, and he was not forced to derive his idea from the confusing patterns of faunal variation on the South American mainland. Fate and the Beagle’s survey itinerary brought him instead to the Galápagos.

  Where he found an iguana that swam like an eel and lived on a diet of seaweed.

  • • •

  “It is a hideous-looking creature, of a dirty black color, stupid, and sluggish in its movements,” Darwin wrote. Here I think he was a little unfair. In the morning of my second day on Santa Cruz, I am out on the black lava rocks of the shoreline again, with the marine iguanas, watching them as they gape stolidly out at the returning tide, and wondering what—if anything—is going on behind those inscrutable hooded eyes. Are these animals truly stupid and sluggish? Or are they just dignified and calm?

  Amblyrhynchus cristatus is the world’s only oceangoing lizard, an interesting distinction for several reasons. Like most other iguanas, it is a vegetarian, which means that it needs to feed longer and more voluminously for the same amount of nourishment as a carnivore gets from one protein-rich meal. Also like other iguanas, it is ectothermic (dependent on external sources of body heat) and lacking in stamina. Since there is no better way to sap a body of heat and stamina than by dunking it in seawater, the marine iguana seems to have chosen a strange path. It feeds on algae exposed at low tide or, more athletically, swims out into the sea and dives down to graze underwater. Herpetologists who have studied its physiology (which seems scarcely different from the standard iguana physiology, suggesting that its marine adaptation is purely behavioral) continue to sound puzzled that Amblyrhynchus can live its life as it does.

  The iguanas I’m watching this morning stand aloof from that scientific conundrum. They stare at the surf, beneath which their pastures of algae are now buried by the risen tide. They shift their positions as the sun shifts, as their bodies warm, and seldom otherwise move. I want to see them swim. But they don’t oblige. Occasionally the big male bobs his head, a quick series of three or four jerky nods that seem to say: Believe it, Jack, I’m the baddest dude on this piece of beach. At one point two females come face-to-face and blow salt out their nostrils at each other. Mainly they all bask. They are poised and opaque. They seem utterly indifferent to time’s passage, boat traffic nearby, the herpetological puzzlement they have inspired, and the large pink creature sitting among them again today with a ring notebook and sunburned ears.

  One hundred and eighty million years ago, while dinosaurs were flourishing on land, the seas also were full of reptiles. Not anymore. Nowadays there are still a few species of sea turtle and estuarine crocodile (both of which tend to be much larger than iguanas, one way of mitigating heat loss), and some sea snakes, but lizards that exploit the seacoast environment, in or out of the water, are rare. In the Philippines is a gecko that reportedly hunts crabs; on the island of Cerralvo, off California, is an iguana-type lizard that sometimes does too; on a Colombian island called Malpelo is a skink that preys on crustaceans in the zone between low and high tides; and on Nosy Bé, just off Madagascar, another intertidal skink. It’s no accident that these anomalies are all native to islands, where necessity and opportunity can be so exceptional. Amblyrhynchus cristatus is the only lizard, though, that clambers straight into the ocean for an underwater meal.

  After long patience, I am rewarded: I see a big male iguana come swimming past, six feet offshore in the churning surf. He parallels the line of the coast for a hundred yards, moving strongly, only his dark head showing. He surges through the water, neck craned, breasting along like a labrador in a duck pond. Then he swings away from shore to cross a narrow channel of open water, headed toward another part of the bay. Mesmerized, I follow his progress for ten minutes.

  To me this large swimming lizard seems a small miracle. But the other iguanas ignore him, and no one else is around.

  • • •

  In 1866 Joseph Hooker, a botanist and a close friend of Darwin, delivered a lecture before a British scientific society on the subject of island biology. Darwin himself had already given the subject some attention in The Origin of Species (of which one section is titled “On the Inhabitants of Oceanic Islands”), and Alfred Russel Wallace (mainly remembered as the co-discoverer, with Darwin, of natural selection) later published an important book called Island Life. But Hooker’s lecture, a less famous performance by a less famous man, is notable for having articulated a handful of factors inherent to the biology of islands. Those factors are still recognized as essential clues, not just for understanding evolution as it happens on islands, but for understanding how island oddities helped Darwin (and have continued to help biologists to this day) understand evolution as it happens everywhere.

  Among the points Hooker discussed were: impoverishment, disharmony, dispersal ability, loss of dispersal ability, size change, and extinction.

  Impoverishment was one of the factors that Darwin had already noted in The Origin: “The species of all kinds which inhabit oceanic islands are few in number compared with those on equal continental areas.” Even a tropical rainforest on an island like New Guinea, impressively rich as it may be, will not harbor nearly the number of different species as will an equal area of rainforest in the Amazon. Likewise, a square mile of island desert will contain fewer drought- and heat-tolerant species than a physically similar square mile in Arizona. This sort of impoverishment is closely entangled with the matter of disharmony.

  Disharmony means that the relative proportion of various species and groups of species—the profile of the ecosystem—will be different between an island and its most proximate continent. South America, for instance, has a glorious abundance of snakes and amphibians; but the Galápagos have only three native snake species, all from the genus Dromicus, and no amphibians whatsoever. South America is also full of terrestrial mammals; but until humans arrived (bringing dogs and goats and other forms of ecologic catastrophe), the Galápagos had no terrestrial mammals except a few species of rodent and bat. On the other hand, the Galápagos even today are exceptionally well endowed with different finch species. Disharmony.

  Good dispersal ability is common to the lineages—both plant and animal—that occupy islands. The ancestors of insular species had to be hardy travelers, after all, or they never would have arrived. Life forms didn’t just appear on islands (notwithstanding what the creationists want us to believe); they had to get there somehow, and that implies long pioneer crossings of salt water. Since salt water is inimical to the metabolism of most terrestrial and freshwater animals, since it also destroys the viability of many plant seeds, the lineages of flora and fauna that have established themselves on islands tend to be those that can fly great distances, or at least ride along in the feathers or the intestinal tract of a bird, or float passively on the air, or endure long periods of metabolic dormancy while being carried by currents to an island landfall. So on the Galápagos you find an abundance of ferns, whose spores are lighter than wind. You find frigate birds and albatrosses, capable of soaring endlessly with almost no effort. You find great tortoises, which can float clumsily but comfortably on the waves and go for months without food or drink. You find very few plants whose seeds are large and heavy. You find no freshwater fish. You find no frogs.

  Loss of dispersal ability, on the other hand, is something that often happens to island lineages after arrival. Hardy travelers though their ancestors may have been, the islan
d-dwelling descendants in some cases evolve toward being more earth-bound, sedentary, stranded. Birds lose the use of their wings, atrophied over generations into comical little flippers. Insects lose their wings entirely or (some beetles) find their outer wing cases fused shut so that the functional wings can’t be unfolded. Plant seeds lose the little parachutes or the burr-like hooks that originally helped them cross oceans. The Hawaiian archipelago, for example, harbors two hundred species of flightless beetle, plus a number of other flightless insects. On the island of Tristan da Cunha in the South Atlantic is a flightless moth and a flightless fly. And on the Galápagos is Halmenus robustus, a flightless grasshopper. Among birds the phenomenon is just as pronounced and maybe a bit more familiar. The dodo, of course; it was one species of a group of large flightless birds, all related to pigeons, that were native to small islands in the Indian Ocean. On Madagascar were the elephant birds, and on New Zealand the moas, huge and grounded and both now extinct. But New Zealand still has its kiwis and Madagascar also still has a few smaller flightless birds. Cassowaries in New Guinea, emus in Australia, and a different group of flightless (or nearly flightless) rail on each of twenty oceanic islands, including a species in the Galápagos. More prominent among the Galápagos bird life, though, is Nannopterum harrisi, the world’s only flightless cormorant. I could make the list longer—but what’s going on here? Why has evolution in island situations transformed all these fliers to pedestrians?