The Song of The Dodo, page 14
That confused record of Geochelone in the Indian Ocean is valuable for what it says about evolution. It says, among other things, that the Galápagos Islands are not so extraordinary after all.
Though the marvels of Galápagos biology are an old subject by now, familiar to anyone who can find PBS with a remote control, those peculiar finches and iguanas and tortoises have become widely renowned without being well understood. People have been led to believe that the Galápagos archipelago is somehow mystically different from other places—different even from other oceanic islands. One author of a popular book about Darwin and his Beagle voyage, Alan Moorehead, has given strong but not atypical voice to that misleading view of the Galápagos: “The fame of the islands was founded upon one thing; they were infinitely strange, unlike any other islands in the world.” No. The real significance of the Galápagos is almost exactly opposite: their fundamental resemblance to other places. They are representative. They are prototypically ordinary. What made them instructive to Charles Darwin, and what makes them instructive to us, is that they are strange in precisely the same ways that other islands tend to be strange.
Islands in general are biologically anomalous. The Galápagos, being anomalous, conform to pattern. They are unique and therefore they are normal.
The clearest evidence of this truth is that you don’t have to go to the Galápagos to find a native population of giant tortoises. You do have to go to an island. Several centuries ago there was an array of alternatives, including Mauritius, Rodrigues, and the granitic Seychelles. But today you would look toward a place called Aldabra.
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THREE DISCRETE groups of giant tortoises survived at the start of the sixteenth century. The three groups together accounted for more than a half-dozen species. They all grew as big as bears and they were all closely related, assignable to the genus Geochelone. In addition to those three sets of giants, the same genus also encompassed a number of smaller tortoises native to the South American mainland, Africa, Madagascar, and southern Asia. The giants, by no coincidence, were entirely confined to small islands.
One group inhabited the Galápagos. Another group lived on the Mascarene Islands of the Indian Ocean, the cluster of forested volcanic nubs that includes Mauritius, Rodrigues, and Réunion. The third group occurred in the Indian Ocean too, but remote and distinct from the Mascarene animals. This third group was scattered across seven hundred miles of oceanic wilderness, from the granitic Seychelles southwestward, beyond Farquhar, beyond Cosmoledo, to the tiny coral atoll known as Aldabra. Aldabra sat lost in its own singularity, far off the coast of Tanzania.
Aldabra was nowhere, hundreds of miles from anywhere. It was not a destination or even a stopover point on any preferred sailing route. It consisted of a low ring of limestone, a corona of coralline sand, and some scrub vegetation, all surrounding a shallow lagoon. The noon sun was blistering and the dry season was longish and grim. Not many kinds of animal could live there. With no high land, no decent harbor, hardly any food or fresh water or timber that might attract human voyagers, it was the most desolate and unapproachable of all tortoise islands. Unlike some other small oceanic outcrops, it didn’t even contain guano resources worth harvesting. Scarcely anyone coveted Aldabra, scarcely anyone visited. So its tortoises survived. By the start of this century, all other giant tortoises of the Indian Ocean were virtually extinct. Some of those other populations may have left a few hybrid offspring, in translocated meat herds or as mascots held captive at botanical gardens, but they were gone from the wild. On Aldabra, meanwhile, a real population of giant tortoises persisted.
The Aldabran population belongs to the species that taxonomists now call Geochelone gigantea. It’s probably the same species as those lost populations from the granitic Seychelles. Down in the Mascarenes, by contrast, each island seems to have harbored a distinct species, and in some cases there were two distinct species coexisting on the same island.
Mauritius had Geochelone inepta and Geochelone triserrata. The anatomical differences between these two species were subtle; the ecological differences can only be guessed at. G. inepta may have been more tolerant of dry coastal habitat, while G. triserrata may have favored the wetter zones of the island’s interior. Both species seem descended from the same ancestral species, possibly having arrived on Mauritius in two separate episodes of colonization. Each colonization may have been accomplished by just a tiny number of individuals, maybe only one—a single pregnant female who found herself washed off a beach in Rodrigues, say, and floated passively across to the neighboring island. Giant tortoises are capable of oceanic crossings, riding the waves like an inflatable raft, head up, patient, enduring days or even weeks adrift. Maybe the Rodrigues population was itself descended from colonists who, centuries earlier, had been washed off a beach in Mauritius. Such back-and-forth colonization is what makes the patterns of evolution on archipelagos so complicated.
Still, it’s safe to say that the tortoises of Mauritius, Rodrigues, and Réunion were all closely related. Taxonomists now lump them within a single subgenus, Cylindraspis, but I prefer to think of them simply as the Mascarene cousins. Various factors contributed to their evolutionary history on the Mascarene Islands, of which one was crucial: absence of humanity. Each of those three volcanic nubs was spared the presence of Homo sapiens until just a few hundred years ago. By some quirk of chance (probably related to distance and to the prevailing currents of water and wind), no ocean-voyaging humans had arrived there during earlier millennia. While the islands of Polynesia and Melanesia were harboring rich Neolithic cultures, while places as remote as New Zealand and Hawaii and Easter Island were being colonized by adventurous men and women in canoes, the Mascarene Islands (like the Galápagos) remained uninhabited. That lack of two-legged predators allowed the tortoises to survive and evolve.
But it couldn’t last. Portuguese ships touched at Mauritius as early as 1507. In 1598 a Dutch expedition landed, beginning an era of frequent contact, and by 1638 there was a Dutch settlement on the island. Eighty years later Mauritius was taken over by the French. The French, the Dutch, and the Portuguese all seem to have used the place as a resting point during long voyages, and as a larder. The Portuguese introduced goats, pigs, and chickens, presumably hoping those meat animals would go feral and multiply; they or the Dutch, by accident, also introduced rats; cats too were introduced, probably by the Dutch during their settlement period in a misguided attempt to control the rats. Nobody knows who introduced monkeys, and nobody can fathom why. In addition, the Dutch slaughtered native animals, most notably dodos and tortoises. Seagoing explorers in those days spread ecological mayhem without dreaming it made any difference, and they ate what they found.
A visiting Englishman in 1630 was impressed by the size of the tortoises, “so great that they will creepe with two mens burthen,” but he was less impressed by their culinary appeal. He called them “odious food,” and in light of English cuisine he presumably knew odious when he tasted it. The Dutch seem to have found tortoise meat tolerable as expeditionary fare. The French, of course, found ways to make delicacies of what the Dutch took for granted and the English despised. French settlers on Mauritius butchered thousands of tortoises, salting the flesh or rendering it for fat. This was before the world’s navies and whaling fleets had made a discovery that doomed Geochelone further: Giant tortoises could be stored alive. Their reptile metabolism and their behemoth endurance allowed the animals to linger for months, without food or water, in the hold of a ship. To prevent wandering and induce stoic surrender, they could be turned upside down. Their physiological dormancy made up for the lack of meat freezers.
That phase of exploitation came later. A more immediate style of butchery prevailed during the seventeenth century and into the eighteenth on Réunion and Rodrigues as well as Mauritius. One traveler reported from Réunion: “The Land Turtles are also some of the Riches of the Island. There are vast Numbers of them: Their Flesh is very delicate; the Fat better than Butter or the best Oil, for all sorts of Sawces.” Another witness, a Frenchman who spent perhaps too many weeks on Rodrigues in the course of an expedition, remembered “soupe de tortue, tortue en fricassée, tortues en daube, tortues en godiveau, oeufs de tortue, foie de tortue,” and grumbled that everything he ate seemed to be only more tortoise stew. By about 1780, the tortoises of Rodrigues, Réunion, and Mauritius were reduced to the point of extreme rarity, most of them having been eaten. Within another generation they were extinct in the wild, possibly extinct altogether. No one knows the fate of the last purebred Geochelone triserrata, or of the last ghost of the species preserved in a translocated hybrid. In 1836 Charles Darwin himself visited Mauritius, on his homeward journey aboard the Beagle. With his Galápagos experience so recent, giant tortoises must have been fresh in his mind. If he had spotted any in Mauritius, presumably he would have said so. But there’s no mention of a Mauritian tortoise in his Journal.
The Aldabra-Seychelles species, Geochelone gigantea, was not quite so closely related to the Mascarene cousins as the cousins were related to each other. Probably G. gigantea had descended from a separate branch of the genus, and taxonomists now place it within its own subgenus, Aldabrachelys, another piece of expendable nomenclature that you have my blessing to forget instantly. The subgenus also includes some giant tortoises that once lived on the great island to the southwest, Madagascar. Those Madagascan species have been extinct for some time, possibly several millennia, and are known today only from shell remains, which look very similar to the shell of G. gigantea. So it may be that this species originally colonized Aldabra along sea currents from the north coast of Madagascar. Its distinctness from the Mascarene group shows in a few curious traits of skull structure. G. gigantea has a more pointed snout. It has a peculiar arrangement of nasal passages, in which a flaplike ridge seems to stand in defense of the olfactory chamber, leaving a clear channel from the nostrils to the esophagus. The ridge could support a fleshy valve (though apparently no scientist has dissected an Aldabran tortoise to find out), which might seal off the olfactory chamber at will. Those structural traits, combined with the field observation that G. gigantea sometimes draws water in through its nostrils, suggest an interesting possibility: Maybe the species is adapted to survive droughts by quenching its thirst in deep narrow potholes—stretching its neck down, drinking through its nose. The compacted coral rock of Aldabra happens to be pitted with deep narrow potholes.
Droughts were part of its evolutionary experience, but nothing prepared G. gigantea to survive the arrival of humans. Aldabra was protected by its remoteness; the other sites were more accessible, so the other populations were more vulnerable. In 1609 an expedition discovered the granitic Seychelles and their most conspicuous species. A participant in that expedition, one William Revett, recorded “lande turtles of so huge a bignes which men will think incredible; of which our company had small luste to eat of, being such huge defourmed creatures and footed with five claws lyke a beare.” He meant that the tortoises, not the members of his company, were “huge defourmed creatures,” I think. Revett was another squeamish Englishman whose distaste for reptile meat wasn’t shared by the people who came later. Although the Seychelles didn’t support a permanent settlement until 1778, by the end of that century the colony’s biggest export was tortoises. G. gigantea turned out to be just as toothsome as the Mascarene species. In fact, a large portion of the exported animals went to Mauritius, where folks maintained a traditional hunger for tortoise but had nearly exhausted their own supply. Customs records from Mauritius testify to shiploads of tortoises brought in from the Seychelles during the peak of the trade, totaling five thousand animals or more. Additional thousands were taken off the Seychelles by naval ships. Making matters worse, cats and rats had come ashore with the humans, and those animals preyed on tortoise eggs and hatchlings. G. gigantea soon disappeared (at least as a wild and unhybridized species) from the granitic Seychelles. Big shipments of tortoises were still passing from the Seychelles to Mauritius in the early nineteenth century, but by then the Seychellois tortoise mongers were just middlemen. The granitic Seychelles had become an import destination, and the imported tortoises must have come from Aldabra; they could only have come from Aldabra. In this ocean, there was no other remaining source.
A further threat arose around 1870, when someone got the notion to lease Aldabra for woodcutting. Severe habitat loss, combined with direct harvest of animals, probably would have extinguished the Aldabran population. Even without the woodcutting, that population was already badly depleted—as indicated by the fact that in 1878 a party of sailors spent three days hunting and found only one tortoise. Scientists back in England grew so concerned over what they were hearing that an illustrious group, including Charles Darwin and Joseph Hooker, signed a letter to the governor of Mauritius asking that Aldabra be protected. Some protective measures were put into force, but the tortoise population continued its decline. Rats and cats had infested the atoll by now, raising the mortality rates of tortoise hatchlings and eggs, and those rates had been naturally high under even the prior conditions. A naturalist who visited just before World War I, spending four months camped there, found the tortoises scarce and concluded that “it would be possible to live for years on Aldabra and never see a specimen.” The reason, he felt, was simply that they were shy and retiring. The governor of the Seychelles was less upbeat: “No plan will effectively prevent the final extinction of these curious survivals in a wild state in their natural habitats.” Not long afterward, as though to prove the governor wrong, the tortoise population began a comeback.
That trend has continued during this century. From its historic low, the tortoise population on Aldabra has increased dramatically. Why? Well, they were left mostly alone, protected from the meat harvesters, their scrub-vegetation habitat still intact. Probably they were also helped by the arid, severe conditions of the atoll, which discouraged a population explosion of cats or rats. After a passing threat in the 1960s that the British government might put an air base on Aldabra, and a public outcry against that bad idea much like the outcry that Darwin had joined earlier, the Royal Society of London assumed protectorship of the atoll. An Aldabra Research Station was built; other developments were kept out. In 1976 Aldabra was incorporated within the Republic of Seychelles, which had emerged as an independent nation, and in 1981 the Seychelles government designated Aldabra a special reserve. According to recent reports, the population of G. gigantea is doing well.
Possibly it’s doing too well. About 150,000 tortoises now live on Aldabra, and they may be destined for a natural crash in the near future, when they have succeeded in disastrously overeating their own resources.
The story of G. gigantea is complicated, but at its core is one simple fact: geographical isolation. In the case of Aldabra, that isolation is extreme. And it isn’t measured only in mileage. In the era of the shrinking planet, Aldabra’s margins of isolation haven’t shrunk. Its remoteness is uncompromised. Today you can board an airliner in Miami and step off within hours in the central Amazon; you can fly out to the Galápagos, connect with a cruise ship, and wander the individual islands on the coattails of a licensed guide; you can book an “adventure travel package,” that oxymoronic commodity, to Antarctica; you can even reach Krakatau, cinderlike Krakatau, for a handful of rupiahs paid to a Javanese fishing-boat captain. Aldabra, no. Not so easily attained. Aldabra stands in a different category. It’s the place you can’t get to from here.
If you’re a scientist—or even just a plausible scientific journalist, so I’m told—you might talk your way onto the biennial Aldabra expedition of the Smithsonian Institution, and have the privilege of spending a sun-stricken month watching serious people collect polychaete worms from the potholes. Alternatively, you might charter a somewhat reliable boat in the granitic Seychelles and venture out on the seven-hundred-mile sea journey toward Aldabra, which your compass-guided Seychellois captain might succeed in finding, with luck, though perhaps not on the first try. That charter would cost about ten thousand dollars, I’m told; and when you did reach the atoll, if you did, you might not be legally entitled to step ashore. Having heard all these things, I elect not to test my own plausibility or my own luck.
The published record is obscure but it’s substantial, and this one I’ll take on faith. I can content myself with having seen G. gigantea in a botanical garden in Mauritius. The wild tortoises of the Indian Ocean have been harried enough. Let them have their refuge, their privacy, on that one little desolate atoll. If Aldabra is the epitome of isolation, I figure, then what better way to highlight that fact than by staying the hell away from it myself?
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GEOGRAPHICAL isolation is the flywheel of evolution. It’s also a controversial subject, and has been for more than a century. The controversy has developed along with the development of biology itself, as classic Darwinian evolutionism became fused with paleontology and twentieth-century genetics, resulting in an amalgam of theory known as the modern synthesis, which has lately been further enriched by molecular biology. In recent decades, geographical isolation has been variously considered (1) extremely important to evolution, (2) surpassingly important to evolution, or (3) just flat-out essential. The parties to this controversy spend much of their zeal brandishing data from islands.
Ernst Mayr, a German-born ornithologist and taxonomist based at Harvard, stands as the preeminent authority on the subject. Mayr was born in 1904 and by the late 1920s had made his first ornithological expedition to New Guinea. His career has been long and wide. He is one of the most respected biologists of our time, author of an influential book titled Systematics and the Origin of Species (published in 1942), and a co-creator of the modern synthesis that brought genetics and Darwin together. He holds an unfair advantage over virtually everyone else in the debate about geographical isolation because, besides being a researcher and a theorist with his own strong opinions, he is also a historian of science. Still more unfair, he’s a good writer.
