The Monkey's Voyage

by Alan de Queiroz

  31When Alfred Russel Wallace argued in his “Sarawak paper” that species with close taxonomic affinities are usually found in geographic proximity, he was basically using geographic structure to argue for evolution—see Chapter One.

  32The land bridges themselves also eventually disappeared beneath the sea, but climate would have separated the Old and New World ranges of warm-adapted plants such as the woody legumes before the actual land connections were broken.

  33Renner, coincidentally, was in the Smithsonian group, mentioned earlier, that was stranded for ten days on the Cerro de la Neblina, the tepui where D. meristocaulis grows. Even though she was pregnant at the time, it apparently wasn’t a big ordeal for her. “We had plenty of sugar and coffee,” she says. (Renner, email to the author, October 4, 2011.)

  34Crisp et al. (2009) concluded that there have been more than one hundred overwater colonizations of New Zealand by plants. However, since I could not obtain comparable numbers for relict groups from that study, I am using the smaller compilation from Wallis and Trewick (2009). I tallied seed plants from the Wallis and Trewick study but excluded ferns and mosses, as these plants are generally seen as having great dispersal abilities and I did not want to “inflate” the dispersal list.

  35In a nutshell, they tried to come up with the best combination of vicariance (splitting of lineages because of splitting of areas), dispersal, and extinction of lineages to explain, for each group, how the particular evolutionary tree, with species found in the observed geographic areas, could have come about. Each vicariance, dispersal, or extinction event was assigned a cost, and the combination of those events that explained the evolutionary/geographic pattern and had the lowest total cost was considered “best.” Their method may have been biased to choose vicariance scenarios over dispersal scenarios, because vicariance events were given a cost that was much less than that of dispersal events, with the result that scenarios that relied on vicariance tended to be preferred.

  36This dichotomy between plants and animals is obviously an oversimplification that obscures a lot of variation in dispersal ability within those groups. For instance, some animals, such as certain kinds of dragonflies, spiders, and birds, are very adept at long-distance dispersal, and are thus “plant-like” in that sense.

  Fossils of elephants have been found on many islands, including Malta, Crete, Cyprus, Sulawesi, Flores, Timor, Okinawa, and the Northern Channel Islands off California. Through the 1970s, many researchers surmised that the existence of elephant fossils meant that these islands must have been joined to continents by land bridges at some point; otherwise, how could a large animal like an elephant have reached them?37 Such land connections were sometimes assumed even when nothing in the geological record seemed to indicate their existence.

  However, in 1980, a biologist named Donald Johnson, as part of his doctoral dissertation, compiled a large number of reports indicating that both African and Indian elephants are very good swimmers, thus calling into question the need for land bridges to explain the island species. Elephants apparently swim with a porpoise-like, lunging motion, using their trunks as snorkels, and can do so for hours at a time. One observer recounted how elephants in a Cambodian lake, being hunted for use as work animals, swam “wildly in every direction, their glistening black heads and bodies emerging like those of sperm whales,” and noted that they could swim for several hours or more. These elephants were harpooned through the ears and chained to the tops of trees emerging from the flooded landscape, producing a final spectacle of “elephants swimming in circles round every tree over a large area.”

  Johnson found many firsthand accounts of elephants swimming to islands very close to the east African and Sri Lankan coasts. Another report had seventy-nine elephants traversing parts of the very wide Ganges River, in one case swimming for six hours, resting on a sandbank, and then continuing for three hours more. Using swimming speeds taken from other accounts, Johnson estimated that these seventy-nine elephants covered more than five miles during the six-hour segment of their Ganges journey. The record distance for a swimming/floating elephant was considerably farther; an individual lost overboard from a ship off the South Carolina coast managed to reach land thirty miles away, in a heavy gale no less.

  37Most of these island elephants were pygmy species that probably evolved to be smaller in size after colonizing the islands. They represented instances of the “island rule,” whereby small mammals, such as mice, tend to get larger on islands, and large mammals, such as elephants, tend to get smaller.

  Chapter Eight

  A FROG’S TALE

  THE “YELLOW SNAKE” OF SÃO TOMÉ

  John Measey was looking for the cobra bobo. His six-foot-two frame was folded onto a tiny 125 cc motorbike, knees up to the handlebars, and he was riding around in the rainforests and cocoa plantations of São Tomé, an island off the west coast of Africa. The place seemed like paradise, with stunning forested mountains, beaches with turquoise water, and friendly locals who would engage Measey in philosophical discussions late into the night, despite his fractured Portuguese (Brazilian Portuguese at that, quite different from the Portuguese creole spoken on the island). And, best of all, there was the possibility, the certainty, according to Bob Drewes, of finding not just one but many cobras bobos.

  Measey, a young British biologist who was working at the Institut de Recherche pour le Développement (IRD) in Paris, had talked to Drewes at a scientific meeting in Kenya in April 2002. Drewes has been a curator of herpetology at the California Academy of Sciences in San Francisco for more than thirty years, but, despite his job location, is one of the world’s leading authorities on African reptiles and amphibians. He had recently begun a massive project on islands in the Gulf of Guinea, especially São Tomé and Príncipe (the two islands making up the country of the same name), bringing together experts on amphibians, reptiles, fishes, mammals, insects, spiders, sea slugs, corals, barnacles, echinoderms, fungi, diatoms, and plants to catalog the biota of the islands and, down the line, study its evolution. São Tomé and Príncipe rival the Galápagos in their assemblage of unique species, from the world’s smallest ibis to the world’s largest begonia. They do not rival the Galápagos in how well they’ve been studied, however; before Drewes and his colleagues started their work, very little research on the islands’ biota had been done since the nineteenth century. Correcting this oversight had become Drewes’s passion, a “mission from God,” as he says.

  8.1 John Measey. A legless, burrowing amphibian called the cobra bobo lured him to São Tomé and Príncipe and, eventually, to a study of chance, overwater dispersal.

  At the meeting in Kenya, Drewes lounged by the hotel swimming pool, beer in hand, and talked for hours about São Tomé and how easy it was to find the cobra bobo there, applying some of his missionary zeal to convince Measey to make a trip to the island. Cobra bobo means “yellow snake” in Forro, the island’s Portuguese creole, but the animal in question is actually a caecilian, a kind of legless, burrowing amphibian. That was the weak spot that Drewes was counting on: a snake would not have drawn Measey to São Tomé, but a caecilian, now that was something else entirely. Measey has a passion for caecilians and is one of a handful of biologists in the world who specialize in studying these obscure amphibians.

  8.2 Bob Drewes, director of a massive study of the unique biota of São Tomé and Príncipe, with a cobra bobo in hand. Photo by Dong Lin. Courtesy of California Academy of Sciences.

  After the meeting, Drewes continued his assault on Measey with emails about the cobra bobo. Eventually, maybe inevitably, Measey gave in and booked a flight to São Tomé. So there he was, six months after the Kenya meeting, puttering around on his undersized motorbike and making long treks on foot through the forest. He had a few troubles—he came down with an especially serious form of malaria, and his field assistant was briefly arrested for disobeyi
ng a police order—but he was not disappointed. Almost everywhere he went, he found the cobra bobo, Schistometopum thomense, which, being bright yellow, stood out against the dark soil. To most of us, these animals would look like giant earthworms, admittedly strange, but probably not worth a special trip to see. Measey, however, sounding like a lovestruck teenager, says he “had eyes for nothing else.” There’s a common frog on São Tomé, Ptychadena newtoni, that he would eventually have great reason to remember, but on this trip, P. newtoni and the other amphibians barely registered.

  Measey set up bags filled with earth and added some cobras bobos and soil invertebrates to see what effect the caecilians would have on populations of their prey, but all but one of the caecilians escaped, ruining the experiment. Measey didn’t seem to care that much. Just seeing so many cobras bobos was enough.

  When Measey got back to Paris, he couldn’t get something out of his mind, and it wasn’t the image of dozens of bright yellow worm-like amphibians slithering on the dark earth, or at least it wasn’t just that. It was the fact that São Tomé and Príncipe even have amphibians at all, including the cobra bobo. How in the world did they get there?

  To understand why this question intrigued John Measey, we first have to consider the deep history and geography of the Gulf of Guinea islands. That history begins with a series of volcanoes, called the Cameroon Line, running roughly southwest to northeast and crossing both the Equator and the west coast of Africa. Over the past 80 million years, these volcanoes have poured out lava at various regions along the line, now in one place, now in another. On the continent, that lava formed a discontinuous arc of highlands (some still containing active volcanoes), while, in the Gulf of Guinea, the lava piled up to form a set of four islands. Bioko is the largest of the four, the farthest north, and the closest to the African coast. Several kinds of amphibians live there, but their existence on the island is no great mystery. Bioko is separated from the mainland by a channel less than two hundred feet deep, so, during various ice ages, sea level dropped far enough to connect the island to the mainland; amphibians could have walked, hopped, or slithered over to the island at those times. The island on the southwestern end of the line, Annobón, presents no problem for amphibian dispersal either, because it doesn’t have any amphibians.

  The middle islands in the chain, Príncipe and São Tomé, are the enigmas for Measey and other amphibian biologists. These two islands are more than 130 miles from the African coast and are separated from the continent by seas more than a mile deep. They are true oceanic islands, meaning that, since their emergence from the ocean—some 31 million years ago in the case of Príncipe and about 13 million years ago for São Tomé—they have never been connected to the mainland. Amphibians had to cross that saltwater barrier to get to these islands, and therein lies the problem: amphibians are not supposed to be able to disperse that far across salt water. At least that was what Measey thought until he ran into the amphibians of the Gulf of Guinea islands. That was what Charles Darwin, among many others, had thought also.

  “Batrachians (frogs, toads, newts),” Darwin said in The Origin of Species, “have never been found on any of the many islands with which the great oceans are studded.” Darwin made this statement at a time when many British intellectuals had rejected the story of Noah’s Ark (and its implication that all species had dispersed from Mount Ararat) and instead believed that God had placed each species in its native range, an area where that species was especially fit to live. Buffalo were fit for roaming the plains of North America, tigers for stalking Asian jungles, and amphibians, well, each amphibian species was fit to live wherever it occurred naturally. Under this view, Darwin argued, the absence of amphibians from oceanic islands was a paradox. Why, for instance, should continental rainforests be teeming with frog species, whereas apparently similar island forests had no native frogs at all? One couldn’t argue that the island environments were unsuited for frogs, because many frog species flourished when introduced on islands. Instead, to rescue the “special creation” view, one had to imagine a quixotic God conjuring up continental amphibians left and right but deciding that these creatures just didn’t belong on remote islands. However, the absence of amphibians from oceanic islands made perfect sense if such islands were originally devoid of life and had to be colonized naturally from continents. Amphibians aren’t out there for a simple reason: they can’t get there. This was and remains a compelling argument for a naturalistic explanation of the distributions of living things. It’s also one step in a powerful argument for evolution, because, if island forms (all those various nonamphibians) originally came from continents, and yet are obviously distinct from any continental species, then descent with modification is implied, either of the island forms, the mainland forms, or both.

  But I’m digressing—although almost unavoidably, since it would be difficult to quote Darwin’s observation about “batrachians” without explaining the context. What we’re really interested in here is not Darwin’s argument for evolution, but one of the assumptions of his argument, namely, that amphibians are hopeless ocean voyagers, so dismal at making ocean journeys that all of the “many islands with which the great oceans are studded” are beyond their reach. In The Origin, all Darwin said by way of explaining this claim was that “these animals and their spawn are known to be immediately killed by sea-water,” a statement that isn’t strictly true but was good enough for his purposes. There is probably no amphibian that could survive a journey from, say, the West African coast to São Tomé or Príncipe if it were simply floating in the ocean. Still, there is a bit more to say about amphibians as poor oceanic dispersers.

  The problem, in a nutshell, is that the skin of most amphibians is very permeable to water. My adviser in graduate school, Harvey Pough, a herpetologist and physiologist, was fond of saying that a frog placed on a table would dry out as fast as an uncovered bowl of water. This means that for most amphibians, a long voyage on a dry raft is out of the question. Many kids who keep frogs and salamanders as pets (as my brothers and I did) have experienced the vulnerability of these creatures firsthand; if their pet gets out of its moist terrarium, it will often be found, perhaps only a couple of days later, as a dried-up husk of an amphibian. (The husk I particularly remember was a California newt.) Drifting in the ocean itself or on a raft with ocean waves breaking over it is no good, either, because, when immersed in salt water, most amphibians rapidly lose water by osmosis across the skin. In other words, like air, salt water quickly dehydrates them.

  There are amphibians on islands, to be sure, but for the most part, they seem to be the exceptions that prove the rule that these creatures are poor ocean travelers. If the amphibians are native, the islands are ones like Madagascar, Borneo, or the Seychelles, which were once connected to a continent. If the islands were never connected to a continent, the amphibians turn out to be introduced species. For instance, Hawaii has a poison-dart frog, a giant toad, and a handful of other amphibian species, but they were all introduced by humans, some to control insects and others accidentally. In short, when you find amphibians on an island, it’s usually easy to explain how they got there without having to invoke an implausible natural ocean crossing.

  As John Measey had learned, however, São Tomé and Príncipe seem to be real exceptions—they are islands that have never been connected to a continent and yet have amphibian species that are apparently native. The existence of these species contradicts Darwin’s confident assertion—there are native “batrachians” on some oceanic islands. More to the immediate point, they’re an affront to vicariance biogeographers, who have used amphibians and other hopeless oceanic dispersers as indicators that vicariance must have occurred. Specifically, the argument goes, if you find related, native amphibians on landmasses separated by salt water, you can assume that there was once a land connection between those regions. The history of connection and disconnection might have something to do with the movement of tectonic plates (tha
t’s why there are frogs and caecilians on the Seychelles with relatives in India), or rising sea levels (that’s why there are frogs and caecilians on Borneo with relatives in mainland Southeast Asia), but, one way or another, a formerly continuous distribution was broken up by the formation or spread of oceans or seas. In other words, the explanation for these cases is vicariance, not oceanic dispersal. However, if frogs, caecilians, or salamanders sometimes do cross saltwater barriers, then the assumption of vicariance, even in such apparently cut-and-dried cases, is not an entirely safe one. It would be yet another blow to the vicariance worldview.

  Without planning it, Measey had found himself thinking about a key case in the debate over dispersal versus vicariance.

  The case for the natural dispersal of amphibians to São Tomé and Príncipe wasn’t entirely convincing though, not yet. In a way, the extremeness of the case was both its strength and its weakness, what made it both hard to deny and hard to completely believe. It was extreme because of the number of amphibian species involved—six frog species plus the cobra bobo, all of them unique to the islands and, in total, requiring five independent colonizations. This was a strength because it was difficult to imagine that all of these species existed but remained undiscovered on the mainland; therefore, it was unlikely that all of them had been introduced by humans. And yet, if it was hard to believe that even one amphibian species could have reached the islands naturally, a moderate handful of such dispersals sounded borderline absurd. From the start, Measey was thinking that natural overwater dispersal was the answer, but he also figured that he needed more evidence to strengthen the case.