The Flight of the Iguana (24 page)

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 island-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?

Biologists suspect two possible causes. The first is absence of predation. Having left their natural enemies behind on the mainlands, island animals can
afford
to dispense with flight and devote their metabolic resources to other anatomical or behavioral needs. The second factor is that flightlessness tends to keep individual creatures (plant or animal) in the particular island's gene pool. Those seeds or birds or insects that can fly well are exactly the ones most likely to disappear, heading back out over the ocean; those that fly poorly are most likely to stay and breed. Neither of the two causes has been experimentally verified, but together they probably account for the widespread pattern.

Size change is another island phenomenon, this one especially familiar in the Galápagos context. Evolution tends to dictate that insular species be either bigger or smaller than their mainland relatives. Most species of land snails on Pacific islands are unusually small. The beetles of Hawaii are small. So are the beetles of St. Helena. Nosy Bé off Madagascar harbors a chameleon the size of an ant. By contrast, St. Helena also offers the world's largest (and therefore most disgusting) earwig, more than three inches long. Those extinct moas of New Zealand stood twelve feet tall, and the Madagascan elephant birds, not quite so tall but heftier, probably reached weights of a thousand pounds. (Sheer magnitude like that is no doubt another good reason they didn't fly.) And finally now we come to
Geochelone elephantopus,
the species that gives the Galápagos Islands their name and their famous image. “Galápagos” is the Spanish for tortoise, and no living thing represents this place so vividly as the giant tortoises.

The Galápagos tortoises are, like the Egyptian pyramids, a
marvel of which the dramatic effect can't be spoiled even by overfamiliarity. No matter how many zoo specimens you have seen, no matter how many photographs, these animals are still magically impressive in their native land. They are huge, they are gentle, and as a half dozen of them munch their way through a field of pangola grass in the highlands of Santa Cruz Island, they are astounding. What they are
not,
however, is incomparable.

Part of understanding island biology—and part of appreciating, therefore, the Galápagos—is to recognize that those Galápagos giants aren't alone. On the island of Aldabra in the Indian Ocean survives a related tortoise species called
Geochelone gigantea,
roughly the same size as the Galápagos animals and produced (insofar as biologists can guess) by the same evolutionary conditions. Having escaped mainland predators and mainland competitors, these island tortoises were free to become enormous, moving into the large-herbivore niche that elsewhere is occupied by moose, rhino, wildebeest.

Isolation and escape are what give island ecosystems their peculiarity. Isolation and escape are the preconditions that allow those other characteristic conditions (impoverishment, disharmony, the arrival of good-traveler lineages, the retention of bad-traveler offspring, changes of size) to appear. The isolation compels an inbreeding population to shape themselves, genetically, toward the new challenges of this new island habitat; and the escape from predators and competitors allows that same population a great latitude of experiment, transformation, aggrandizement. A tortoise grows huge. A finch takes on the life-style of a woodpecker. A prickly pear cactus stiffens and raises itself into a stout tree.

The exceptional becomes routine. An observer as keen as Darwin might even see an iguana fly through the air.

•   •   •

No sensible person travels all the way to the Galápagos just to sit on one pile of lava rock, staring at iguanas. And neither have
I. Leaving behind that stretch of coastline on Santa Cruz, I board a boat and spend seven days cruising around the archipelago.

To visit a number of these islands in brisk succession, taking note of the differences in vegetation and wildlife from one to another, the differences in relative concentration of those various species, the differences in physical terrain, is an essential part of the experience. Just as it was for Darwin: “I have not as yet [noted] by far the most remarkable feature in the natural history of this archipelago; it is, that the different islands to a considerable extent are inhabited by a different set of beings.” My own week of island cruising will pass too quickly, too full of amazing sights for adequate appreciation of each, but at least I have come prepared. Despite having already destroyed my binoculars (in a maneuver so clownish as not to bear recounting), I am equipped with a field guide, a snorkel mask with prescription lenses, a packet of seasickness medicine, a silly hat, a Royal Robbins shirt that washes well in salt water, and a paperback copy of
The Voyage of the
Beagle. Most helpful of all, I am accompanied by my own personal consulting biologist, with whom I share a fondness for homely arthropods and a wedding anniversary. She is here mainly to scout for new species of fiddler crab, but consents also to show an interest in birds, plants, reptiles. After a day at sea we reach Española Island, nesting site for virtually the world's entire population (twelve thousand pairs) of the waved albatross.

In a quiet bay off another island, Santa Fe, we see spotted eagle rays, their leopard-pattern fins swirling out of the water as they frolic (or possibly mate) near the surface. On the island called South Plaza we see land iguanas, fat mustard-colored lizards that are cousins to
Amblyrhynchus cristatus
but even larger, and that sustain themselves on this arid slab with a diet of prickly pear pads. On Tower Island, far up in the northeast corner of the archipelago, we see male frigate birds with their scarlet throat pouches inflated for mating display, and red-footed boobies
roosting in bushes, webbed feet wrapped clumsily over the thin branches. Just off the shore of James Island we witness a feeding frenzy of blue-footed boobies, falling out of the sky in formation to make their plunge dives—
whop, whop, whop, whop
—like a mortar attack hitting the water. On Seymour we see prickly pears that grow horizontally, resting their pads on the ground, an indulgence they can afford partly because Seymour has no tortoises to feed on them. And then already the boat tour, this dizzying pageant of exotica, is over.