Why does this happen? What factors encourage a species to alter its dimensions on islands? What, in short, determines whether a creature will get Brobdingnagian or Lilliputian?
The Island Rule
The first attempt to account for this apparent evolutionary roulette was made in 1964 by a young biologist named J. Bristol Foster. Fresh out of a doctoral program at the University of British Columbia, Foster published a brief but influential paper in the journal Nature entitled "Evolution of Mammals on Islands." Claiming "much confusion and contradiction" in the scientific literature over the size of mammals on islands, Foster undertook a survey of 116 insular (island-dwelling) species or subspecies living mostly off the coasts of western North America and Europe. He summarized his findings in the table below, which indicates whether an island critter is smaller, similarly sized, or larger than its presumed mainland ancestor.
The table reveals some interesting trends. Rodents tend toward gigantism, while carnivores, lagomorphs (rabbits and hares), and artiodactyls (deer, hippos, and other even-toed ungulates) are more likely to become dwarfed. Overall, amongst mammal species that colonize islands, big ones have a tendency to shrink while small ones are apt to enlarge. Biologists have come to call Foster's generalization the "island rule."
Foster went on to offer tentative explanations as to how at least some of these extraordinary transformations occurred. Islands, he argued, contain fewer species than mainlands and thus fewer numbers of both predators and competitors that might stress a newcomer. "In such situations," he wrote, "it appears that the larger rodent has an advantage."
But how to explain those species that diminish in size? Foster offered one possible answer, and for just one group, the artiodactyls. While rodents are able to control their populations in the absence of predators, hippos and deer and their kind cannot. As a result, artiodactyls are "especially susceptible," Foster felt, to exhausting food resources and occasioning malnutrition and stunting in their young. If, in succeeding generations, smaller individuals met with greater reproductive success, then eventually evolution might begin to favor them, leading to dwarfism.
Foster's modest paper represented, as David Quammen writes in The Song of the Dodo, "a sort of prerevolutionary innocence, standing on the distant side of a major upheaval." That upheaval was the 1967 publication of The Theory of Island Biogeography, by Robert MacArthur and Edward O. Wilson. The book not only launched an entirely new field of scientific endeavor—the study of how insular plants and animals got to be where they are today—but spurred a host of young biologists to tackle the gigantism/dwarfism question.
Refining the Island Rule
One of those young scientists was Ted Case. Like Foster, Case published a seminal paper early in his career (1978) in a leading scientific journal (Ecology). But his paper was much longer—18 pages compared to Foster's two—and, against Foster's lone table, it offered numerous graphs, tables, and complex mathematical equations. Case acknowledged Foster's pioneering work, then pointed out where his predecessor's analysis came up short. Perhaps most notably, Case, who studies an iguana-like reptile known as the giant chuckwalla, stressed glaring exceptions to the island rule: how the same lizard or rodent could be relatively large on some islands but not on others, and how one island may have gigantic forms of one type of lizard or rodent and dwarf races of another.
As an example of the latter scenario, Case cited the curious instance of two rattlesnake species that cohabit Angel de la Guarda, one of the sun-baked desert islands in the Gulf of California where he studies the chuckwalla. On the nearby Mexican mainland, Crotalus ruber is roughly twice the size of C. mitchelli, but on Angel de la Guarda, the situation is exactly reversed, with C. mitchelli about two times as big as C. ruber. How did this happen? Judging from a close look at the two species, C. mitchelli appears to have diverged more from its mainland progenitor than has C. ruber, Case says, which implies that C. mitchelli arrived first on Angel de la Guarda. In order to make use of all available prey, C. mitchelli went in for a larger size. When C. ruber finally reached the island, it found the big-rattlesnake niche already taken. So it had to accept the little-rattlesnake niche, and it evolved a smaller frame to do so.
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Bearing such anomalies in mind, Case concluded in his paper that "[a]ny theory proposing to account for these insular size trends must also be consistent with their numerous exceptions." In those 18 pages of small print, Case elaborately spun out such a theory. The chief factor that underlies all modifications in body size, he argued, is the net amount of energy that an animal can gain in a given amount of time. All changes proceed from that, yet all manner of other factors also come into play to decide whether its kind shrinks or grows in size.
And, practicing what he preached, Case allowed for exceptions to his modification of Foster's rule. For example, if an ever-increasing size in species undergoing gigantism eventually were to interfere with, say, that creature's ability to fly (in birds) or climb (in geckos) or burrow (in rodents), then that species would enlarge only up to the point at which these other factors become of overriding importance.
Blazing the Trail
The trail that Foster and Case began cutting decades ago has been further cleared by a growing band of biogeographers. These workers, too, have come up with generalizations, and they, too, grant exceptions. Like any scientists in a nascent field, they still vociferously argue the particulars, but most agree on some general conclusions.
Large size, for one, appears to confer a number of selective advantages. Bigger creatures can exploit a wider range of resources; larger predators, for example, can feed on big as well as small prey. Because of that ability, they can give birth to larger litters or clutches. Heftier individuals tend to dominate others of their species in territorial skirmishes and other conflicts over resources. And because they have greater stores of energy and water, they can better survive famine and drought.
Small size, too, has its advantages, however. Smaller animals need fewer resources to survive and reproduce. That's important on islands, where resources are more limited than on continents. They are more efficient at absorbing nutrients and energy. They can hide from predators in tighter spaces. And they can better cope with stressful environmental conditions.
Researchers have made other discoveries in their pursuit of answers as to why islands breed giants and dwarfs. For one, size changes can occur with astonishing speed. In a mere 6,000 years after it found itself isolated on Jersey, one of the Channel Islands 15 miles off the coast of France, the red deer dwarfed to one-sixth its size on continental Europe. The Wrangel Island mammoths went from six tons to two tons in just 5,000 years. In a 2006 examination of the published literature, paleontologist Virginie Millien confirmed what many biogeographers had suspected—that island species evolve faster than mainland species, particularly over shorter time intervals of years to thousands of years.
Another finding: For mammal species arriving on islands, a certain body size exists above which their body sizes tend to decrease over many generations and below which they're apt to increase. That size is just under nine ounces, about the weight of a red squirrel. (Median size of mammals diminishes as land area increases; thus, in Madagascar, it is a tad over eight ounces, in Australia 7.6 ounces, and in North America just under three ounces.)
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Today, human beings, as they're wont to do, can throw a wrench into the works. While studying evolutionary trends in Australian mammals, Tim Flannery, a research scientist at the Australian Museum, identified something he calls "time dwarfing." Humans are thought to have first colonized the island-continent 40,000 to 60,000 years ago, and Flannery reported that over the past 40,000 years, body sizes of most Australian marsupials have decreased. He hypothesizes that aboriginal people, out to maximize their harvest of meat, probably hunted larger species, and larger individuals within those species. Over time, this would have diminished the fitness of relatively large individuals, Flannery posits, causing dwarfing in surviving species.
Despite all the work in the three and a half decades since Foster first took an intellectual machete to the tangle of questions surrounding the gigantism/dwarfism question, much awaits illumination. As biologists James Brown and Mark Lomolino conclude in their classic textbook Biogeography, "the generality of the island rule and its corollaries ... remain promising areas for future studies."
New studies might also help clear up certain evolutionary conundrums. No one knows, for instance, whether the Seychelles giant tortoise became humungous before or after it arrived in the archipelago. No one knows why island-dwelling bears show only a slight degree of dwarfism despite their bearish build and carnivorous habits. And no one knows why ducks tend toward dwarfism. Many birds in evolutionary history have become gigantic (and flightless)—the great auk, the ostrich, the elephant birds of Madagascar. Why has evolution never produced a giant flightless duck? "A question," muses Quammen, "to lie awake over."
One of the most intriguing enigmas comes from Flores, an island in Indonesia. It concerns the Komodo dragon, which lives on Flores as well as on nearby Komodo. By all appearances, this voracious monitor lizard represents an archetypal case of gigantism. Thought to have grown huge on a diet of Stegodon, an extinct elephant-like creature that became a pygmy on Flores, the Komodo dragon today can grow up to 10 feet long and weigh 330 pounds; it can drag down and devour deer, water buffalo, and people. Yet while it is the world's largest lizard, a much heftier monitor lived in Australia during the Pleistocene—a 23-foot-long, 1,370-pound monster.
Could the Komodo dragon, the most fiercesome lizard on Earth, actually be an island dwarf? Biogeographers: Break out that machete.