Gigantism and Dwarfism on Islands
by Peter Tyson
Why are the "little people" of Flores so little? Were their
ancestors originally much bigger and diminished in size over
many generations due to competition for limited resources and
other factors? If so, H. floresiensis, as the hobbit is
known scientifically, would represent the first known instance
of this tendency happening in our close relatives—the
tendency, that is, for certain animal species that colonize
new islands to become either dwarfed or gigantic.
Island giants are aplenty: Komodo has its dragons. Madagascar
has its giant hissing cockroach. Until about 1,000 years ago,
New Zealand had its colossal bird, the moa. Of dwarves, the
world has witnessed everything from foxes, rabbits, and snakes
that are smaller than their mainland counterparts, to that
ultimate oxymoron, the pygmy mammoth, which once existed in
various forms from California's Channel Islands to Wrangel
Island in the Siberian Arctic.
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.
This table from Foster's paper shows relative sizes of
island-dwelling mammal species and subspecies as compared with
their presumed mainland relatives.
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.
Why has evolution never produced a giant flightless duck?
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.
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.)
Could the Komodo dragon actually be an island dwarf?
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.
Nagging questions
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—and
it doesn't concern the hobbit. Rather 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.
Further Reading
