"Shrunken" Boas Pose Question: Nature or Nurture?

Brian Handwerk
for National Geographic News

Updated March 14, 2003

Nothing fascinates people quite like a snake—unless it's a really,
really big snake. One of the biggest is the boa constrictor (Boa
constrictor), the legendary non-venomous strangler found throughout
much of Central and South America.

But it's not the large size of boas that interests Auburn University herpetology graduate student Scott Boback. It's the smaller size of the boas found on certain Central American islands. In the Snake Cayes, a group of small islands just off the coast of Belize, boas grow to only a fraction of the size of their mainland relatives.

"You have snakes on the islands that are completely different in size," Boback told the National Geographic Channel. "The mainland snakes are at least twice as long and four or five times as heavy [as those on the islands]." The island snakes are no small fry, averaging some six feet (1.8 meters) in length, but their mainland relatives can grow up to 12 feet (3.6 meters) long or more. Why the discrepancy? No one knows, but Scott Boback hopes to find out. If he can, he might just shed some light on the evolution not only of reptiles—but many other island-dwelling animals as well.

Will Identical Diet, Habitat Produce Identical Size?

Boback's plan requires research subjects, willing or otherwise. That means the arduous collection of a lot of boa constrictors—specifically females. With his professor, Craig Guyer, and several research assistants, Boback has already gathered 16 female boas from both the islands and the mainland and brought them back to his research lab in Auburn, Alabama, to carefully watch them give birth.

It's the lab-reared generation of snakes that may help Boback to solve this "nature versus nurture" dilemma. He plans to study the offspring carefully to determine whether varying diets or genetics lie behind the differences in island and mainland boa constrictors. By feeding snakes from both locations identical food, raising them in the same environment, and carefully charting their growth, Boback hopes to learn more about the factors determining their differing sizes.

"The questions that I'm asking are critically important to understanding snake biology and evolution in general," he explained.

Island snake populations often exhibit different sizes than their mainland relatives. But the changes are hardly predictable. Some snakes are known to reach giant proportions in island ecosystems, while others take on smaller sizes. The discrepancies make it even more difficult to pinpoint the root causes of such tendencies.

For some time, the herpetological community has been aware of island boas such as Hog Island boas, Corn Island boas, and Crawl Caye boas. Some of these animals are prized in the hobby trade for their smaller and more manageable sizes.

The reptiles' size may be determined by the availability of their prey. Most boas prey on birds, small mammals, and lizards. In mainland Belize, mammals and iguanas are in plentiful supply. But that's not the case on the islands. Boback theorizes that the island boas could be smaller because their diet is composed chiefly of birds. While gathering specimens for his research on the Snake Cayes, he did notice the island snakes dining nocturnally on feathered fare.

"It's awesome to be able to finally see [them] out here at night foraging for birds that are sleeping in the trees," Boback told the National Geographic Channel. "This is really cool. It's really cool to finally see it."

Examining the Many Quirks of the "Island Rule"

The unique evolution of island species has challenged science since the time of Darwin. For years, biologists have recognized that island animal populations tend to acquire different sizes than their mainland counterparts. Examples of such dwarfism or gigantism include the giant tortoises of the Seychelles islands, Indonesia's Komodo dragons, and the boas of the Belizean Snake Cayes.

Both larger and smaller sizes have some innate advantages. Big creatures have a wider choice of food and tend to dominate others of their species. Smaller creatures, on the other hand, need fewer resources to survive—which may be important in the more limited ecosystems of many islands.

Perhaps the first attempt to explain these developments was a 1964 paper published in Nature entitled, "Evolution of Mammals on Islands," prepared by a young biologist named J. Bristol Foster. Foster's research led him to theorize that large mammals tended to evolve to smaller sizes after colonizing islands, and smaller mammals tended to grow larger. His generalizations became known as the "island rule," and were soon followed by the landmark 1967 book The Theory of Island Biogeography by Robert H. MacArthur and Edward O. Wilson.

Though it represented an important step into little explored scientific territory, the "island rule" is riddled with exceptions and unanswered questions. The same animals, for example, sometimes grow to large size on some islands but not on others. Certain islands also exhibit both gigantic and dwarf forms of various reptile and lizard species. The reasons for such variations are a matter of fascinating debate—and a promising field for many researchers like Boback.

The riddle of the Belizean boas may have its origins in the complex and unique ecosystems of the Snake Cayes, within the genetic makeup of the reptiles themselves, or perhaps a combination of both. Either way, snake wrangler Scott Boback hopes that his research can provide the key.

This adventure in the National Geographic Channel's Snake Wranglers series, The Boas of Belize, airs in the United States tonight, March 14, at 7.30 p.m. Watch snake researcher Scott Boback as he investigates the boa constrictor stranglers found on Belize's mainland and islands.