Some tiger snakes on islands grow to be twice their normal size because they… (Fabien Aubret / French National…)
Tiger snakes on some islands around Australia have evolved to reach twice the normal size of their mainland counterparts, while those on other islands have not or have even gotten smaller, a finding that has perplexed some researchers. Biologists have generally argued that comparisons between island and mainland species should be based on adult size, but a French herpetologist now argues that it is birth size that is most important -- at least for snakes. If the available prey on an island is larger than their normal prey, then those snakes born with bigger mouths -- required because the reptiles ingest their food whole -- have the best chance of surviving.
British biologist J. Bristol Foster first formulated what is known as the Island Rule in 1964. Studying pygmy elephants and giant rats, Foster concluded that big animals isolated on islands tend to get smaller because of limitations in food supply, while small animals tend to get bigger because there are fewer other species to prey on them. Many exceptions to the rule have since been noted, and the situation is clearly more complicated than that simple formulation suggests. Most researchers, however, have considered only adult body size when attempting to produce generalizations of the rule.
Herpetologist Fabien Aubret of France's National Center for Scientific Research in Saint-Girons and the University of Sydney in Australia decided to test the importance of infant size by studying tiger snakes. On the mainland, these venomous reptiles grow to a maximum of about 35 inches long. They live in swampy areas and subsist primarily on frogs. But some of the snakes have also been isolated on nearby islands for as long as 10,000 years due to rising sea levels. Some specimens from that population are as much as 70 inches long, while others are shorter than normal.
Aubret toured 12 islands around the continent and measured 597 adult snakes. He released the males and nonpregnant females and brought 72 pregnant snakes back to the lab. After the snakes gave birth, he measured each of the 1,084 babies they produced, then compared mouth size at birth to the size of available prey on their island of origin. He reported in the journal American Naturalist that mouth size at birth was the key indicator of how large the snakes would grow.
On islands where the primary prey of snakes was birds ranging in size up to sea gulls, the snakes were born with larger mouths. Simply put, if they did not have large mouths at birth, they would be unable to eat and could not survive, a situation that favors snakes born with big mouths and leads to evolutionary change. On islands where the main prey was small lizards, however, the snakes did not require large mouths at birth and tended to be smaller. In this case, food supply appears to be the limiting factor.
"This study confirms that adult size variations on islands may be a nonadaptive consequence of selection acting on birth size," Aubret said. "Animals may become either giant or dwarf adults on islands for the simple fact that they were born either unusually large- or small-bodied."