Some populations of tiger snakes stranded for thousands of years on tiny islands surrounding Australia have evolved to be giants, growing to nearly twice the size of their mainland cousins. Now, new research in The American Naturalist suggests that the enormity of these elapids was driven by the need to have big-mouthed babies.
Mainland tiger snakes, which generally max out at 35 inches (89 cm) long, patrol swampy areas in search of frogs, their dietary staple. When sea levels rose around 10,000 years ago, some tiger snakes found themselves marooned on islands that would become dry and frog-free. With their favorite food gone, the island snakes "are now thriving on an altered diet consisting of skinks, rodents, and nesting oceanic bird chicks," said study author Fabien Aubret of La Station d'Ecologie Experimentale du CNRS Moulis.
Along with the dietary shift came dramatic changes in the snakes' adult body sizes. On some islands, the snakes shrank, becoming significantly smaller than mainland snakes. But other islands have produced giants, measuring 60 inches (1.5 meters) and weighing as much as three times more than mainland snakes.
Aubret hypothesized that the size of available prey on each island was driving the variation in body size. Snakes are gape-limited predators, meaning they swallow their prey whole and can only dine on animals they can wrap their mouths around. This gape limitation would be most pronounced in newborn snakes, when their mouths are at their smallest. Simply put, baby snakes born too small to partake of the local cuisine would have little chance to survive. Where prey animals are larger, selection would favor larger newborn snakeswith larger mouths. That head start in size at birth could be the reason for larger size in adulthood.
To test his idea, Aubret took field expeditions to 12 islands, collecting and measuring 597 adult snakes. He released the males and non-pregnant females, and brough
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