Big-Mouthed Babies Drove Evolution of Giant Island Tiger Snakes

Posted on May 15, 2012

Australian Tiger Snake

Some populations of tiger snakes stranded for thousands of years on tiny islands surrounding Australia have evolved to be giants. Some island tiger snakes are nearly twice the size of their mainland cousins and weigh up to three times as much. Mainland tiger snakes, which generally max out at 35 inches (89 cm) long, patrol swampy areas in search of frogs. When sea levels rose around 10,000 years ago, some tiger snakes found themselves marooned on tiny islands that eventually become dry and free of frogs, their favorite prey.

Study author Fabien Aubret of La Station d'Ecologie Experimentale du CNRS a Moulis says that with the frogs gone, the island snakes "are now thriving on an altered diet consisting of skinks, rodents, and nesting oceanic bird chicks."

Along with this sudden 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 theorized that the size of available prey on each island is what drove the variation in body size. Snakes are gape-limited predators. They swallow their prey whole and can only consume animals they can wrap their mouths around. This gape limitation is 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 have little chance to survive. Where prey animals are larger, selection favors larger newborn snakes with larger mouths. This 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 brought 72 pregnant snakes back to the lab. After the snakes gave birth, he measured each of the 1,084 babies they produced. He then looked for correlations between snake size at birth and the size of prey animals available on each island. He also tested for correlations between birth size and adult size.

Aubret says, "The results were unequivocal: snake body size at birth tightly matches the size of prey available on each island."

Aubret's research paper, "Body-Size Evolution on Islands: Are Adult Size Variations in Tiger Snakes a Nonadaptive Consequence of Selection on Birth Size?," was published here in The American Naturalist.

Photo: Fabien Aubret