Large Mouths Drove Island Snakes to Gigantism

Big-mouthed baby snakes are to blame for the evolution of giant snakes on the islands around Australia, new research suggests. Island life meant limited food choices, which in some areas selected for larger mouthed offspring, which made these tiger snakes about twice as large as their mainland cousins.

"The results were unequivocal: snake body size at birth tightly matches the size of prey available on each island," study researcher Fabien Aubret of the French National Centre for Scientific Research in France. If the snakes were smaller, they would have a harder time trying to eat the large prey on some of these islands.

The study was published in the June 2012 issue of the journal The American Naturalist.

Mainland tiger snakes generally max out at 35 inches (89 centimeters) long and patrol swampy areas in search of frogs. When sea levels rose around 10,000 years ago, some tiger snakes found themselves marooned on frog-free islands. With their favorite food gone, some of the snakes had to survive off of skinks, rodents, and nesting oceanic bird chicks, the researchers said.

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 prey available on each island was driving the variation in body size, since the snakes swallow their prey whole. This limitation would be most pronounced in newborn snakes, with their small mouths, so islands with larger prey would encourage large-mouthed babies.

"Mean adult body size has always been used as a traditional measure in the literature," Aubret said. "On the other hand, patterns of variation for body size at birth in island populations have received, to my knowledge, no attention at all."

So, Aubret collected and measured 597 adult tiger snakes from 12 islands around Australia. 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.

As predicted, where prey animals were bigger, newborn snakes were bigger and they grew up to be bigger adults. Where prey animals were smaller, newborn snakes followed suit, leading to smaller adults.

"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."