May 16, 2012
Tiger Snake Size Depends On Offspring Mouth Size
Tiger snakes in Australia are of varied size, with some isolated island populations being twice the size of those on the mainland.
New research conducted by Fabien Aubret of La Station d'Ecologie Experimentale du CNRS Ã Moulis, published in “The American Naturalist,” theorizes that the mainland snakes eat mainly frogs and always have. When sea level rose after that last Ice Age, some tiger snakes were stranded on islands where frogs could not survive because of dry conditions. These snakes now eat skinks, rodents, and bird chicks.
Snakes swallow food whole, so they shrank or grew according to the food they now had available. Baby snakes were too small to eat the new prey because they were limited by the size of their mouths. The larger the prey available, the faster and more pronounced the change in mouth size and body size to match.
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 writes, "Mean adult body size has always been used as a traditional measure in the literature. On the other hand, patterns of variation for body size at birth in island populations have received, to my knowledge, no attention at all."
Aubret's work shows that selection isn't necessarily acting on adult body size.
"This study confirms that adult size variations on islands may be a nonadaptive consequence of selection acting on birth size," he 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."