The Secret to Success Is Giant-Jawed Snake Babies

When coming face-to-face with a wriggling, freshly born pile of poisonous snakes, most of us wouldn't linger for a close look. But it was by looking into these living linguini platters that one biologist found a new answer to an old question: Why does island life make animals such freak shows?

Some big-bodied species shrink when they move from the mainland to an island habitat, a phenomenon that's created pygmy sloths, miniature mammoths, and possibly even a dwarf hominid that's now extinct. Some small-bodied species, meanwhile, grow enormous on islands. This category includes a 3-inch-long earwig, various ungainly and flightless birds, and a giant rat (living on Flores, the same island where the miniature people were, unfortunately for them).

Scientists have explained these fun-house transformations with a lack of resources on an island (keeping animals smaller) or a lack of predators (allowing them to grow bigger). Other factors, such as distance to the mainland or one sex's preference for extreme traits in a mate, could be at work too.

French researcher Fabien Aubret wondered whether scrutinizing the sizes of adult animals was making scientists miss another important variable: the size of babies. A newborn animal that can't find its first meal will quickly exit the gene pool. In snakes, this could be a simple matter of not being able to get one's mouth all the way around one's prey to swallow it.

Aubret studied twelve populations of tiger snakes, some living on mainland Australia or Tasmania and others on nearby islands. Among the island exiles, some groups have grown giant--up to 1.5 meters long, rather than the usual 0.8 or 0.9 meters--while others have shrunk. Most of the island populations were stranded by rising seas six to ten thousand years ago, leaving them with a different selection of prey animals than on the mainland.

Armed with a measuring tape, Aubret asked whether the changes the snakes' bodies have undergone since then can be entirely explained by the need for newborns to get their jaws around a meal. Tiger snake mothers give birth to live young rather than laying eggs, popping out a dozen or more at a time. On the mainland, these snakes and their parents swallow frogs for most of their meals. But on the islands, their prey can range from little lizards to large nesting seabirds.

Aubret captured almost 600 adult snakes from the various populations, measuring their length and weight before releasing all of them except the pregnant females. When the tangles of baby snakes emerged, he monitored the newborns' sizes for six months while feeding them a standard diet. For each study site, he calculated the average weight and circumference of animals on the prey buffet. (Weight because first a snake must subdue the unfortunate gecko or skink, and circumference because the animal must fit down the gullet.)

The size of baby snakes from each site--and the size of their jaws--was closely tied to the weight and circumference of the prey animals available there. Baby snakes from sites with large prey also grew faster.

Aubret says the pressure on newborn snakes to swallow available prey might be the only explanation necessary for the various body sizes tiger snakes have evolved on different islands. Adult body size, though of course it's related to the size of newborns, might be mainly irrelevant.

This gives biologists a new clue to the puzzle of how island life makes animals shrink or grow. While they wrap their heads around that, the tiger snakes will continue to wrap their own heads around any slow-moving animal that fits.

Fabien Aubret (2012). Body-Size Evolution on Islands: Are Adult Size Variations in Tiger Snakes a Nonadaptive Consequence of Selection on Birth Size? The American Naturalist, 169 (6)


Image: Not actually a tiger snake, by batwrangler/Flickr