August 03, 2008

Tiny Snakes and Natural Selection on Offspring Size

Happy Herp Day! One of Tycho's very distant cousins is making news in zoological circles today. It's the world's tiniest snake, which has just been discovered living in a tiny patch of habitat on a tiny island. Cuteness factor aside, Leptotyphlops carlae also tells us a tiny bit more about how natural selection is generally manifested in snakes that are too small to have numerous offspring capable of surviving on their own.

World's smallest snake found in Barbados

Barbara Kennedy

The world's smallest species of snake, with adults averaging just under four inches in length, has been identified on the Caribbean island of Barbados. The species -- which is as thin as a spaghetti noodle and small enough to rest comfortably on a U.S. quarter -- was discovered by Blair Hedges, an evolutionary biologist at Penn State. Hedges and his colleagues also are the discoverers of the world's smallest frog and lizard species, which too were found on Caribbean islands. The most recent discovery will be published on 4 August 2008 in the journal Zootaxa.

Hedges found the new snake, a type of threadsnake, in a tiny forest fragment on the eastern side of Barbados. He believes the species is rare because most of its potential habitat has been replaced by buildings and farms...

Hedges determined that the Barbados species is new to science on the basis of its genetic differences from other snake species and its unique color pattern and scales...

According to Hedges, the smallest and largest species of animals tend to be found on islands, where species can evolve over time to fill ecological niches in habitats that are unoccupied by other organisms. Those vacant niches exist because some types of organisms, by chance, never make it to the islands...

In contrast to larger species -- some of which can lay up to 100 eggs in a single clutch -- the smallest snakes, and the smallest of other types of animals, usually lay only one egg or give birth to one offspring. Furthermore, the smallest animals have young that are proportionately enormous relative to the adults. For example, the hatchlings of the smallest snakes are one-half the length of an adult, whereas the hatchlings of the largest snakes are only one-tenth the length of an adult. The Barbados snake is no exception to this pattern. It produces a single slender egg that occupies a significant portion of the mother's body.

"If a tiny snake were to have two offspring, each egg could occupy only half the space that is devoted to reproduction within its body. But then each of the two hatchlings would be half the normal size, perhaps too small to function as a snake or in the environment," said Hedges. "The fact that tiny snakes produce only one massive egg -- relative to the size of the mother -- suggests that natural selection is trying to keep the size of hatchlings above a critical limit in order to survive..."

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