Lefties have the advantage in adversarial situations

Contrary to traditional wisdom, being a leftie promotes survival from attacks, at least in the world of snails and crabs, according to research.
A snail shell with a counterclockwise spiral.

Contrary to traditional wisdom, being a leftie promotes survival from attacks, at least in the world of snails and crabs, according to a report by researchers at Yale and Cornell in the Biology Letters of the Royal Society, UK.

While rare in nature, many organisms including humans have some, but few, members that are lefties. Author Gregory P. Dietl, post-doctoral fellow in geology and geophysics at Yale has studied the survival advantages or disadvantages of being a “leftie” by working with snails and their predatory crabs.

One of the conclusions we draw from the interaction between crabs and these snails is that in the adversarial mode, lefties have a competitive advantage as long as they remain rare,” said Dietl. This parallels some social interactions in human cultures, such as sporting competitions in which left-handed players enjoy an advantage over their right-handed opponents.

The functional advantage of this rare reversal in shell-coiling direction, while a topic of fascination for scientists and laypersons alike, has evaded explanation. Persistence of handedness in snails is most often associated with a mating advantage. In this case, snails mate most effectively with snails whose shells coil in the same direction.

The overwhelming majority of snail species are right-handed — their shells coil clockwise. Dietl studied a species of snail that are lefties, and have shells that coil counterclockwise.

The left-handed advantage is realized when snails interact with predators of opposite handedness. Some predatory crabs are “righties” — and have a specialized tooth on their right claw that acts like a can opener to crack and peel the snail shells. “The ‘sinistral advantage,’ or advantage to being left-handed, is that it would be like using a can opener backwards for the crab to crack and peel the snail shell,” said Dietl.

They measured the frequency of unsuccessful crab predation left as repair scars on the snail’s shell in specimens from the fossil record. Ten out of the 11 species pairs showed more scars on the right-handed shells, suggesting that crabs are attacking them in preference to their left-handed counterparts. These traces of failed predation indicated to the researchers that left-handed coiling enhances survival from attacks by right-handed crabs. A right-handed crab often abandons a left-handed snail before it breaks the shell.

The authors’ conclusion, “In the balance of selection processes, the survival advantage of sinistrality may be as important a selection pressure as mate selection. It is not all about sex all of the time.”

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Janet Rettig Emanuel: janet.emanuel@yale.edu, 203-432-2157