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Symmetrical Crabs Come Out on Top

Winners of World Wrestling Federation matches win cash prizes and the admiration of millions of fans. In the undersea world of crabs, the stakes are almost as high: Winners in the frequent male-versus-male fights get first dibs on food sources and mates. Now it seems that an especially symmetrical pair of hind legs--like chiseled quads for pro wrestlers--gives a big advantage in such bouts. The find, reported in the current Animal Behaviour, sheds light on how minor asymmetrical imperfections can influence an animal's success in life.

For more than a decade, researchers have collected evidence that symmetrical individuals seem to get more out of life than asymmetrical ones. Birds with tail feathers of equal length tend to have more offspring, for example, and humans who are more symmetrical are judged to be better looking. No one knows what controls these often subtle differences in symmetry, however, or why being asymmetrical is a disadvantage.

To see how asymmetry might affect competition, evolutionary biologists Lynne Sneddon of the University of Glasgow and John Swaddle, now of the University of Bristol in the U.K., arranged fights between 16 pairs of shore crabs caught off the coast of Scotland. Fighting pairs differed by less than a millimeter in claw size and body size. The winner was the crab who managed to climb on top of the other or who forced the opponent to retreat repeatedly. After the fights, the researchers killed the crabs and measured their limbs. The tale of the tape showed that symmetry in the front legs made no measurable difference in fight outcomes, but crabs with symmetrical hind legs were more likely to win fights.

The researchers propose that a symmetrical pair of hind legs gives a crab more stability during a fight. Like wrestlers, fighting crabs often try to push each other over by rearing up on their hind legs-a position in which added stability is an advantage. Some scientists have theorized that symmetry is a general sign of an individual's health and "good genes." But Sneddon and Swaddle say their study suggests that sometimes, a simple mechanical disadvantage may make all the difference.

"This is the kind of analysis that has never been made," says evolutionary biologist Frederik Nijhout of Duke University in Durham, North Carolina. "Here you have a direct link" between a symmetrical body and an evolutionary advantage, he says.