Hakai Magazine

Coastal science and societies

By ditching its oversized claw to evade predators, a male fiddler crab can have its cake and eat it too. Photo by Ivan Kuzmin/Alamy Stock Photo

When Being Big, Strong, and Sexy Comes at a Cost

Male fiddler crabs’ big claws are attractive, but they weigh them down. Fortunately, they have a workaround.

Authored by

by Richa Malhotra

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A male fiddler crab’s massive pincer serves a claw full of purposes. It’s useful in fighting off males and wooing females, but being big and heavy also makes it a liability. Fortunately, if caught by a predator, the crab can shed this hefty claw and scuttle to the safety of its burrow. Research shows this escape act might be facilitated by the fact that, after losing the claw, the crab runs almost a third faster.

Benjamin Martin, a doctoral student at the University of Wisconsin-Madison, discovered this burst of speed when he set Atlantic sand fiddler crabs on a sand-and-mud track and chased them around with his index finger, measuring the speeds at which they sprinted to the finish line. With both their claws intact, male crabs clocked in at speeds of 0.09 to 0.38 meters per second. But without their oversized claw, they ran at 0.13 to 0.47 meters per second.

Of the 64 crabs that ran along the one-meter track, 56 were faster after losing the claw. “The claw comprised 40 percent of their body weight. So, strictly from a theoretical standpoint, it should not come as a surprise,” says Martin.

Yet this finding, as intuitive as it is, runs counter to those of previous experiments. In one, scientists found no increase in sprint speeds of clawless crabs on a flat surface. In another, while a slight increase in speed was observed, the results were not statistically significant. Martin attributes this to a difference in methodology. Unlike researchers before him, he raced the same crabs both with and without their big claw. He also tested a larger number of crabs than previous researchers.

But because Martin let his crabs rest for an hour after he tugged at their claws to prompt shedding, his results may be limited in scope. Given that the crabs had this little breather before being chased, their faster running speeds may not reflect the speeds at which they would scuttle away from a predator immediately after shedding the claw. “That is one caveat,” Martin acknowledges. He thinks that the clawless crabs might benefit from the increase in sprint speeds for a month—until the claw grows back. However, this remains to be tested.

“Overall, I have no problem believing that there is a speed advantage associated with losing the major claw,” says Bengt J. Allen, a marine biologist at California State University who was not involved in the study. “That said, it’s worth remembering that this is still only a single study, done on one species of fiddler crab.” Whether the increase in sprint speeds is a general phenomenon in crabs still remains to be seen, Allen says.