Why do fiddler crabs have such ridiculously large claws?

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Male fiddler crabs have a single giant claw that looks like the evolutionary equivalent of going through a mid-life crisis and buying a super expensive sports car. The truth, it turns out, is only slightly less ridiculous than that.

Fiddler crabs are one of the more spectacular violators of symmetry in nature, with their major claw being many times larger than their minor claw. Only the males possess this unique anatomical arrangement - females have two normal-sized claws. Previous research has demonstrated that the crabs uses the claws in mating rituals, but beyond that it's hard to figure out just why the crabs would have ever evolved the things in the first place.

University of Texas researcher Dr. Zachary Darnell explains the extensive problems posed by this claw arrangement:

"The large claw is metabolically costly, it hinders feeding because it is cumbersome for this task, and it reduces endurance capacity when crawling on the sand. Male crabs are both heat-stressed and hungry while on the surface, foraging and performing the claw-waving display."


So yeah, that's a whole lot of problems for such a big claw. But, according to Darnell's research, it would actually be even worse to go without the claw. He and his colleagues tested a bunch of male fiddler crabs, both those who still had their major claws and those who had lost theirs. When they shone lights on the crabs to heat them up, they found that those who still had the major claws were able to cool themselves down far more effectively than their peers without claws.

Darnell explains that the major claw serves as basically one giant heat regulator, and for all its obvious drawbacks that's still enough to make it a worthwhile adaptation:

The major claw likely functions like a heat sink, with heat being transferred from the body to the claw and dissipated into the surrounding air through convective heat transfer. With the large claw acting as a heat sink male fiddler crabs can remain on the surface longer, foraging and performing the waving display. A thermoregulatory function of the major claw should change the way scientists think about this structure, and reconsider the costs and benefits associated with the claw and other [sexual] traits."


Via BBC News.