This story appears in the August 2016 issue of National Geographic magazine.
All around her, guys wave seductively, beckoning her to their beach homes. How will the female fiddler crab pick a mate? By the quality of his lodging, the allure of his wave—and, especially, the size of his claw.
The female fiddler crab has two small, symmetrical claws; the male has one small and one oversize. “The large claw is all about the initial attraction,” says marine biologist Zachary Darnell of Louisiana’s Nicholls State University. After checking out several males and the love-nest burrows they’ve dug, a female “will find a male whose claw she really likes,” he says. “Maybe a crab with a large claw relative to his body size, and a bit higher wave than others”—because if he can tote and swing a claw that’s up to half his body weight, he’s probably a physically fit sire.
After the pair has sex in the beach burrow, the female stays there while her eggs develop; the male goes back to waving, and often brings home other females. On the hot sand, his claw is more than just a chick magnet, Darnell’s research has found. It’s a thermoregulator, as air passing over it seems to dissipate heat and lower body temperature.
A big claw is also the male crab’s best weapon: He uses it to fight rivals and keep intruders away from his burrow. After a few weeks, the pregnant females will emerge from that love lair and head for the waterline, where they’ll release the larvae.