Female Crabs Are Constantly at Risk of Being Raped, Study Says

In the harried process of finding a suitable mate, female fiddler crabs hold much of the power. Often alone, they will crab-walk amongst a herd of males that subsequently crabcall them—by waving their pinchers in a show of sexual fitness—in the hopes of catching their attention. The goal, for the males, is to get a lovely lady to come back to their bachelor burrow, inspect it for suitability, and mate. In a study published in Plos One, Australian researchers suggest that male fiddler crabs employ forceful and sneaky tactics to make this happen.

After observing 144 mating rituals at East Point Reserve in Darwin, Australia, the researchers observed that once a male crab gets a female to the entrance of his burrow, he will proceed in one of two ways: The male will either enter first so that the female follows, or he will step aside and wait for the female to enter, enticing her in with more of the claw waving. In the second scenario, the researchers found that females were three times less likely to enter the burrow, but when they did, they were three times more likely to stay. Probably because they are often trapped.

Though only 41 percent of female crabs were willing to enter first, the researchers found that some males were up to take that risk, using the "step aside" method to then trap the female within their burrow. They hypothesized that males with shoddy burrows used the tactic to prevent the females from leaving them for a crab with a better stereo system.

"We know that females look at the quality of habits in which they're going to lay their precious eggs," Marah J. Hardt, a marine ecologist and author of the book Sex in the Sea, told me over the phone. "It makes sense that these males with the shittier burrows will use this tactic to lure her in."

Coercion like this is common in the animal kingdom. "Some are more aggressive," Hardt said. "In nurse sharks, the males will chase a female into the shallows and bite her fin and roll her over [until she submits to reproduction]. She really doesn't have much choice in the matter."

The female fiddler crab, once trapped, still has some degree of control at this point—she has to voluntarily open up their abdomen to expose her vulva. But the shitty male crab, it seems, just sort of waits it out, pinning the female with his body weight and pinchers, until she gives up. "It appears that females who are trapped inside the burrow by the male are prevented from leaving until they relent and allow the male to copulate," the researchers explain in the study.

"I'm not surprised that this behavior occurs, but [with animals that burrow for reproductive purposes] you usually see the opposite happen," Hardt added. "Usually males will gift resources to females to entice them with additional benefits."

Indeed, trapping a female without access to food in a substandard love den until she submits her eggs is probably not the best reproductive strategy. The researchers cite evidence that this coercive technique is detrimental to female crabs in the long run—as well as to the health of their embryos.