Researchers from The Australian National University in Canberra found that male fiddler crabs will happily defend a female neighbor against intruders - partly because the females will dole out sex in return.
"The fact that the neighbor comes over and helps to defend another territorial individual is pretty unusual," said Michael Jennions, who helped conduct the study, the results of which were published in the journal Biology Letters.
Jennions and fellow ANU researchers Richard Milner and Patricia Backwell studied the behavior of fiddler crabs living in mud flats off the African country of Mozambique in October and November 2008. Male fiddler crabs have giant claws to defend themselves, but the researchers wanted to see how female crabs - which only have two small feeding claws - protect their homes.
Fiddler crabs are territorial and live in burrows. The researchers gathered crabs from distant parts of the mud flats and tethered them near new, occupied burrows. In 21 trials involving male intruders, the researchers found that male crabs would scuttle over to fight off the invaders on a female neighbor's territory 95 percent of the time. But in 20 trials involving female intruders, the males crabs only fought off the invaders 15 percent of the time.
That suggests the male crabs preferred to keep females nearby, largely because they will almost always have sex with their male neighbors, Jennions said.
Most of the time, female fiddler crabs are selective about their partners and choose to mate in the male's burrow. But the researchers also found females mating on the surface - and 85 percent of the time the surface sex was with a neighbor. The researchers speculated the female crabs were having the neighborly sex in exchange for some sort of benefit. In this case, that benefit appeared to be protection, Jennions said.
Orpha Bellwood, a lecturer of marine and tropical biology at James Cook University in Townsville, said she was particularly interested in the motivations behind the crabs having sex on the surface, which is unusual and makes them vulnerable to predators.
Bellwood wonders whether it might just be the proximity of the crabs to their own homes that allows them to feel safe enough to mate in the open, or whether the females are indeed gaining some sort of protective advantage by doing so.
"It opens up a lot of those questions," she said.
Peter Davie, senior curator of the Queensland Museum in Brisbane, said the males are known to use their claws to protect themselves.
"But to have that sort of encompass the territories of the females as part of his sexual territory sounds quite interesting," said Davie, who has spent 30 years studying crabs.
Swapping sex for favors is not unheard of in the animal kingdom. Antarctica's Adelie penguins exchange sex for highly coveted stones used for nest building.
Another reason the crabs might help fight off their neighbors' intruders is to keep a familiar comrade next door, Jennions said. Even for crabs, he said, sometimes it's a case of "better the devil you know than the devil you don't."