Australian researchers have discovered that female fiddler crabs rely on a male partner's large claw to protect them from other suitors. In fact, selling sex is the key to survival in the seedy world of fiddler crabs, according to the researchers at the Australian National University’s School of Biology located in Canberra.
Scientists studying the burrow-dwelling, highly territorial creatures found that male fiddler crabs fought off intruders to protect female neighbors—who lack the male's huge claw—far more often than they would aid male neighbors, and apparently received sex in return.
"Males protected their female neighbors in 95 percent of instances where the intruder was male, and only 15 percent of instances where the intruder was female," one researcher said. "This suggests that males don't care who their neighbor is, as long as they are female."
The female fiddlers are physically no match for attacking males. But Richard Milner, Professor Michael Jennions and Dr. Patricia Blackwell found the females had something that can work in their favor – offering sex to neighboring males in exchange for protection.
"In fiddler crabs, both males and females defend territories that are essential to their survival," Jennions said.
Size apparently is key to the deal.
"Males have a weapon in the form of a giant claw that is the largest weapon relative to body size in the animal kingdom," Jennions said.
The rearchers 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.
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.
That suggests the male crabs preferred to keep females nearby, largely because they will almost always have sex with their male neighbors, Jennions said.
"The fact that the neighbor comes over and helps to defend another territorial individual is pretty unusual," Jennions said. "This study shows, for the first time, that in exchange for sex and other benefits, males protect their female neighbors from territory-seeking male intruders. The paper provides the first evidence of 'defense coalitions' between territorial males and females."
Trading sex for material benefit is not unheard of in the animal world. For female red-winged blackbirds, mating with an extra-pair male opens the chance to forage on his territory. Another example is the Adelie penguin, where females exchange sex for highly sought-after stones used for nest building.
It appears the safe sex message that applies to the fiddler crab is not so much a matter of public health, as these cads of the crustacean world have an eye for the ladies and are a rather promiscuous lot.
These sexual offerings lead to neighborhood coalitions, where a male crab will protect female neighbors from homeless males seeking a new territory.
"Males protected their female neighbors in 95 per cent of instances where the intruder was male, and only 15 per cent of instances where the intruder was female," Jennions said. "This suggests that males don't care who their neighbor is, as long as they are female."
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.
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."
The complete research results are found in the paper, “Safe sex: male-female coalitions and pre-copulatory mate-guarding in a fiddler crab,” that was published today in Australia’s journal Biology Letters.
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