Female crabs 'offer sexual favours in exchange for protection'

Melbourne, Nov 4: An Australian study has found that female fiddler crabs have sex with their male neighbours in exchange for protection against wandering male intruders.

Both male and female fiddler crabs shelter in burrows which they both must defend from intruders.

But while males have an extremely large claw that can be used as a weapon, female crabs have just two small feeding claws.

So to show female crabs defend their territory, lead author Patricia Backwell from the Australian National University and colleagues based their new study on previous work that showed that under certain circumstances, males would help protect a neighbouring male from an intruder.

In the new study, the researchers found that males will also defend neighbouring females - apparently in return for sex.

The researchers first established the background mating rate of Uca annulipes fiddler crabs on mudflats in Durban Harbour, South Africa.

They found that most of the time females mate with a carefully chosen mate in his burrow but sometimes they are willing to mate with other neighbouring male, on the surface of the mudflat.

Co-author Michael Jennions said that given how fussy females normally are with their mate choice, there must be some benefit they get out of mating with the average male neighbour.

The researchers set a number of trials in mudflats in Mozambique to study whether a male would protect a female neighbour when confronted by an intruder.

They super-glued a tether to the shell of a crab, placing it near the entrance of a female burrow to simulate an intruder.

When the intruder was male a neighbouring male rushed in to defend the female 95 percent of the time, but when the intruder was female protection only occurred 15 percent of the time.

Jennions says it makes sense for a male crab to defend a female neighbour.

"Females are a weak neighbour and it's good to have a weak neighbour. In addition, you have the added bonus that as a male that you can mate with a neighbour if she's a female," ABC Online quoted Jennions as saying.

The findings have been reported in the Royal Society journal Biology Letters.

--ANI