Australian female fiddler crabs are taking up prostitution to prevent against raids from randy males.

Researchers from The Australian National University 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.

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.

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 neighbours, 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 neighbour.

The researchers speculated the female crabs were having the neighbourly sex in exchange for some sort of benefit. In this case, that benefit appeared to be protection, Jennions said.