"The fact that the neighbour comes over and helps to defend another territorial individual is pretty unusual," Michael Jennions, who helped conduct the study, said to the media
Over a period of two months - October and November 2008 - Jennions and a group of fellow researchers at the Australian National University in Canberra studied the behaviour of fiddler crabs living in burrows in mud flats in Mozambique. They discovered how female crabs - which only two small feeding claws - protect their homes when compared to their male counterparts, which have one very large claw (measuring 65% of its total body weight) to defend themselves.
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 neighbour'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 be able to mate", Jennions said.
He added that female fiddler crabs are usually 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 added.