Fear not, fair neighbour, I’ll protect you…

Female fiddler crabs mate with their neighbours in exchange for protection.

The discovery of the sex-for-security trait helps to explain a surprising quirk: how it is that females defend their territory just as successfully as males despite their smaller claws. It is also the first known case of male and female neighbours teaming up to defend territory in any species, according to lead researcher Richard Milner of the Australian National University in Canberra.

Fiddler crabs live in burrows and often fight to protect them from would-be squatters. Males have one giant claw, sometimes as heavy as the rest of their body, which they use in fights. Females have two much smaller claws, yet are just as good at holding on to their territory.


A willingness to offer sex to male neighbours seems to be behind this success, says Milner. He studied crabs on South African and Mozambican beaches and found males frequently defending the burrows of neighbouring females when rival males approached. Milner and colleagues also found 85 per cent of the mating they observed was between crabs with neighbouring burrows.

Females of other species trade sex for material benefits. Red-winged blackbirds do so for the right to forage on a male’s territory, and Adelie penguins exchange sex for stones to build their nests.

Journal reference: Biology Letters, DOI: 10.1098/rsbl.2009.0767