06 November 2009

How do girl fiddler crabs defend their territory?

Boy fiddler crabs have one enlarged pincer that can be bigger than their body. This humungous pincer is so large they cannot feed with it.
Orange fiddler crab (Uca vocans)
Boy fiddler crabs have one tiny pincer to feed with. Girl fiddler crabs, however, have two tiny pincers so they can feed twice as fast.

But how do girl fiddler crabs then defend their territory from other thuggish fiddlers? It appears they get their big pincered boyfriends to help. In exchange for something all boys want. Sigh.

Crabs trade sex for protection
Jim Giles, New Scientist 4 Nov 09;
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


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