杏吧原创

AFRICAN fiddler crabs will put their claws on the line to help defend their neighbours from an intruder 鈥� but only because it suits them.

Defence coalitions ought to be common in the wild, says Michael Jennions of the Australian National University in Canberra, but in practice they have only been seen in two fiddler crab species, and European rock pipits.

Jennions and colleagues tethered 鈥渋ntruder鈥� crabs next to the burrows of two established crabs, then released them. The neighbouring crabs formed alliances that significantly increased their chances of repelling the intruder. Alliances were strategic. In most cases, crabs with burrows would only assist their neighbour if they were bigger than both the intruder and the neighbour, and had a good chance of winning.

鈥淭he best explanation is better the devil you know,鈥� he says. 鈥淏y retaining a familiar neighbour you don鈥檛 have to re-negotiate territorial boundaries.鈥� Crabs with unfamiliar neighbours had more border disputes than those that knew their neighbours, and when attacked by intruders, were less likely to form territorial coalitions. 鈥淚f you lose your neighbour it is likely to be to a larger neighbour, that is going to need more territory,鈥� says Jennions. Plus, larger fiddler crabs get the females, he adds.