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Crabs lend a helping claw

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Although it has long been predicted by theorists, there is no compelling evidence from studies of animal territoriality to show that a resident will strategically help a neighbour to defend its territory against an intruder. Patricia Backwell and Michael Jennions have, however, recently shown that territory-owning male fiddler crabs will assist a neighbour to defend his territory against an intruder. This cooperation supports the prediction that animals may sometimes find it less costly to assist a familiar neighbour than to renegotiate boundaries with a new, and possibly stronger, neighbour. Pat and Michael found that males only assisted their neighbours in certain contexts. Specifically, a male helped his neighbour when this was most likely to have a beneficial effect.

In crabs, body size equals strength so larger crabs tend to win fights. In 94% of cases where assistance was provided, the resident being challenged was smaller than the intruder. This was only true in 51% of cases when no assistance was given. Males therefore provided assistance when their neighbour was more likely to lose his territory. In addition, the male that helped was generally larger than the intruder and therefore more likely to defeat the intruder than was his neighbour. So the circumstances under which assistance was provided give the impression that crabs make judicious decision about the relative strength of the three participants. In reality, it is likely that the crabs use very simple 'rules of thumb' to determine when to intervene in a fight between a neighbour and an intruder. That this form of cooperation between territorial rivals occurs in an invertebrate, but has still not been reported in birds or mammals, suggests that territorial coalitions depend more on appropriate circumstances than on advanced cognitive skills. This work was carried out in tropical Darwin, where the A.N.U. has a research station ( N.A.R.U.) that is now the base for BoZo's fiddler crab research.

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