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Fiddler crabs help weaker neighbors

July 21, 2004 By Michael Hopkin This article courtesy of Nature News.

Males weigh up the advantages of pitching in.

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It is good political strategy to surround your territory with the weak. And it turns out that a species of Australian crab is expert at working out how worthwhile it is to protect its feeble neighbours for the sake of a quiet life.

If the next-door territory comes under attack, a crab will fight to help defend it, report Patricia Backwell and Michael Jennions of the Australian National University in Canberra. But the crustacean weighs up the odds first, and only provides backup for neighbours who are smaller than itself.

The researchers studied fiddler crabs (Uca mjoebergi) living on seaside mudflats in Darwin, Australia. These crab communities are densely packed and the average male marshals a territory just 10 centimetres across. "Everyone knows his neighbour," says Backwell.

But not all males boast a bachelor pad, and lack of one can seriously hamper success with the ladies. Homeless crabs roam the community, attacking homeowners and attempting to steal their patch.

In some cases, however, the bandit gets more than he bargained for, Backwell and Jennions report in this week's Nature1. In around 6% of the fights they documented, a larger crab from next door weighed in to see off the intruder.

It all sounds very sweet: the strong guy looking out for his puny mate. But crabs join the fight strictly in their own interest, says Backwell. "The big male is in it for himself. He doesn't give a damn about the little guy."

His apparent benevolence is actually an attempt to avoid the hassle of negotiating borders with a new neighbour, Backwell explains. Territory size is linked to a crab's physical bulk, so the arrival of a new, larger neighbour may trigger an aggressive attempt to claim more ground.

Getting crabby

Territorial boundaries are initially established by a series of "nagging" fights, in which a smaller crab repeatedly shoves his larger neighbour, ultimately winning enough respect to allow the pair to reach a pact over who owns what.

But when the researchers deliberately replaced a crab's neighbour with a stranger, the resulting fights were far more vicious. The males locked their large claws, each attempting to twist the other off his feet. "Neighbours just don't do that to each other," says Backwell.

Of course, a crab that dashes off to help next door leaves his own territory briefly unguarded and he must include this risk in his calculations. But, says Backwell, such crabs are always bigger than average. "He'll whack anyone who tries anything," she says.

Theory predicts that territorial animals should look out for their neighbours, says Lee Alan Dugatkin, who studies animal behaviour at the University of Louisville, Kentucky. But this study goes a step further in showing that the crabs weigh up the combatants' sizes and make a strategic decision about whether to pile in. "It says a lot about the crabs' cognitive ability," he says.

Deciding when to help a weaker ally is also a key process in human warfare, Backwell points out. During the first Gulf War, for example, the United States leapt to Kuwait's aid when it was invaded by Iraq. "It's a huge part of military strategy," she says.


  1. Backwell P. R. Y. & Jennions M. D. Nature, 430. 417 (2004).


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