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Science & Space

Neighbor, can you lend a claw?

Fiddler crabs form coalitions to protect turf

By Marsha Walton
CNN

Scientists found male fiddler crabs often aid their neighbors when intruders threaten to move in.
Scientists found male fiddler crabs often aid their neighbors when intruders threaten to move in.
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(CNN) -- There's no formal neighborhood watch program, but fiddler crabs in Australia know a thing or two about preserving a safe place to live.

Scientists have found that when an "outsider" male threatens to take over another male's burrow, nearby residents help their neighbor defend his territory.

And in most cases, that cooperation helps the neighbor hold his ground. Males most often help out when the neighbor is smaller than the intruder, and therefore more likely to lose the burrow.

Scientists say the invertebrates somehow figured out that it is less costly to help out a familiar neighbor than it is to set up new boundaries with a different, perhaps more menacing crab.

Or, as a Chinese proverb puts it, "Better the devil you know than the devil you don't."

"This study shows you don't have to have a big brain to be capable of some very complex behavior," said biologist Patricia Backwell of the Australian National University in Canberra. She has been studying fiddler crabs and their "neighborhoods" for a dozen years. Her research is detailed in this week's edition of the journal Nature. It's the first time such helping behavior has been documented in any animals.

Fiddler crabs make communities on mud flats on the edge of mangrove forests in tropical climates all over the world. Backwell says these communities, which often have thousands of residents, tend to be stable and interesting to watch.

So what prompts these burrow brawls in the first place? It's not a surprise that it's tied to the courting process.

When females are ready to mate, they carefully check out potential partners' burrows.

Because females will spend about two weeks in the burrow, most are as particular about their pad as they are their partner.

Often a female will check out 10 to 20 possible homes before making a commitment. Backwell says she has seen finicky females visit 27 before moving in.

After mating, the female stays alone in the burrow until she is ready to release her eggs in the ocean. It's during that time that the post-mating, now homeless males (known as "floaters") must head out to find some new real estate: Hence the attempts at burrow-busting.

Backwell says there's much to be learned from the actions of these seemingly simple animals.

Fiddler crabs defend neighbors to maintain boundries.
Fiddler crabs defend neighbors to maintain boundries.

"We expect complex behavior from really advanced animals, like birds and mammals," Backwell said. So the fact that fiddler crabs can somehow create such complex scenarios is worthy of further study. Complex social behavior and sophisticated societies have been observed in other "simple" animals, including bees, ants and other insects.

Backwell says the world's diplomats and politicians often follow very similar strategies to those of the fiddler crab who lends a claw to a neighbor.

"You're not really doing them a favor, you're doing the favor because it suits you," Backwell said.

For example, she says a superpower helping out a little country may get nothing out of it immediately, but that country is likely to be an ally down the road. Likewise, the helpful crab may get nothing beneficial out of fighting, but he does help maintain an orderly neighborhood.


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