There are rules in fiddler crab fight club

When conditions are right, territorial crustaceans will go to bat for their rivals

Fiddler crabs will do battle to help a punier rival, a report in the May American Naturalist concludes. But it isn’t out of kindness.

CRABBY Male fiddler crabs (Uca annulipes) grapple in fast, furious fights over ownership of burrows, and some of these territorial clashes may turn rivals into temporary allies. Tanya Detto

Ecologists already knew that male fiddler crabs will sometimes take on an attacker who is trying to invade a neighbor’s territory. By setting up a series of bouts between crabs of the species Uca annulipes on a beach in Mozambique, Australian researchers have found that the crustaceans defend only smaller neighbors. And they do so only when an intruder is intermediate in size between their neighbor and themselves.

Size has a lot to do with which crab is going to win a fight. So the rules of crab engagement suggest it’s not a mutual defense pact that motivates a male to defend his neighbor. By the rules, the little guy next door isn’t going to help his big defender. But the big guy still benefits by preventing a larger rival from moving in next door and instigating the customary series of border clashes, said study coauthor Michael Jennions of Australian National University in Canberra.

This kind of behavior, males aiding a territorial rival, has been observed in only two species before. Coauthor Patricia Backwell, also at Australian National University, had described the same behavior in an Australian fiddler crab in 2004. Before that, biologists had observed helpful rivals among European birds called pipits.

Among the fiddler crabs, fights break out over deep burrows exhaustingly excavated in the sand. Crabs need a good hole for hiding when incoming tides bring predatory fish and for romancing when a female is checking out a male and deciding if his burrow meets her standards.

Should one male get evicted, the fastest cure for homelessness is to go find a slightly smaller male and evict him.  Fights typically last less than a minute. After a claw-to-claw tap to get started and then shoving and grappling between males, “one does a judo throw and flicks over the other one,” Jennions says. The thrown crab, disoriented, flees.

Males intervening judiciously to save less-threatening neighbors makes sense as an explanation, evolutionary biologist Tom Sherratt of Carleton University in Ottawa. “Everything seems to fit perfectly. I’m not a military historian, but I suspect there have been wars waged for similar reasons,” he says.

“This paper provides a fascinating case example of cooperation in the field,” Sherratt says.

Earlier observations had hinted at the size relationships, but Jennions says only tests could confirm them. The researchers tethered intruders in carefully selected spots with neighbors of different sizes nearby to see who would help and who wouldn’t. In the 20 cases with big allies, intermediate intruders and puny targets, 16 allies intervened. But when the attacker outsized all the others, only four allies tried to intervene.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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