Bluffing crabs

Like poker players, some fiddler crabs bluff when they have a weak hand, or in this case, claw.

The crabs are famed for their huge claws on one side and dramatic waving as they show them off. To other males, it is a display of strength, a suggestion of who should not be messed with. To females looking on, the claw-waving goes hand-in-hand with the male’s burrow location in helping them to determine the quality of a mate.

Claws, however, can fall off. Or, more likely given the pugnacious nature of many crabs, can be violently ripped off. And this is the start of a problem for male crabs. Yes, they can brilliantly grow the missing limb back, but it is weaker than its previous incarnation. Claw-waving stand-offs that escalate to combat could expose the weakness of the re-grown claw and leave the male crab in trouble. It could lose territory and potential mates, along with another long period of recuperation as the lost limb slowly returns.

So, as discovered by researchers in Japan, these individuals turn to deception. If they have been forced to regenerate a limb in their lifetime, they bluff. Firstly, re-grown claws were often larger than original claws, suggesting a dishonest physical signal. Secondly, the behavioural bluff: the dishonest crabs start contests with a large amount of aggression, hoping the opponent will retreat, but, if an actual brawl looks imminent, avoidance becomes the better option, and they quickly back down. A fantastic example of dishonesty and deception in the natural world.

“The findings are in line with current theories that predict that animal signals are generally honest, but each signalling system allows for deception,” said author Tsunenori Koga.


Originally published in Biosphere Magazine Issue 16.

Daisuke Muramatsu, Tsunenori Koga. (2016) Fighting with an unreliable weapon: opponent choice and risk avoidance in fiddler crab contestsBehavioral Ecology and Sociobiology,; DOI: 10.1007/s00265-016-2094-2


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