Male fiddler crabs fight aggressively over territory using their one giant claw, which can grow on either their left or right side. These claws can often get damaged or entirely removed, requiring the owner to regenerate the claw before entering into more fights.

A new behavioural study of wild milky fiddler crabs (Uca lactea) by Daisuke Muramatsu of Kyoto University and Tsunenori Koga of Wakayama University in Japan shows how males that have regenerated their claws are more likely to rely on deception than strength to win their battles.

138 territorial disputes between fiddler crabs were filmed in Japan’s Waka River estuary and the footage was analysed to inform the researchers about which crabs were more likely to win fights and at which stage in the conflict. Once a fight had concluded, the participants were collected and various body and claw measurements were taken.

Regenerated males would initially act aggressively but quickly back down if a physical brawl seemed likely. The researchers believe that this is because the regenerated claws are more fragile than the original claws and are more likely to be damaged during a fight, so the regenerated males rely on bluffing their strength to win fights early on.

It was also found that regenerated males would choose opponents that were smaller and were opposite-handed, as this also helped to reduce the chance of their fragile claws being involved in the fight.

“Fiddler crab males adapt their fighting tactics and choice of opponent depending on whether they have lost their major claw,” summarises Muramatsu, also adding that crabs are able to change their tactics whenever appropriate during their lifetime.

Muramatsu and Koga believe that these bluffing tactics are often successful at winning fights before they become physical brawls, as regenerated males weren’t engaged in fights any more often than the original males. However, they did lose significantly more fights than original crabs once they were physically engaged.

Finally, the researchers found that regenerated males tended to grow claws that were often as big, or even bigger than the claws of original males. By increasing the size of the regenerated claw, the crabs may be relying on a dishonest physical signal to other crabs that suggests bigger claws are stronger claws.

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


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


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