Fighting with an unreliable weapon: opponent choice and risk avoidance in fiddler crab contests

Abstract

Current signaling theories predict that animal signals are generally honest, but each signaling system allows some admixture of deception. Male fiddler crabs fight aggressively through use of their greatly enlarged major claw, which grows on the left or right side. Some males have fragile regenerated claws (regenerated males) and others have robust original claws (original males), but crabs cannot visually discriminate between the two types. In the present study, we conducted field observations in a population of Uca lactea to investigate how regenerated males fight with their inferior weapons, and how other males deal with the potential deception. Regenerated males employed bluffing tactics; they pretended to be aggressive to deter opponents, but surrendered when the fight escalated. Regenerated males tended to choose smaller and opposite-handed opponents, probably because claws can be grappled tightly in same-handed fights. The bluffing tactics seemed to be partially successful, because regenerated males were not selectively challenged by other males and defeated original males in 41.7 % of the cases. However, original males developed counter-bluff tactics, such as choosing same-handed opponents and persisting in contests even when the opponent was larger. Consequently, original males defeated regenerated males equipped with longer a claw in 42.9 % of the cases. In the most striking case, an original male evicted a resident male from his burrow despite the 42.7 % longer regenerated claw. This counter-bluff tactic decreases the reliance on the signal of strength (weapon size) and limits the benefit of bluffing. Significance statement: Male fiddler crab (Uca lactea) fight with their major claw, which grows on either the left or right side. However, males with fragile a regenerated claw (regenerated males) which were similar in size to an original claw made deception in the fights. Regenerated males employed bluffing tactics; they behaved aggressively to deter opponents, but surrendered when the fight escalated. They chose smaller and opposite-handed opponents, because claws can be grappled tightly in same-handed fights. As a result, they defeated males with an original claw (original males) in 41.7 % of the cases. Interestingly, however, original males developed a counter-bluff tactic: they chose same-handed opponents and persisted in contests even when the opponent was larger. Consequently, 42.9 % of original males defeated regenerated males equipped with a larger claw. This counter-bluff tactic serves to “call the bluff” and limit the benefits of bluffing.