Virile crayfish escalate aggression according to body size instead of weapon size

Many crustaceans use their large claws to signal fighting ability during territorial contests. However, the size of a claw does not always reflect its strength or the bearer's ability to fight. If strength predicts the outcome of a fight, the mismatch between the size and strength of a claw creates dishonest signalling. Still, only a handful of species have been studied, leaving a gap in our knowledge of the taxonomic diversity and evolutionary history of dishonest signalling. To help fill this gap, we studied signalling and fighting between virile crayfish, Faxonius virilis. First, we measured the relationship between the size and strength of a claw in males and females. Then, we staged encounters between members of the same sex to determine how carapace length, claw size and claw strength influence the probability of fighting and the probability of winning. Both males and females possessed claws whose size poorly predicted their strength, especially at larger sizes. Unlike other species, which escalate aggression based on claw size, virile crayfish escalated aggression based on relative body size. When fighting occurred, however, both body size and claw strength determined the winner. Likely, virile crayfish ignore claw size because body size provides a more reliable signal of dominance. Our results stress the importance of observing behavioural dynamics in addition to functional relationships when studying the potential for dishonest signalling.