Claw size predicts dominance within and between invasive species of crayfish.
During aggression, animals signal their physical abilities to deter opponents from fighting. Competitors that have a coevolved system of communication can resolve the outcome of a fight without paying the cost of engaging in potentially injurious fighting. Invasive species complicate the dynamics of competition, because such species may not have a coevolved system of communication. For example, many species of crayfish have invaded freshwater ecosystems, where they compete with species never encountered before. We studied the traits used to escalate aggression within and between two species of invasive crayfish in Arizona, Procambarus clarkii and Faxonius virilis. Through staged encounters, we found that individuals of both species escalated aggression according to their relative claw size. Furthermore, claw size (instead of claw strength) determined the outcome of interactions that escalated to physical combat. Despite P. clarkii and F. virilis presumably not having a coevolved system of communication, claw size predicted the outcome of interspecific aggression. In crayfish and other species that have invasive potential, future investigations into intraspecific versus interspecific communication dynamics are warranted as they may shed light on the successes and failures of past and present species introductions.