Ritualized submission and the reduction of aggression in an invertebrate

Ritualized behaviors that signify acceptance of a dominance relationship and reduce aggression between rivals are a common feature of vertebrate social behavior [1, 2]. Although some invertebrates, including crayfish [3], lobsters [4], and ants [5], display dominance postures, more complex dominance rituals and their effects on fitness have not been reported. We found that crayfish display such a complex ritual, when two males engaged in pseudocopulatory behavior to signify their dominance relationship. This was followed by a reduction in aggression and an increased likelihood of the subordinate's survival. Pseudocopulation was initiated by the eventual dominant and could be accepted or refused by the eventual subordinate. The frequency of aggressive behavior declined significantly during the first hour in all pairs that pseudocopulated but remained high in pairs that did not. Whereas all the subordinate members of pairs that pseudocopulated survived the initial 24 hr of pairing, half of subordinates that did not pseudocopulate were killed during that time. This differential mortality indicates that the reduction of aggression induced by the pseudocopulatory ritual directly enhances the differential survival of male crayfish that engage in this behavior.