Assessment of strength and willingness to fight during aggressive encounters in crickets*

Game theory predicts that at least some of the behaviour patterns displayed during aggressive encounters are used to assess asymmetries in variables that indicate fighting ability and resource value. Game theoretical models such as the sequential assessment game see assessment as the major activity during a fight. However, while these models acknowledge the existence of physical and motivational assessment parameters, there are only a few examples where a mechanism for the assessment of fighting readiness has been shown. In staged encounters between male Mediterranean field crickets, Gryllus bimaculatus, fighting behaviour follows a stereotyped escalation cascade with ritualized displays in the beginning and physical combat towards the end. Despite their larger size, heavier animals lost 30% of the encounters even if weight asymmetry was large. To examine whether the contestants provide assessment cues that might explain this surprising result, we analysed two stereotyped displays in detail (antennal fencing and mandible spreading). The duration of antennal fencing, which is necessary to initiate a fight, was independent of experience and weight asymmetry between the contestants, but was prolonged after shortening the antennae by almost 90%. Fights escalated only when antennal movement frequencies were high in both contestants. In blinded crickets few contests were settled by another ritualized display, mandible spreading, and fights that escalated beyond this stage were significantly shorter than in untreated crickets. We suggest that antennal fencing may be used to assess fighting readiness of the opponent, while mandible spreading may indicate fighting ability. We conclude that high willingness to fight may help crickets to overcome inferior fighting ability.