Fighting for harems: assessment strategies during male–male contests in the sexually dimorphic Wellington tree weta

Females often aggregate at particular sites for feeding or shelter, thus giving adult males the opportunity to defend harems and increase male reproductive success. Rival males compete for control of harems via ritualized displays or direct combat using weaponry. Contests for harems or the resources required by females can be settled based on asymmetries in fighting ability or resource ownership. Males that accurately assess a rival's fighting ability prior to engaging in potentially costly combat should be favoured by selection. Game theory and optimality models provide three models to explain how individuals decide to persist in or flee from a fight. These models are the energetic war of attrition, the sequential assessment model and the cumulative assessment model. Using staged contests in the laboratory, I tested predictions of these models using the Wellington tree weta, Hemideina crassidens, a sexually dimorphic insect native to New Zealand. Male H. crassidens use their enlarged mandibles as weapons in fights for access to adult females that reside in cavities in trees. My results supported a prediction common to each assessment model: contest duration was negatively correlated with the asymmetry in opponent's weapon size. The sequential assessment model of contest settlement was partially supported but the strongest support was for the cumulative assessment model. Predictions of the latter model were supported because: (1) fights are apparently settled based on own-size assessment; (2) fights occur in a single phase and escalate; and (3) contests involve physical combat and injury. I suggest that, in nocturnal species, cumulative assessment will generally be most applicable.