Combat in a cave-dwelling wētā (Orthoptera: Rhaphidophoridae) with exaggerated weaponry

Abstract

Animals sometimes possess extraordinarily enlarged or specialized structures used as weaponry for intrasexual combat. The way in which an animal's mating system leads to the diversity of exaggerated armaments we see in nature is a matter of current and ongoing research. Central to this enquiry is the question of how animal weapons are involved in assessment: how, when and why is the decision made to retreat from a contest by combatants fighting over their future fertilization success? We investigated the agonistic role of highly elongated male hindlegs in an Orthopteran insect found in dense aggregations in New Zealand caves: the cave wētā, Pachyrhamma waitomoensis (Rhaphidophoridae). We found a large degree of sexual dimorphism in the hindlegs. In contests among males in the field, males with longer hindlegs were more likely to win contests, while body size did not influence contest outcome. We also assessed the influence of winner, loser and relative hindleg length on contest escalation, finding that fights among males with greater differences in leg length were resolved by less-escalated contests. In addition, the level of contest escalation was positively correlated with the loser's, but not the winner's, leg length, matching the predictions of self-only models of animal assessment.