Exaggerated male hindlegs function as pure weapons of male-male combat in thorny devil stick insects

Sexually selected weapons can function as both combat tools and agonistic signals of fighting ability, depending on whether and how males assess rivals. We investigated the function of the enlarged male hindlegs in the New Guinean thorny devil stick insects, Eurycantha calcarata in male-male and male-female interactions. Field and lab experiments showed that larger males with proportionally larger hindlegs were more likely to win fights over access to females and subsequently mate. Behavioral sequence analyses and contest cost predictors indicated that males likely use a mutual assessment strategy. Surprisingly, males did not appear to use their hindlegs as signals of fighting ability, relying instead on tactile and chemical cues to assess opponents. Hindlegs were employed only to deliver powerful squeezes in rare, escalated fights. During copulation, males also used hindlegs to stabilize their position, but as females did not appear to resist, we found no evidence for a coercive function. These findings suggest that enlarged male hindlegs in E. calcarata serve purely as force-delivering combat tools rather than signaling structures, even though males assess rivals during contests. These result highlight how understanding the specific functions and contexts of weapon use provides critical insight into the diversification of sexually selected traits.