Identifying a causal agent of sexual selection on weaponry in an insect

In many animal species, males do not seek females directly butinstead locate and defend sites that contain spatially or temporallylimited resources essential to female survival and reproduction.Resident males that successfully repel conspecific rivals canmate with females attracted to these resources. In theory, increasingresource value increases harem size and thus increases the opportunity(I_mates) for and strength of sexual selection on traits crucialto male resource-holding potential and mating success. I experimentallytested this hypothesis in the field using the Wellington treeweta, Hemideina crassidens (Orthoptera: Tettigonioidea: Anostostomatidae),a sexually dimorphic insect in which males use their enlargedmandibles as weapons in male–male contests over accessto females sheltering in tree cavities (galleries). By manipulatinggallery size, I showed that, compared with smaller galleries,larger galleries housed larger harems. Variation in gallerysize was an important determinant of I_mates, but contrary toexpectation, greater opportunity existed in small galleriescompared with large galleries. As predicted, male weapon sizewas under stronger directional selection in large galleriesbecause the fitness benefits were greater under these conditionscompared with small galleries. My results help explain the positiveassociation between average weapon size and average gallerysize observed within and among tree weta populations in NewZealand.