Contemporary sexual selection on sexually dimorphic traits in the ambush bug Phymata americana

Sexual selection is a potent evolutionary force often invoked to explain observed cases of sexual dimorphism. However, evidence of this process operating on existing phenotypic variation is limited. We investigated whether sexual selection could account for sexual dimorphism in size and color pattern in the ambush bug Phymata americana. We considered the alternative hypothesis that dimorphism merely reflects sex differences in habitat use but found no evidence of sex differences in microhabitat during 2 sampling periods in the wild. Although the form of sexual (phenotypic) selection on male lateral color pattern varied between samples, selection consistently favored lateral coloration in males but not size. For females, weight was a consistent predictor of mating status in both the early and the late season. We performed 2 separate laboratory studies to investigate potential proximate mechanisms of sexual selection that might account for the field data. Although we found that male weight predicted male success in direct male-male competition and male courtship intensity predicted success in male-female interactions, we did not detect any role of male color pattern in either laboratory study. These data suggest that visual signaling is unlikely to play a role in the evolution of color pattern dimorphism in this species. Consistent with the field data, our laboratory results also found that female weight predicted the probability of copulation, possibly indicating that female receptivity coincides with female reproductive cycle (i.e., egg maturation).