Darwinian balancing selection: Predation counters sexual selection in a wild insect

The potential viability costs of sexually selected traits are central to hypotheses about the evolution of exaggerated traits. Estimates of these costs in nature can come from selection analyses using multiple components of fitness during the same time frame. For a population of tree crickets (Oecanthus nigricornis: Gryllidae), we analyzed viability and sexual selection on male traits by comparing Oecanthus prey of a solitary wasp to those that survived, and comparing mating individuals to solitary males. We measured forewing width (sexually size dimorphic and used for singing), head width, pronotum length, and size of hind jumping legs as potential targets of selection. Supporting the hypothesis that sexually selected traits have viability costs, we found that significant directional sexual selection for wider heads was opposed by significant viability selection for narrower heads. Nonlinear selection revealed that individuals with wide heads and small legs were most attractive, but individuals with narrow heads, large legs, and intermediate pronotum length were most likely to survive. Successful mating may put males at greater risk of predation, especially if copulation per se is risky. Such balancing selection in tree crickets may have constrained the evolution of sexual dimorphism in head size—a condition seen in other gryllids and orthopterans.