Sexual selection on aggressiveness in females: evidence from an experimental test with tree swallows

While much research has focused on the evolution of conspicuous male displays, ornaments and behaviours, less is understood about the evolution of similar traits in females. To determine whether females express these traits as nonadaptive by-products of sexual selection favouring the same traits in males or whether females are directly selected to express such traits, the benefits of these traits must be understood. The prevalence of aggression in female–female competition suggests that females may experience direct sexual selection for aggressiveness, but data are lacking. I used the tree swallow, Tachycineta bicolor, to test the hypothesis that selection directly favours aggressiveness in females during intrasexual competition over nesting sites. After assaying the aggressiveness of a population of tree swallows, I experimentally reduced the number of nesting cavities, a resource required for female reproductive success. More aggressive females were more likely to obtain nesting cavities after the experimental reduction, while female age and male aggressiveness did not predict nesting cavity acquisition. Aggressive behaviour in female tree swallows is therefore beneficial in the context of female–female competition for a critical breeding resource. These results indicate that the processes favouring aggressiveness in intrasexual competition among females may not be very different from the processes shaping aggressive behaviour in males.