Adaptation versus pleiotropy: why do males harm their mates?

Recent studies have documented male traits that cause physical harm to their mates during copulation. Such harm has been suggested to either (1) arise as a negative pleiotropic side effect of adaptations that give males a reproductive advantage in another context or (2) represent a male adaptation per se. In other words, male traits that cause harm to their mates may become established despite the fact that they cause harm or because they do so. A critical assumption of the latter hypotheses is that females respond to infliction of harm in a manner that is beneficial to their mates: by reducing their propensity to remate and/or by elevating their current reproductive rate. In the present study, we test this assumption by experimentally inflicting various forms of harm to females immediately after copulation in three different insect species. We reveal that females do not delay remating or increase their reproductive rate after being harmed but, on the contrary, remate sooner and lay fewer eggs in some cases. We conclude that selection for infliction of harm to females per se is unlikely. Instead, available empirical evidence supports the hypothesis that harmful male traits arise as negative pleiotropic side effects of adaptations that yield other selective advantages to males during reproductive competition.