The curse of being single: both male and female Gammarus pulex benefit energetically from precopulatory mate guarding

Abstract

Precopulatory mate guarding (PCMG) is frequently presented as a classic case of sexual conflict between partners. For instance, long-lasting PCMG is regarded as an adaptive male strategy to secure a female in a context of strong intrasexual competition, while females guarded for a long time are assumed to bear many costs. This assumption has been derived from guarding systems where females obviously resist males' attempts to initiate early guarding. However, females of some species such as the freshwater amphipod Gammarus pulex do not seem to possess adaptations to reduce PCMG duration, which remains to be explained from an evolutionary perspective. In this model organism for sexual conflict research, a male grasps a female several days before her sexual receptivity. Here we tested the hypothesis that G. pulex females might benefit from being passively transported by their partner during PCMG, whereas the male alone bears the costs of swimming while carrying his mate. We therefore compared the energetic states of paired and single individuals and found that, after 5 days of PCMG in controlled conditions, paired individuals contained more protein, lipid and glycogen reserves than single individuals in both sexes. Our results suggest that PCMG might be energetically beneficial not only to the female, but also to the male. We discuss overall fitness consequences of PCMG for both partners given the mutual benefits we highlighted here. We plead for a more precise estimation of the cost/benefit ratio for each sex to improve our understanding of how sexual conflict shapes guarding patterns.