Why do male Callosobruchus maculatus harm their mates?

• Males of the bruchid beetle Callosobruchus maculatus have spines on their intromittent organs that puncture the female reproductive tract during mating. Females kick their mates during copulation. If females are prevented from kicking the males, copulations last longer and the injuries females sustain are more severe. We tested whether or not these injuries represent real fitness costs that can be mitigated by kicking and also what males gain by inflicting them. Our results show that females do indeed suffer lowered lifetime fecundity if they are prevented from kicking. However, we could find no evidence that males gain benefits through harming their mates. It has been suggested that the way females respond to the harm may benefit the male causing it. Injured females may be less willing to remate to avoid sustaining further injuries, or they may respond by increasing their rate of oviposition if they perceive the injuries as a threat to their survival. In our study, however, females that were prevented from kicking did not respond by delaying remating or increasing their rate of oviposition. Furthermore, preventing females from kicking during their second copulation did not make their second mates more successful in sperm competition. This suggests that the spines have evolved for other reasons than harming the females, such as serving as an anchor during copulation, and that the harm they cause is a side effect of a male adaptation and is not itself adaptive for either sex.

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