The fitness cost to females of exposure to males does not depend on water availability in seed beetles

Access to multiple males can benefit a female in terms of increased fecundity and/or offspring performance. However, the presence of more males can also impose costs on females that arise from an elevated mating rate (e.g. due to increased genital damage, loss of feeding opportunities) and/or increased harassment. Different environments might influence the relative magnitude of these costs and benefits, because they can influence how often males and females encounter each other as well as the nature of these encounters. In the seed beetle, Callosobruchus maculatus, water is a limiting resource for females that can be obtained from male ejaculates. Here we explored whether the net fitness of female seed beetles is affected by breeding in either a dry or a wet environment when housed with differing numbers of males (none, one or four). Consistent with costly male harassment, females housed with four males laid significantly fewer eggs than those housed alone or with a single male, but there was no effect of the number of males on female egg-laying rate, life span, larval development rate or egg–adult survival of offspring. Although females in the wet environment lived significantly longer, there was only tentative evidence that water availability affected the net fitness cost to females of being exposed to more males. We conclude that to understand the evolution of mating systems it is important to explore how the environment affects female fitness by balancing the costs and benefits of being exposed to males.