Ejaculate size, second male size, and moderate polyandry increase female fecundity in a seed beetle

Abstract

Several hypotheses have been proposed to explain the evolution of polyandry in species that provide nuptial gifts. When nuptial gifts are in the form of nutritional elements in the ejaculate and ejaculate size is correlated with male body size, females can accrue both direct (nutritional) and indirect (genetic) benefits from multiple mating. We examined remating decisions in females of the seed beetle Stator limbatus and, using path analysis, examined the effects of male body size on the size of his ejaculate, the amount of ejaculate that was successfully transferred to females, and the overall effect of these variables on female fecundity. Larger males produced larger ejaculates and consequently transferred a larger ejaculate to females, but the effects on female fecundity differed between the females' first and second mates. Both larger first and second males were able to transfer more of their ejaculate to females than were smaller males. Both the total amount of ejaculate transferred by these males and polyandry (number of matings) were positively correlated to female fecundity independently of each other. However, larger second males were more successful at stimulating female fecundity independently of how much ejaculate they transferred. We also provide evidence that females are choosy during their second mating opportunity. Both female choosiness and higher female investment after mating with larger second males suggest that females may benefit from both direct and indirect effects from multiple mating. We also conclude that male body size is under both directional fecundity selection and directional sexual selection.