Larval competition causes the difference in male ejaculate expenditure in Callosobruchus maculatus

The seed beetle Callosobruchus maculatus larvae exhibit two types of resource competition: scramble, in which a resource is shared, and contest, in which the resource is monopolized. This difference in larval behavior results in different adult densities. Under contest competition, adult density remains constant regardless of larval density, but under scramble competition, adult density increases with larval density. This in turn affects mating frequency during adulthood, and thus, the intensity of sexual selection operating on males. In this study, we examined the relationship between larval competition types and male reproductive investment in mating. We assessed the male ejaculate expenditure per mating across geographic strains of C. maculatus. The male investment (ejaculate expenditure) increased with the degree of scramble competition and decreased with the degree of contest competition. We therefore suggest that males experience different selective pressures depending on the type of larval competition: scramble type males are selected for increased reproductive investment.