A trade-off between pre- and post-copulatory sexual selection in a bean beetle

It is now generally recognized that it is necessary to examine how sexual selection operates across the lifespan of a male, in order to understand the total sexual selection in action. However, less attention has been paid to the fact that selection pressures can change in response to varying environmental conditions. Here, we examine male allocation to trait subject to pre- and post-copulatory sexual selection in Callosobruchus chinensis. We find evidence of a trade-off between dispersal ability and testes size and sperm transfer, across geographical strains. In addition, male mating success of strains with larger testes and better dispersal ability, respectively, changed in response to environmental conditions. Males with better dispersal ability had greater success in securing mates, compared to males with larger testes, under conditions where it was harder to find females. There was no difference in male ability to secure mates when it was easy for them to find females. However, we suggest that males with larger testes are likely to have an advantage in sperm competition under conditions where it is easier to find females, and when females are thus likely to mate multiply. Our results indicate that environmental fluctuations and trade-offs can work in conjunction to maintain alternative male behavioral reproductive strategies.