Male density and rapid evolution of genital morphology in the seed beetle Callosobruchus maculatus

Male reproductive structures are known to be extremely diverse, particularly in insect taxa. Male genital structures are thought to be some of the fastest evolving traits, but the processes responsible for this pattern remain unclear. In the present study we manipulated the mating regimes of Callosobruchus maculatus, a seed beetle, to determine if male genital structures would be altered under forced monogamy and polyandry. Males in this species have an intromittent organ that contains spines that are known to puncture the female reproductive tract. We measured both testes size and genital spine length in monogamous and polyandrous treatments over seven generations. We found that testes size was not significantly different between treatments, but that genital spine length was significantly longer in the polyandrous treatment within seven generations. These results highlight the fact that evolution can occur rapidly when under strong sexual selection, a process that has been implicated in leading to morphological differences in male genitalia.