Level of sperm competition promotes evolution of male ejaculate allocation patterns in a moth

The risk and intensity of sperm competition can influence male adaptations that ultimately affect male fertilization success, for example investment in sperm production and strategic sperm allocation patterns. Previous studies have examined male behavioural responses to variation in immediate levels of sperm competition. Here we quantified the evolutionary responses of male adaptations resulting from long-term manipulation of the level of sperm competition. By varying the adult sex ratio of populations of the Indian meal moth, Plodia interpunctella, we successfully altered the frequency of male and female copulations, and thus the level of sperm competition experienced by males. As predicted by theory, males evolving under higher levels of sperm competition increased investment in sperm production and differed in their sperm allocation patterns across matings, compared to males evolving under intermediate and low levels of sperm competition. However, we found no evidence for trade-offs between ejaculate components, or between total sperm production and male body size in this species.