Age-related sperm transfer and sperm competitive ability in the male hide beetle

Abstract

The influence of male age on reproductive success after a single mating has been explored widely; however, few studies have investigated whether quantitative or qualitative differences in male sperm are responsible for the observed patterns. Moreover, the role of male age on sperm competitive ability has been largely ignored. We examined the importance of male age on the probability and amount of sperm transferred during a single mating and explored whether sperm competitive ability varies with male age in the hide beetle Dermestes maculatus, a species where sperm viability does not vary with male age. We also investigated whether sperm transfer rates varied with female age. We found that the probability of sperm transfer and the amount of sperm transferred varied with male, but not female, age. All males performed behaviorally successful copulations, but intermediate-age and old males were more likely to transfer sperm successfully and also transferred a greater quantity of sperm than young males. Old males were less likely to transfer sperm than intermediate-age males, but if they did transfer sperm successfully, they transferred comparable amounts. Sperm competitive ability varied with male age and reflected the quantity of sperm transferred. On average, intermediate-age males achieved greater fertilization success when competing against young or old males than when competing against other intermediate-age males. Old males were poor competitors against intermediate-age males, but they achieved significantly higher rates of fertilization when competing against young males. Our findings suggest that quantitative differences in the amount of sperm transferred determine male success in sperm competition in the hide beetle.