Dissecting the mechanisms underlying old male mating advantage in a butterfly

Selection is expected to maximize an individual’s own genetic reward regardless of the potential fitness consequences for its sexual partners, which may cause sexual conflict. Although performance in holometabolous insects typically diminishes with age, old male mating advantage has been documented in a few species. Whether this pattern arises from female preference for older males based on, e.g., pheromone blends (intersexual selection), or from increased eagerness to mate in older compared to younger males is currently debated. We explore the mechanistic basis of old male mating advantage, using a series of experiments including behavioral as well as manipulative approaches, in the tropical butterfly Bicyclus anynana. Consistent with the residual reproductive value hypothesis, old male mating advantage was associated with a greater eagerness to mate, evidenced by a two times higher flying and courting activity in older than in younger males. In contrast, we found only limited support for a contribution of female preference for older males based on pheromone composition, although male sex pheromones clearly do play a role in mating success. Our results suggest that male behavior may play a primary role in old male mating advantage, and that pheromones are likely of secondary importance only. Male mating success was related to higher overall pheromone titers rather than variation in a single component. A dominant importance of male behavior in determining mating success may result in sexual conflict.