Sperm Competition Cannot Eliminate Protandry

Many butterfly species are protandrous, that is males emerge some time before the females. Models aiming to explain protandry generally assume that females mate only once, while males are capable of multiple matings. This assumption is not generally fulfilled, however. In polyandrous species (females mating repeatedly), protandry is often reduced or absent, and males are relatively large. A possible explanation for the observed pattern is offered by the so-called sperm competition hypothesis. According to this hypothesis, male size is positively correlated with male reproductive success, because mating with large males lowers the propensity of females to remate. After remating, females fertilize eggs with sperm of the latter male. The number of eggs fertilized as the result of a mating therefore, increases with size. According to the sperm competition hypothesis, the selection pressure for large size counterbalances that for early emergence. In this paper, it is shown that the sperm competition hypothesis fails to explain the absence of protandry in polyandrous species. The analysis shows that the proposed mechanism only affects the level of polyandry.