Being Big or Emerging Early? Polyandry and the Trade-Off between Size and Emergence in Male Butterflies

In many butterfly species, females mate only once, whereas males are capable of multiple matings. As a result, males compete intensely for access to females. To maximize their expected mating success, males emerge some time before the females. This phenomenon, known as protandry, is widespread but not ubiquitous. Protandry is negatively correlated with polyandry (i.e., repeated female mating) and male size. Since male size and development time are interdependent, several hypotheses can be advanced to explain the observed correlations. Either large size is especially important in polyandrous species, or early emergence is of lesser importance. So far, models of protandry have neglected the trade-off between large size and early emergence. In this article, I present models that account for this trade-off and the interaction with polyandry. In polyandrous species, females use male-derived nutrients to enhance realized fecundity. Since large males produce large ejaculates, there will be a selection pressure for large male size, counterbalancing the selection for early emergence. Given this hypothesis, the correlations among protandry, polyandry, and relative male size can be explained. Finally, the conditions under which polyandry is likely to evolve are discussed.