SEXUAL SELECTION FOR LARGE MALE SIZE IN A POLYANDROUS BUTTERFLY - THE EFFECT OF BODY-SIZE ON MALE VERSUS FEMALE REPRODUCTIVE SUCCESS IN PIERIS-NAPI

Sexual selection theory predicts that the larger sex should be that for which fitness increases at the faster rate with size. In butterflies, as in most invertebrates, females are usually the larger sex, but previous comparative analysis has shown that relative male size increases with female polyandry among butterflies. In agreement with this pattern, males are larger than females in the strongly polyandrous green-veined white butterfly, Pieris napi L, and in this article we assess the size dependence of reproductive success in both sexes. In an experiment where virgin males and females were released in the field, we found no strong association bet ween size and male mating success. However, laboratory experiments showed that there was a strong correlation between size and the ejaculate that the male delivered to the female at mating and that large ejaculates delayed female remating for a longer time compared to small ejaculates. Moreover, female P. napi utilize male-derived nutrients received at mating to increase their fecundity. Hence, large males sire more offspring both by way of donating more nutrients to female egg production and by way of delaying female remating (given that the last male to mate with the female will father most of the offspring). Laboratory experiments showed that the association between size and fecundity was low, or nonexistent, among P. napi females allowed to mate only once. However, weak size dependence was found for polyandrous females. We hypothesize that size dependence of female fecundity may be especially weak among polyandrous butterflies because a fundamental source of variation in fecundity relates to their ability to find nutrient giving males, an ability which may be unrelated to female size. According to this hypothesis there is a causal association between weak size dependence of female fecundity and polyandry, and a strong size dependence of male reproductive success that may underlie the comparative pattern of positive correlation between relative male size and polyandry.