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 shouldbe that for which fitness increases at the faster rate withsize. In butterflies, as in most invertebrates, females areusually the larger sex, but previous comparative analysis hasshown that relative male size increases with female polyandryamong butterflies. In agreement with this pattern, males arelarger than females in the strongly polyandrous green-veinedwhite butterfly, Pieris napi L., and in this article we assessthe size dependence of reproductive success in both sexes. Inan experiment where virgin males and females were released inthe field, we found no strong association between size and malemating success. However, laboratory experiments showed thatthere was a strong correlation between size and the ejaculatethat the male delivered to the female at mating and that largeejaculates delayed female remating for a longer time comparedto small ejaculates. Moreover, female P. napi utilize male-derivednutrients received at mating to increase their fecundity. Hence,large males sire more offspring both by way of donating morenutrients to female egg production and by way of delaying femaleremating (given that the last male to mate with the female willfather most of the offspring). Laboratory experiments showedthat the association between size and fecundity was low, ornonexistent, 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 maybe especially weak among polyandrous butterflies because a fundamentalsource of variation in fecundity relates to their ability tofind nutrient giving males, an ability which may be unrelatedto female size. According to this hypothesis there is a causalassociation between weak size dependence of female fecundityand polyandry, and a strong size dependence of male reproductivesuccess that may underlie the comparative pattern of positivecorrelation between relative male size and polyandry.