Unpredictable food and sexual size dimorphism in insects

The evolution of sexual size dimorphism is likely to be affected by the am ount of resources each sex invests in offspring. M ale nuptial gifts, occurring in m any insect species, might reduce the value of large size in females and increase the value for males. For large nuptial gifts and an accom panying shift in dimorphism to evolve, males with larger gifts should be rewarded, in effect trading the larger gift for more offspring. We suggest that food variability, causing some males to have much to provide and some females to be in great need, would be conducive to the evolution of such a m ating system, and we present com parative data on butterflies supporting the suggestion. In a gift-giving m ating system, growing male and female juveniles should react differently to food shortage. A female m aturing at small size can to some extent buffer her disadvantage through nuptial gifts, whereas a male m aturing at small size will suffer from his inability to provide substantial gifts. Thus, males benefit more than females from continued growth in the face of food shortage, leading to a shift in size dimorphism. Here we confirm this prediction in a butterfly, Pieris napi, with large nuptial gifts.