UNPREDICTABLE FOOD AND SEXUAL SIZE DIMORPHISM IN INSECTS

The evolution of sexual size dimorphism is likely to be affected by th e amount of resources each sex invests in offspring. Male nuptial gift s, occurring in many insect species, might reduce the value of large s ize in females and increase the value for males. For large nuptial gif ts and an accompanying shift in dimorphism to evolve, males with large r gifts should be rewarded, in effect trading the larger gift for more offspring. We suggest that food variability, causing some males to ha ve much to provide and some females to be in great need, would be cond ucive to the evolution of such a mating system, and we present compara tive data on butterflies supporting the suggestion. In a gift-giving m ating system, growing male and female juveniles should react different ly to food shortage. A female maturing at small size can to some exten t buffer her disadvantage through nuptial gifts, whereas a male maturi ng 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. Her e we confirm this prediction in a butterfly, Pieris napi, with large n uptial gifts.