UNPREDICTABLE ENVIRONMENTS, NUPTIAL GIFTS AND THE EVOLUTION OF SEXUALSIZE DIMORPHISM IN INSECTS - AN EXPERIMENT

Many insects have a mating system where males transfer nutrients to fe males at mating, which are often referred to as 'nuptial gifts'. Among butterflies, some of the characteristic features of these species are polyandry (females mate multiple times), and relatively large male ej aculates. When males produce part of the resources used for offspring, the value of body size might then increase for males and decrease for females. The male/female size ratio is also observed to increase when the degree of polyandry and gift size increase. Butterfly species whe re gift-giving occurs are generally more variable in body size, sugges ting that food quality/quantity fluctuates during juvenile stages. Thi s will cause some males to have much to provide and some females to be in great need, and could be conducive to the evolution of a gift-givi ng mating system. In such a system, growing male and female juveniles should react differently to food shortage. Females should react by mat uring at a smaller size since their own lack of reproductive resources can partly be compensated for by male contributions. Males have to pa y the full cost of decreased reproduction if they mature at a small si ze, making it more important for males to keep on growing, even when g rowth is costly. An earlier experiment with the polyandrous and gift-g iving butterfly, Pieris napi, supported this prediction. The pattern i s expected to be absent or reversed for species with small nuptial gif ts, where females do not benefit from mating repeatedly, and will thus be dependent on acquiring resources for reproduction on their own. To test this prediction, we report here on an experiment with the speckl ed wood butterfly, Pararge aegeria. We find that growth response corre lates with mating system in the two above species, and we conclude tha t differences in environmental conditions between species may act as a n important factor in the evolution of the mating system and sexual si ze dimorphism.