Body Size Evolution in Butterflies

Sammanfattning: Life history research deals with the scheme of resource partitioning to a wide spectra of processes and the trade-offs shaping these events. One of the most fundamental life history trade-offs is the one of at which age- and size an organism should start to reproduce; reaching a large size at maturity is often advantageous in terms of high adult survival and reproductive potential, while to attain a larger size the organisms must prolong juvenile development which is assumed costly in terms of mortality. In holometabolous insects, a larger size usually confers increased fitness to females in terms of fecundity. Moreover, insect larvae have the capacity for fast size increase. So, it seems that there are substantial fitness benefits associated with a choice to prolong development. Surprisingly, there is a great lack of empirical support for costs of such a choice and by incorporating these observations into a life history framework one arrives at the conclusion that insect body sizes should be several times larger than observed. I study body size evolution in butterflies by closely examining the fitness consequences of variation in the age- and size at maturity. By combining both laboratory and field measures of size-fitness relationships with standard life history modelling, I arrive at the main conclusions; 1: positive size dependent predation on larvae might a) significantly increase the cost of attaining a larger size at maturity, or b) induce risk sensitive foraging responses so to slow larval growth rates and thereby restrict size at maturity, 2: ecological factors might constrain female fecundity by inducing time limitation on large females that need more time to convert all their resources into offspring, making reproductive value increase at a slower rate than body size with increased larval growth effort. These mechanisms may help to explain the inconsistency between natural observations and theoretical predictions of life history variation in insects.