GOTTHARD, K; Stockholm University: Optimal body size and growth strategies in temperate butterflies

Adaptive variation in rates of juvenile development and growth is often present both within and between populations, indicating that natural selection tends to optimize these rates rather than maximize them. Larval growth strategies of butterflies in the tribe Pararginii (Satyrinae: Nymphalidae) seems to be adapted to variation in seasonal time horizons that is typical for temperate environments. Individual larvae use the photoperiod to estimate how much time they have available for larval development and adjust their growth rates in relation to this information. The adjustment of larval growth in relation to time stress also interacts with the ambient temperature during growth so that the relationship between growth rate and temperature depends on the seasonal state of individual larvae. These examples indicate that when individual larvae have a relatively longer time period available for growth they reduce their growth rates rather than try to reach the larger pupal size that constant growth maximization would allow. This is surprising since adult size is often positively correlated with reproductive success in insects. One explanation why larvae use extra time for growth to reduce growth rates rather than to increase in final size is that high growth rates are costly in terms of lower starvation endurance and in higher predation risks. However, in relation to current life history theory it seems unlikely that costs of high growth rates and long development times are the only explanations for why many insects with exponential growth do not become bigger than they do. A plausible additional explanation lies in the relationship between body size and fitness, which often is not known in any detail. I hope to discuss the present knowledge of this relationship and how it may relate to the evolution of adaptive growth strategies.