Take-off performance under optimal and suboptimal thermal conditions in the butterfly Pararge aegeria.

Realized fitness in a fluctuating environment depends on the capacity of an ectothermic organism to function at different temperatures. Flying heliotherms like butterflies use flight for almost all activities like mate location, foraging and host plant searching and oviposition. Several studies tested the importance of ambient temperature, thermoregulation and butterfly activity. Here, we test the influence of variation in flight morphology in interaction with differences in body temperature on locomotor performance, which has not been thoroughly examined so far. Take-off free flight performance was tested at two different body temperatures in males and females of the speckled wood butterfly Pararge aegeria. We found that both males and females accelerated faster at the optimal body temperature compared to the suboptimal one. The multivariate analyses showed significant sex-specific contributions of flight morphology, body temperature treatment and feeding load to explain variation in acceleration performance. Female and male butterflies with a large relative thorax (i.e. flight muscle investment) mass and large, slender wings (i.e. aspect ratio) accelerated fast at optimal temperature. However, high aspect ratio individuals accelerated slowly at suboptimal temperature. Females of low body mass accelerated fast at optimal, but slowly at suboptimal body temperature. In males, there was an interaction effect between body and relative thorax mass: light males with high relative thorax mass had higher performance than males with a low relative thorax mass. In addition, relative distance to the centre of forewing area was positively related to acceleration at both temperatures in males. Males and females with higher feeding loads had lower levels of acceleration. Finally, males that were able to accelerate fast under both temperatures, had a highly significantly heavier relative thorax, lower body and abdomen mass. More generally, this study shows that the significance of butterfly flight morphology in terms of flight performance is at least partially dependent on body temperature.