Morphological and physiological determinants of local adaptation to climate in Rocky Mountain butterflies

Flight is a central determinant of fitness in butterflies and other insects, but it is restricted to a limited range of body temperatures. To achieve these body temperatures, butterflies use a combination of morphological, behavioural and physiological mechanisms. Here, we used common garden (without direct solar radiation) and reciprocal transplant (full solar radiation) experiments in the field to determine the thermal sensitivity of flight initiation for two species of Colias butterflies along an elevation gradient in the southwestern Rocky Mountains. The mean body temperature for flight initiation in the field was lower (24-26 degrees C) than indicated by previous studies (28-30 degrees C) in these species. There were small but significant differences in thermal sensitivity of flight initiation between species; high-elevation Colias meadii initiated flight at a lower mean body temperature than lower-elevation Colias eriphyle. Morphological differences (in wing melanin and thoracic setae) drive body temperature differences between species and contributed strongly to differences in the time and probability of flight and air temperatures at flight initiation. Our results suggest that differences both in thermal sensitivity (15% contribution) and in morphology (85% contribution) contribute to the differences in flight initiation between the two species in the field. Understanding these differences, which influence flight performance and fitness, aids in forecasting responses to climate change.