The Consequences of Sexual Dimorphism in Body Size for Butterfly Flight and Thermoregulation

The body size differences between male and female insects of a species can result in different flight and thermoregulatory abilities, potentially causing marked behavioural differences between the sexes. In the butterfly Euphydryas phaeton Drury (Lepidoptera: Nymphalidae), females are twice the size of males, with greater wing loadings throughout most of their lives. Males are able to fly at lower body temperatures than females, but multiple-regression analysis of body temperature as a function of weather variables reveals that females are better able to maintain elevated body temperatures against shortterm thermal fluctuations. Males perch in exposed sites where they are more vulnerable to convective cooling; heat loss is counteracted by basking for the majority of the daylight hours. Females inhabit a warmer, more sheltered microenvironment and spend less time basking. I argue that the flight of females is sluggish and more constrained by environmental conditions than that of males due to their greater wing loading, yet in spite of the difference in the degree of environmental constraint, males and females spend an equal amount of time in flight. These results show that small behavioural differences can place ectotherms in different thermal environments, dramatically offsetting the physical constraints imposed by a given body size. Key-words: Behaviour, butterflies, Euphydryas phaeton, flight, habitat selection, physiologial ecology, thermoregulation