Sex- and landscape-related differences in flight ability under suboptimal temperatures in a woodland butterfly

1. Active time budgets of flying ectotherms depend on body temperature. Knowledge of flight ability and performance is currently heavily biased towards the range of temperatures favourable for voluntary flight activity. Flight under suboptimal temperature conditions may also be important, however, in terms of fitness. 2. Here we study the ability of organisms to fly at body temperatures colder than those at which flight is initiated voluntarily, and the duration of such flights. Laboratory-reared Pararge aegeria (L.) butterflies that originated from woodland and agricultural landscape were tested at five temperatures (range: 10-21 degrees C) in a common-garden set-up. 3. We predict that males are able to fly at lower temperatures than females as males have lower wing loading (i.e. body mass/wing area). Since woodland is on average cooler than agricultural landscape we also predict that flight ability at low temperature is better developed in woodland individuals. 4. Individuals showed an ability to fly at all tested temperatures and flights were longer with increasing temperature. Males flew for longer than females. There was no difference in flight duration between sexes at the lowest temperature, but an increasing difference with increasing temperature. We showed that woodland butterflies flew for longer and had higher wing loading than agricultural landscape butterflies. 5. Our results shed new light on the thermal ecology of flight at suboptimal temperatures and are discussed from both proximate and ultimate points of view.