Resting Metabolic Rate Does Not Predict Peak Metabolic Rate in the Glanville Fritillary Butterfly.

Peak metabolic rate reflects maximal performance and may have direct fitness consequences, whereas resting metabolic rate (RMR) represents the maintenance cost of the whole animal. These traits may be linked, which has significant implications for the evolution of both traits. In vertebrates, a positive correlation between RMR and aerobic capacity has been proposed to explain the origin of endothermy. However, as studies on the relationship between RMR and aerobic capacity have focused on vertebrates, we know much less about these traits in ectothermic insects. I measured RMR in the Glanville fritillary butterfly (Melitaea cinxia) using two configurations: one optimized for measuring flight metabolic rate and the other optimized for RMR. The relationship between RMR and body mass was similar for the two configurations. Body mass explained 82% of the variation in RMR when it was measured using the "flight" configuration at 32°C, and 91% when using the "rest" configuration at 23°C. The Q10 coefficient calculated based on the two RMR measurements was 2.8. Mass-independent RMR was positively correlated between measurements obtained using the two instrument configurations. However, neither measure of RMR was correlated with peak metabolic rate, which indicates that RMR cannot be used as a surrogate measure for aerobic capacity in the Glanville fritillary. Ectothermic insects may be able to combine high metabolic capacity with no apparent increase in maintenance cost. Even though RMR is among the most frequently measured physiological variables, it may have limited predictive power when it comes to questions related to activity or aerobic capacity, or in the case of butterflies, flight performance.