Mean temperature determines whether winter variability accelerates or buffers energy loss

Winter survival in dormant animals depends on conserving finite energy reserves, yet winter temperatures fluctuate around shifting means. In ectotherms, metabolic rate increases exponentially with temperature, so thermal variability is expected to accelerate energy loss, with consequences for overwinter survival under climate change. Whether dormant ectotherms can compensate physiologically, however, remains unclear. We overwintered Bombus impatiens queens under constant (2, 3, 4°C) or variable (2 ± 6°C or 4 ± 6°C) temperature regimes for 6 weeks, then measured metabolic rates across the temperature range. Thermal experience altered the temperature dependence of metabolism, but responses depended on mean temperature: variability centred on 2°C elevated metabolic rate at 11°C and increased thermal sensitivity of O 2 consumption relative to constant 2°C, whereas variability centred on 4°C reduced baseline metabolism and thermal sensitivity estimated from CO 2 production relative to constant 4°C. Using these responses, we simulated subsequent lipid depletion from a standardized energetic state. Variability around 2°C was predicted to shorten the time to lipid depletion, whereas variability around 4°C produced trajectories similar to those predicted for constant 4°C. Thus, identical thermal variance produced opposite physiological and predicted energetic outcomes depending on the mean temperature around which fluctuations occurred. Integrating both temperature means and variability is, therefore, essential for predicting overwintering energetics in a changing world. Read the free Plain Language Summary for this article on the Journal blog.