Mean temperature determines whether winter variability accelerates or buffers energy loss

Abstract

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 important consequences for overwinter survival and population persistence under climate change. However, it remains unclear whether dormant ectotherms can compensate physiologically for thermal variability. We overwintered Bombus impatiens queens under constant (2, 3, 4{degrees}C) or variable (2 {+/-} 6{degrees}C or 4 {+/-} 6{degrees}C) regimes for six weeks, then measured metabolic rates across a range of temperatures. The temperature dependence of metabolic rate shifted in response to thermal experience, but the direction of compensation depended on mean temperature: variability centered on 2{degrees}C elevated metabolic rate and increased thermal sensitivity relative to all other conditions, whereas variability centered on 4{degrees}C reduced metabolic rate and dampened thermal sensitivity relative to constant 4{degrees}C. We used these metabolic responses to simulate rates of lipid depletion and found that survival trajectories echoed physiological shifts: experiencing variability around 2{degrees}C would reduce subsequent survival time, whereas experiencing variability around 4{degrees}C would preserve subsequent survival even under variable future conditions. Thus, identical thermal variance produced opposite energetic outcomes depending on the mean temperature around which fluctuations occurred. Integrating both temperature means and variability is, therefore, essential for predicting overwintering survival in a changing world.