Keaveny, E. C. | Search bioRxiv

bioRxiv2026

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 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.

physiology↗

bioRxiv2022

Phat queens emerge fashionably late: body size and condition predict timing of spring emergence for queen bumble bees

For insects, the timing of many life history events (phenology) depends on temperature cues. Body size is a critical mediator of insect responses to temperature, so may also influence phenology. The determinants of spring emergence of bumble bee queens are not well understood, but body size is likely important for several reasons. In fall, queens accumulate energy stores to fuel overwinter survival. Accumulation of fat stores prior to and depletion of fat stores during overwintering are likely size-dependent: larger queens can accumulate more lipids and have lower mass-specific metabolic rates. Therefore, larger queens and queens in relatively better condition may have delayed depletion of energy stores, allowing for later spring emergence. To test whether timing of spring emergence is associated with body size and condition, we captured 295 Bombus huntii queens in Laramie, WY, during the 2020 and 2021 growing seasons, weighed them, and measured intertegular width (a size metric unaffected by variation in feeding and hydration state). Early emerging queens were smaller than later emerging queens across years. Mass relative to intertegular width increased as the season progressed suggesting, as predicted, that body condition influences the timing of spring emergence for these crucial pollinators.

physiology↗