Varied Air Temperature and Helium‐Nitrogen Ratios Reveal the Thermal Performance Curve of a Free‐Flying Endotherm

Abstract

While thermal performance curves are central to understanding the thermal biology of ectotherms and are key parameters for predicting effects of climate change, a thermal performance curve has never been described for free flight of any endotherm. This is likely the case for two reasons: 1) it is challenging to vary the body temperature of most flying endotherms, and 2) thermal performance curves require assessment of maximal performance at each body temperature, and maximal performance is challenging to elicit during fight. We overcame both of these challenges by flying honey bees at 23°C and 35°C air temperatures and at air densities ranging from 1.288-0.441 kg·m-3 (21% O2/79% N2 to 21% O2/79% He). Decreasing air density increases lift requirements, providing a way to test maximal aerobic performance during hovering flight. Helium has a six-fold higher thermal conductance than nitrogen, so interactively changing air temperature and He-N2 content provides a system for varying body temperature. Decreasing air density produced an increase in flight metabolic rate of up to 40% (Fig. 1), and the range of air temperatures and He-N2 content produced thorax temperatures that ranged from 29-44°C (Fig. 2). Plotting the maximal performance at each thorax temperature revealed a clear thermal performance curve, with an optimal temperature of 39°C, and maximal performance decreasing by about 5% with increasing or decreasing 1°C thorax temperature (Fig. 2). These findings help resolve past disagreements about how honey bees thermoregulate during hovering flight; some prior studies have found that flight metabolic rates were constant across air temperatures, while others have reported that decreased metabolic heat production at higher air temperatures contributes to thermoregulation. We found that the range of thorax temperatures differed in these prior studies, and our results suggest that honey bees use different mechanisms to thermoregulate during hovering depending on thorax temperature. Because maximal flight performance will determine capacities to carry loads and fly in challenging environments, the thermal performance curve during flight likely affects the fitness of honey bee colonies, and thus may be useful for developing predictions for effects of climate change on these important pollinators. Also, documentation of the strong effects of thorax temperature on flight performance provides a quantitative measure of the benefits of thermoregulation for this partially homeothermic periodic endotherm.