On Estimating Thermal Conductance in Endotherms

TLDR

Thermal conductance quantifies heat exchange between an endotherm and its environment, and when measured below thermoneutrality it equals the slope of the metabolism–temperature curve extrapolated to zero metabolism; however, least‑squares fitted slopes (Cf) are typically lower than the true mean minimal conductance (Cm) because endotherms combine physical and chemical thermoregulation. The authors provide a correction formula, Cm/Cf = 0.060 δT + 1.0, that adjusts least‑squares slopes to true mean minimal conductances for studies conducted between 12–20 °C air temperature and 32–38 °C body temperature. The analysis demonstrates that extrapolated curves overestimate body temperature, and the resulting bias in conductance estimates follows the linear relationship Cm/Cf = 0.060 δT + 1.0.

Abstract

Thermal conductance is a measure of the ease with which heat is exchanged between a body and the environment. It may include or exclude evaporative heat loss. If evaporative heat loss is included, conductance in endotherms should be used only at temperatures below thermoneutrality because evaporation is of little importance only at these temperatures. The slope of the curve of metabolism on air temperature equals conductance if and only if the curve extrapolates to body temperature when the rate of metabolism is zero. Slopes fitted by the method of least squares (Cf) usually are less than the mean minimal conductance (Cm) calculated from individual measurements of metabolism below thermoneutrality, because most endotherms mix physical and chemical thermoregulation at temperatures below thermoneutrality. The extent to which an extrapolation of the fitted curve overestimates body temperature (δT) is related to the error by which fitted slopes estimate mean minimal conductance: Cm/Cf = 0.060(δT) + 1.0. This relation can be used to correct fitted slopes in the literature to mean minimal conductances, as long as the mean air temperature at which minimal conductance is being estimated falls between 12 and 20 C and mean body temperature falls between 32 and 38 C.