How to avoid errors when quantifying thermal environments

Modelling thermal environments at high resolution becomes simpler when using operative temperature, which condenses microclimate and morphology into an index of thermal stress. Operative temperature can be mapped using large numbers of 'operative temperature thermometers', hollow models that duplicate external properties of the animal. As climatologists predict that air will warm by 2-4 °C by 2100, biologists must be able to distinguish climate change from systematic errors in operative temperature of the same magnitude. A systematic error in operative temperature of 2 °C or a similar amount of climate warming can change predicted surface activity and indices of habitat quality, thermoregulatory precision and predation risk by 5-12%, and in some cases more than 30%. As construction details of operative temperature thermometers can affect their accuracy by 2 °C or more, biologists should use detailed physical models calibrated against living animals over potential ranges of postures, orientations and microclimates. Water-filled models do not measure operative temperature correctly, fail to capture thermal extremes and are an unnecessary complication as one can easily compute the body temperature of moving or stationary animals from body mass and the spatio-temporal distribution of operative temperatures.