Use of operative temperature and standard operative temperature models in thermal biology

Operative temperature (Te) and standard operative temperature (Tes) models have been used to address ecological questions about the thermal biology of ectotherms and endotherms for over 25 years. This review focuses on the accuracy and use of Te and Tes models in ecological and physiological studies. The utility of Te and Tes models lie in the fact that they take a multivariate problem involving inputs of air temperature, ground temperature, solar radiation, and wind speed and map them into a single thermal metric on a spatial scale appropriate for the animal. The most reliable Te models are copper casts that mimic the morphology and absorptivity of an animal. Simplified Te models such as cylinders and spheres have been shown to produce errors in Te as large as 12 °C under certain conditions and should only be used after careful calibration against a live animal. The accuracy of heated Tes models has been addressed in much less detail then that of Te models. When calibrated and used under conditions of low solar radiation, heated taxidermic mounts and simplified Tes models produce errors in net heat production on the order of 5% or less. In order to provide reliable data, all types of models must be calibrated over an ecologically realistic range of environmental conditions experienced by the animal. This advice has been largely ignored in the literature, where 61% of the of studies examined failed to properly calibrate the models prior to use. Additionally, studies using these models tend to lack experimental rigor, using only one or two models to make measurements on 1 or 2 days of the active season. When used correctly, Te and Tes models can be powerful tools for integrating the thermal environment experienced by an animal into a single metric that can address questions regarding the ecology, physiology, and behavior of endotherms and ectotherms. However, until investigators make the effort to use these models in a scientifically valid manner with proper calibration and experimental design their value to thermal biologists will be limited.