An appraisal of operative temperature mounts as tools for studies of ecological energetics

Taxidermic mounts are frequently used to quantify the operative temperature or standard operative temperature or standard operative temperature of complex natural environments. These thermal indices are subsequently used to estimate the metabolic heat production of free-living animals. Such use has often entailed violation of two standards for biological data collection: that multiple samples be analyzed and that instruments be calibrated under the range of conditions used in empirical analysis. To explore the consequences of these practices, we compared the responses of both heated and unheated taxidermic mounts with those of live animals when exposed to combinations of wind and radiation that normally could be experienced in nature. Two mammals, the rock squirrel Spermophilus variegatus and the golden-mantled ground squirrel Spermophilus lateralis, as well as a bird, the verdin, Auriparus flaviceps, were studied. For all species, there was substantial individual variation in mount responses. Under a single set of environmental conditions, coefficients of variance for mont response were as high as 71%, and the range of values between mounts typically equaled the change in the group's average standard operative temperature that was produced by two- to fourfold increases in wind speed. For predicting physiological responses of animals, the adequacy of taxidermic mounts varied widely with species, type of mount, and environmental conditions. In some cases, predictions closely matched animal responses. In other cases, predictions using data from mounts produced errors of up to 28.5 degrees C in the estimate of standard operative temperature. The types of error observed, combined with the extensive investment of time and resources required to construct and determine the utility of mounts as operative temperature thermometers for a particular species, greatly reduces the attractiveness of this technique for quantifying the thermal environments of animals in nature.