Operative temperature (T_e) is a useful tool for asking ecological questions concerning the energetic interactions between organisms and their environments. The operative temperature of an animal can be calculated either mathematically or estimated by using a physical model. Both methods integrate measurements of energy flow to produce a single index (T_e) that approximates the steady state body temperature of the animal in a given set of environmental conditions. Absorbed radiation is a key component of the energy budget that influences operative temperature, and is used for both mathematical and physical models. Physical models are typically built to approximate the size and shape of the animal of study and are constructed of a conductive material, such as copper, that can respond rapidly to environmental fluctuations with minimal thermal inertia. Models must also be painted to match the absorbance of the animal. This is typically done by choosing a paint that matches the integrated spectral absorptance of the animal for clear day solar radiation, including the ultra violet, visible, and short-wave infrared components. However, models are often placed in environments that do not receive direct solar radiation, and instead solar radiation may pass through vegetation, cloud cover, or other filters that alter the spectral composition of the incident radiation. This may lead to a discrepancy between the radiation absorbed by the animal and the radiation absorbed by the paint. This is also problematic when using mathematical models, as these too rely on a correct estimate of absorptivity. We measured the incident radiation spectrum under a variety of shrubs and conditions, and investigated the potential error in the absorptance under these scenarios for both painted models, and for mathematical models by integrating their absorptance curves against the different incoming spectra.  

Results/Conclusions

We found that under some cover types, integrated absorbance under clear day solar conditions could differ by 10%. This difference creates a discrepancy using either mathematical or physical operative temperature models in that operative temperature may be estimated incorrectly, thus creating incorrect predictions for a variety of physiological estimates, e.g. time available for activity, preferred temperature availability, field metabolic rates, etc.. Caution must be taken when calculating absorptivity to be sure that the incident solar spectrum for each microhabitat is used to correct the estimate.