The cost-benefit model of thermoregulation does not predict lizard thermoregulatory behavior

Physiological processes are optimized within a narrow range of body temperatures. Reptiles engage in behavioral thermoregulation to achieve the optimal body temperature range. Interestingly, however, thermoregulatory effort varies over time and across species. The cost-benefit model of thermoregulation of R. B. Huey and M. Slatkin is the only conceptual framework that attempts to explain the observed variation. The model postulates that reptiles should engage in thermoregulation when the costs incurred do not outweigh the benefits. The main cost is the thermal quality of the environment: thermoregulation is more costly in habitats of low thermal quality. We used thermal quality of the habitat, measured by the deviations of operative temperatures from the preferred body temperature range, as an indicator of costs and two quantitative indices of thermoregulation as indices of thermoregulatory investment. Regressions of the indices of thermoregulation on thermal quality of the habitat produced no pattern or patterns that were in the opposite direction to the predictions of the cost-benefit model of thermoregulation. These results suggest that the disadvantages of thermoconformity are higher than the costs of thermoregulation in habitats of low thermal quality.