Disentangling thermal preference and the thermal dependence of movement in ectotherms

Many ectotherms thermoregulate by choosing environmental temperatures that maximize diverse performance traits, including fitness. For this reason, physiological ecologists have measured preferred temperatures of diverse ectotherms for nearly a century. Thermal preference is usually measured by observing organism distributions on laboratory thermal gradients. This approach is appropriate for large ectotherms which have sufficient thermal inertia to decouple body temperatures from gradient temperatures. However, body temperatures and therefore speeds of movement of small ectotherms will closely track gradient temperature, making it difficult to distinguish between thermal preference and thermal dependence of movement. Here we develop and demonstrate the use of a patch model to derive the expected thermal gradient distribution given only the thermal dependence of movement. Comparison of this null distribution with the observed gradient distribution reveals thermal preference of small ectotherms.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► We develop a patch model to estimate null gradient distributions of ectotherms. ► Temperature-dependent rates of movement alter the null distribution. ► Comparison of the null with the observed distribution reveals thermal preference. ► We demonstrate use of the model for cohort and individual-based experiments. ► The model suggests that traditional approaches underestimate thermal preference.