Decoupling of behavioural and physiological thermal performance curves in ectothermic animals: a critical adaptive trait.

Thermal performance curves (TPCs) represent an increasingly popular tool in ecology for anticipating species responses to climate change. TPC theory has been developed using species that experience similar temperatures during activity and at rest and consequently exhibit thermal ranges for activity that closely coincide with their physiological thermal tolerances. Many species, however, experience other stressors, such as desiccation, that limit active behaviour at temperatures below the maximum values experienced. As a result, activity is constrained to a narrow thermal window that is a subset of the range of temperatures that can be tolerated physiologically. This results in a decoupling of behavioural and physiological TPCs that does not conform to the present paradigm. To test the generality of TPC theory, we measured thermal responses for behaviour (crawling speed) and physiological tolerance (heart rate) for six rocky shore gastropods spanning a thermal/desiccation stress gradient. We hypothesized a positive relationship between shore level and the degree of decoupling of behavioural and physiological TPCs. This prediction was confirmed, and was explained by the extension of the physiological TPC beyond the range of the behavioural TPC. Decoupling of behavioural and physiological TPCs is central to predicting accurately the fitness dynamics of ectothermic species subject to multiple stressors. We believe that this decoupling should be explicitly considered as an adaptive trait defining an organism's thermal niche.