The two environmental drivers of thermoregulatory costs: Interactions between thermal mean and heterogeneity influence thermoregulation

Abstract

Ectotherms often thermoregulate behaviourally within variable thermal environments in their attempts to acquire optimal body temperatures. Thermoregulation accrues benefits but incurs costs, which are crucial for understanding thermoregulatory behaviour. Costs of thermoregulating are influenced by two key attributes of the thermal environment—heterogeneity of thermal microhabitats and the deviation of the mean temperature from an animal's preferred temperature. However, empirical research has rarely distinguished between these two drivers of thermoregulatory costs or examined them concurrently. Our experiment used a novel thermoregulatory arena to examine the independent and interactive effects of the two environmental drivers of costs of thermoregulation on the thermoregulatory behaviour of a small ectotherm, the jacky dragon (Amphibolurus muricatus). Following previous theory, we predicted that (i) thermoregulation would be higher in environments with greater thermal heterogeneity (body temperatures closer to preferred temperatures, higher thermoregulatory accuracy and higher effectiveness of thermoregulation) and (ii) changes in thermoregulation as a function of mean environmental temperatures would differ depending on thermal heterogeneity. We found support for our prediction that these two environmental attributes had an interactive effect on thermoregulation but not for our prediction that thermal heterogeneity would impact thermoregulation independently. Individuals in highly heterogeneous environments maintained greater thermoregulatory accuracy compared with those in less heterogeneous environments as the mean environment increasingly deviated from the preferred temperature range. The results emphasise the crucial conceptual distinction between thermal mean and heterogeneity as drivers of thermoregulatory costs and how this distinction underpins variation in the thermal behaviour of ectotherms. Read the free Plain Language Summary for this article on the Journal blog.