Distinguishing between active plasticity due to thermal acclimation and passive plasticity due to Q10 effects: Why methodology matters

Characterizing thermal acclimation is a common goal of eco-physiological studies and has important implications for models of climate change and environmental adaptation. However, quantifying thermal acclimation in biological rate processes is not straightforward because many rates increase with temperature due to the acute effect of thermodynamics on molecular interactions. Disentangling such passive plastic responses from active acclimation responses is critical for describing patterns of thermal acclimation. Here, we reviewed published studies and distinguished between different study designs measuring the acute (i.e. passive) and acclimated (i.e. active) effects of temperature on metabolic rate. We then developed a method to quantify and classify acclimation responses by comparing acute and acclimated Q(10) values. Finally, we applied this method using meta-analysis to characterize thermal acclimation in metabolic rates of ectothermic animals. We reviewed 258 studies measuring thermal effects on metabolic rates, and found that a majority of these studies (74%) did not allow for quantifying the independent effects of acclimation. Such studies were more common when testing aquatic taxa and continue to be published even in recent years. A meta-analysis of 96 studies where acclimation could be quantified (using 1,072 Q(10) values) revealed that 'partial compensation' was the most common acclimation response (i.e. acclimation tended to offset the passive change in metabolic rate due to acute temperature changes). However, 'no acclimation' and 'inverse compensation', in which acclimation further augmented the acute change in metabolic rate, were also common. Acclimation responses differed among taxa, habitats and with experimental design. Amphibians and other terrestrial taxa tended to show weak acclimation responses, whereas fishes and other aquatic taxa tended to show stronger compensatory responses. Increasing how long the animal was allowed to adjust to a new test temperature increased the acclimation response, but body size did not. Acclimation responses were also stronger with longer acclimation durations. Collectively, these results highlight the importance of using the appropriate experimental design to investigate and estimate thermal acclimation of biological rates. To facilitate and guide future studies of thermal acclimation, we end with some suggestions for designing and interpreting experiments. A free Plain Language Summary can be found within the Supporting Information of this article.