According to the metabolic theory of ecology (MTE), rates of ecological processes are fundamentally limited by organism metabolic rates. Metabolism has in turn been found to scale predictably with temperature according to the Arrhenius equation for enzyme kinetics, which can be adapted to account for enzyme deactivation at high temperatures (e.g., Sharpe-Schoolfield model). An important outstanding question is how key metabolic parameters, such as the activation energy for metabolism, are influenced by plastic responses of organisms to shifting temperatures, i.e., thermal acclimation responses. In this study, we investigated how thermal acclimation influences key MTE model parameters for adult spring peeper frogs (Pseudacris crucifer), by using the rate of observable respiratory movements as a proxy for metabolic rate. After holding frogs at one of three “acclimation temperatures” for two weeks, we transferred them to one of eight “performance temperatures”. We then used analysis of high-speed video to measure frog respiratory rates at several time points following the transfer. We used statistical model fitting to estimate key MTE model parameters and to determine whether and how thermal acclimation influenced P. crucifer thermal performance.
Results/Conclusions
We found that thermal acclimation did not significantly alter P. crucifer metabolic responses to temperature when the respiration rate was assumed to scale with mass as predicted by MTE (exponent of -1/4). When the mass-scaling exponent was allowed to vary, however, it optimized to a positive value and the effect of thermal acclimation on respiration rate became significant. Upon further investigation, we found that this was caused by a confounding effect of the acclimation treatment on frog mass; i.e., the average mass of frogs held at warmer acclimation temperatures was lower than those held at cooler temperatures. Thus the apparent acclimation effect on respiratory performance appears to have been mediated by treatment effects on frog mass, rather than adaptive plastic responses. This result highlights the importance of accounting for mass-scaling relationships when analyzing the temperature dependence of metabolism. The Sharpe-Schoolfield model provided a better description of the thermal performance curve for P. crucifer metabolism than the Arrhenius model (lower AIC) and provided a slightly higher estimate of the activation energy for metabolism (Arrhenius: 0.27 ± 0.01 eV; Sharpe-Schoolfield: 0.33 ± 0.05 eV).