Individual (co)variation in thermal reaction norms of standard and maximal metabolic rates in wild‐caught slimy salamanders

Standard metabolic rate (SMR) and maximal metabolic rate (MMR) are fundamental measures in ecology and evolution because they set the scope within which animals can perform activities that directly affect fitness. In ectotherms, both SMR and MMR are repeatable over time when measured at a single ambient temperature (Tₐ). Many ectotherms encounter variable Tₐ from day to day and over their lifetime, yet it is currently unknown whether individual differences hold across an ecologically relevant range of Tₐ (i.e. thermal repeatability; RT). Moreover, it is possible that thermal sensitivity of SMR and MMR are important individual attributes, and correlated with one another, but virtually nothing is known about this at present. We measured SMR and MMR across an ecologically relevant Tₐ gradient (i.e. from 10 to 25 °C) in wild‐caught salamanders (Plethodon albagula) and found that RT was significant in both traits. SMR and MMR were also positively correlated, resulting in a lower RT in absolute and factorial aerobic scopes (AAS and FAS). We found significant individual differences in thermal sensitivity for both SMR and MMR, but not for AAS and FAS. The intercept (at Tₐ = 0 °C) and the slope of the thermal reaction norms were negatively correlated; individuals with low MR at low Tₐ had a higher thermal sensitivity. Finally, individuals with a high thermal sensitivity for SMR also had high thermal sensitivity for MMR. Our results suggest that natural selection occurring over variable Tₐ may efficiently target the overall level of – and thermal sensitivity in – SMR and MMR. However, this may not be the case for metabolic scopes, as the positive correlation between SMR and MMR, in addition to their combined changes in response to Tₐ, yielded little individual variation in AAS and FAS. Our results support the idea that organisms with low metabolism at low Tₐ have a high metabolic thermal sensitivity as a compensatory mechanism to benefit in periods of warmer environmental conditions. Hence, our study reveals the importance of considering within‐individual variation in metabolism, as it may represent additional sources of adaptive (co)variation.