Individual variation in metabolic reaction norms over ambient temperature causes low correlation between basal and standard metabolic rate.

Basal metabolic rate (BMR) is often assumed to be indicative of the energy turnover at ambient temperatures (Ta) below the thermoneutral zone (SMR), but this assumption has remained largely untested. Using a new statistical approach, we quantified the consistency in nocturnal metabolic rate across a temperature range in zebra finches (N=3213 measurements on 407 individuals) living permanently in eight outdoor aviaries. Foraging conditions were either benign or harsh, and body mass and mass-adjusted BMR (BMRm) and SMR (SMRm) were lower in individuals living in a harsh foraging environment. The correlation between SMRm at different Ta was high (r=0.91), independent of foraging environment, showing that individuals are consistently ranked according to their SMRm However, the correlations between BMRm and SMRm were always lower (average: r=0.29; range: 0<r<0.50), in particular in the benign foraging environment. Variation in metabolic response to lower Ta at least in part reflected differential body temperature (Tb) regulation: early morning Tb was lower at low Ta, and more so in individuals with a weaker metabolic response to lower Ta Our findings have implications for the use of BMR in the estimation of time-energy budgets and comparative analyses: we suggest that the use of metabolic rates at ecologically relevant Ta, such as the easily tractable SMR, will be more informative than the use of BMR as a proxy for energy turnover.