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

Basal metabolic rate (BMR) is often assumed to be indicative of the energy turnover at ambient temperatures (T_a) 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 (BMR_m) and SMR (SMR_m) were lower in individuals living in a harsh foraging environment. The correlation between SMR_m at different T_a was high (r=0.91), independent of foraging environment, showing that individuals are consistently ranked according to their SMR_m However, the correlations between BMR_m and SMR_m 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 T_a at least in part reflected differential body temperature (T_b) regulation: early morning T_b was lower at low T_a, and more so in individuals with a weaker metabolic response to lower T_a 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 T_a, such as the easily tractable SMR, will be more informative than the use of BMR as a proxy for energy turnover.