Multi-level variation of instantaneous metabolism across activity and time in a songbird

Abstract Metabolism and activity should be tightly linked; metabolism provides energy for activity, and general activity may often affect the rate at which animals acquire resources that drive metabolism. However, little is known about the ways metabolism and activity covary in ecologically germane contexts in natural populations. To explore this in more detail, we collected repeated measures of instantaneous metabolic rates and associated changes in activity levels in house sparrows (Passer domesticus) in response to shifts between stimulated and unstimulated contexts. Using a reaction norm approach and mixed-effects models, we examined among-individual variation in metabolism during inactivity (intercept/resting metabolic rate; RMR) and the rate of change in metabolism associated with changes in activity and time since a transition in the level of external stimuli (slope/metabolic plasticity). Individuals exhibited significantly repeatable differences in both inactive metabolism and metabolic slopes with respect to time (but not activity) during stages of the experiment designed to ramp up or ramp down responses. Individuality persisted after accounting for potential causes of variation. Our results indicate consistent differences in how individuals alter metabolic rate across time after changes in stimulation, raising questions about underlying processes contributing to and maintaining such variation. Similar analyses incorporating physiological and life history traits may reveal important relationships that allow for the maintenance of individual variation in metabolic plasticity. Our framework of treating metabolic rate as a reaction norm across time and activity uncovers ecologically relevant individual variation that holds promise for further investigations into the context-dependent link between metabolism and behavior.