Environmental, ecological and mechanistic drivers of avian seasonal metabolic flexibility in response to cold winters.

Small birds inhabiting regions with cold winter climates show seasonally flexible metabolic phenotypes, with the winter phenotype characterized by increments of summit metabolic rate (M) and cold tolerance. In the study reported here, we focused on variations in M as a metric of metabolic performance because it is positively correlated with cold tolerance in birds and positively related to overwinter survival in small mammals, although the latter has yet to be demonstrated in birds. Temperature appears to be a prominent driver of seasonal metabolic phenotypes in birds, as evidenced by the correlation between inter- and intra-seasonal variation in M and temperature variation, by recent temperature variables serving as better predictors of M variation than long-term climate variables, and by the induction of M variation by experimental cold exposure. In contrast, photoperiod and social status do not appear to be prominent drivers of metabolic flexibility in birds studied to date. Because skeletal muscle is the primary thermogenic tissue in birds, studies of the mechanistic underpinnings of metabolic flexibility have focused on skeletal muscles, particularly flight muscles. At the level of the skeletal muscle, two potential mechanisms exist for increasing thermogenic capacity, namely, muscle hypertrophy and elevated cellular metabolic intensity. Correlative studies suggest consistent winter increments in flight muscle size, with a potential regulatory role for the muscle growth inhibitor myostatin. Recent experimental studies in small birds, including modification of flight costs, cold acclimation, and exercise training, also suggest that muscle size is an important driver of metabolic flexibility in birds. Therefore, the focus of our study was on the seasonal regulation of muscle size and its contribution to metabolic flexibility. Future studies should address fitness consequences of M variation, the relative roles of muscle hypertrophy, and other factors (e.g., oxygen and substrate transport, cellular metabolic intensity) promoting M variation, as well as the molecular mechanisms underlying seasonal phenotypes. [ABSTRACT FROM AUTHOR]