Morphological and physiological correlates of among- individual variation in basal metabolic rate in two passerine birds.

Basal metabolic rate (BMR) has been shown to be a highly phenotypic flexibility trait within species. A significant proportion of an individual's energy budget is accounted for by BMR, hence among-individual variation in this trait may affect other energetic processes, as well as fitness. In this study, we measured BMR, organ mass, mitochondrial respiration capacities and cytochrome c oxidase (COX) activities in muscle and liver and circulating levels of plasma triiodothyronine (T) in Chinese bulbuls (Pycnonotus sinensis) and Eurasian tree sparrows (Passer montanus). Our results showed that heart and kidney mass was positively correlated with BMR in Chinese bulbuls, whereas liver and kidney mass was positively correlated with BMR in Eurasian tree sparrows. Regarding metabolic biochemical markers of tissues, state 4 respiration and COX activity in the muscles of the Chinese bulbuls was correlated with BMR, while state 4 respiration in the muscle and liver was correlated with BMR in Eurasian tree sparrows. T was significantly and positively correlated with BMR in Chinese bulbuls and Eurasian tree sparrows. Consistent with the above results, our findings suggest that T levels play an important role in modulating BMR in Chinese bulbuls and Eurasian tree sparrows. Moreover, individual variation in BMR can be explained partly by morphological and physiological mechanisms.