Relationships between interspecific differences in the mass of internal organs, biochemical markers of metabolic activity, and the thermogenic properties of three small passerines

The capacity for thermogenesis is considered part of an animal’s adaptive strategy for survival, and basal metabolic rate (BMR) is one of the fundamental physiological standards for assessing the energy cost of thermoregulation in endotherms. BMR has been shown to be a highly flexible phenotypic trait both between, and within, species, but the metabolic mechanisms involved in the regulation of BMR, which range from variation in organ mass to biochemical adjustments, remain unclear. In this study, we investigated the relationship between organ mass, biochemical markers of metabolic tissue activity, and thermogenesis, in three species of small passerines: wild Bramblings (Fringilla montifringilla), Little Buntings (Emberiza pusilla) and Eurasian Tree Sparrows (Passer montanus), caught in Wenzhou, southeastern China. Oxygen consumption was measured using an open-circuit respirometry system. Mitochondrial state-4 respiration and cytochrome c oxidase (COX) activity in liver and pectoral muscle were measured with a Clark electrode. Our results show that Eurasian Tree Sparrows had significantly higher BMR, digestive organ mass, mitochondrial state-4 respiration capacity and COX activity in liver and muscle, than Bramblings and Little Buntings. Furthermore, interspecific differences in BMR were strongly correlated with those indigestive tract mass, state-4 respiration and COX activity. Our findings suggest that the digestive organ mass, state-4 respiration and COX activity play an important role in determining interspecific differences in BMR.