The role of temperature as a driver of metabolic flexibility in the Red-billed Leiothrix (Leiothrix lutea)

Background The thermoregulatory ability of animals is strongly influenced by the temperature of their environment. Acclimation to cold requires a range of physiological and morphological adjustments. In this study, we tested the hypothesis that a small passerine, the Red-billed Leiothrix (Leiothrix lutea), can maintain homeothermy in cold conditions by adjusting the physiology and biochemistry of its tissue and organs and return to its former physiological and biochemical state when moved to a warm temperature.

Methods Phenotypic variation in thermogenic activity of the Red-billed Leiothrixs (Leiothrix lutea) was investigated under warm (35 ℃), normal (25 ℃) or cold (15 ℃) ambient temperature conditions. Oxygen consumption was measured using an open-circuit respirometry system. Mitochondrial state-4 respiration and cytochrome-c oxidase (COX) activity in liver, kidney heart and pectoral muscle were measured with a Clark electrode.

Results Birds acclimated to an ambient temperature of 15 ℃ for 4 weeks significantly increased their basal metabolic rate (BMR) compared to a control group kept at 25 ℃. Birds acclimated to 35 ℃ decreased their BMR, gross energy intake (GEI) and digestible energy intake (DEI). Furthermore, birds acclimated to 15 ℃ increased state-4 respiration in their pectoral muscles and cytochrome-c oxidase (COX) activity in their liver and pectoral muscle, compared to the 25 ℃ control group. Birds acclimated to 35 ℃ also displayed lower state-4 respiration and COX activity in the liver, heart and pectoral muscles, compared to those kept at 25 ℃. There was a positive correlation between BMR and state-4 respiration, and between BMR and COX activity, in all of the above organs except the liver and heart.

Conclusions Our study illustrates that the morphological, physiological, and enzymatic changes are associated with temperature acclimation in the Red-billed Leiothrix, and supports the notion that the primary means by which small birds meet the energetic challenges of cold conditions is through metabolic adjustments.