Seasonal acclimatization and temperature acclimation in small passerine birds is achieved via metabolic adjustments

Temperature and other environmental factors play an integral role in the metabolic adjustments of animals and drive a series of morphological, physiological, and behavioral adaptions essential to survival. However, it is not clear how the capacity of an organism for temperature acclimation translates into seasonal acclimatization to maintain survival. Basal metabolic rate (BMR), evaporative water loss (EWL), and energy budget were measured in the Chinese Hwamei (Garrulax canorus) following winter and summer acclimatization, and in those acclimatized to 15 ​°C (cold) and 35 ​°C (warm) under laboratory conditions for 28 days. In addition to the above indicators, internal organ masses, as well as state 4 respiration and cytochrome c oxidase (COX) activity were also measured for the liver, skeletal muscle, heart, and kidney. Both winter-acclimatized and cold-acclimated birds exhibited significantly higher BMR, EWL, and energy budget, as well as organ masses, state 4 respiration, and COX activity compared with the summer-acclimatized and warm-acclimated birds. This indicated that the Chinese Hwamei could adapt to seasonal or just temperature changes through some physiological and biochemical thermogenic adjustments, which would be beneficial to cope with natural environmental changes. A general linear model showed that body mass, BMR, GEI, state 4 respiration in the liver and kidney, and COX activity in the skeletal muscle, liver, and kidney were significantly affected by temperature and acclimation. A positive correlation was observed between BMR and each of the other parameters (body mass, EWL, energy budget, heart dry mass, kidney dry mass, state 4 respiration) in the muscle, heart, and kidney and also between BMR and COX activity in the muscle and kidney. The results suggested that similar to seasonal acclimatization, Chinese Hwameis subjected to temperature acclimation also exhibited significant differences in metabolism-related physiological and biochemical parameters, depending on the temperature. The data also supported the prediction that metabolic adjustment might be the primary means by which small birds meet the energetic challenges triggered by cold conditions.