Tissue-specific Thermal Acclimation in Chinese Bulbuls: Differential Physiological Response Rates to Temperature...

In nature, survival is affected by many ecological factors, with ambient temperature (Ta) being one of the most important, as it determines the energy metabolism of animals. In endothermic bird species, exposure to colder temperatures significantly increase metabolic thermogenesis. However, whether prior exposure to cool temperatures exacerbates birds' response to warm temperatures and affects tissuespecific physiological response rates in the context of temperature fluctuations remains unclear. Our study examined the effects of cool-warm thermal acclimation on metabolic thermogenesis in different tissues of Chinese bulbuls (Pycnonotus sinensis). Adult Chinese bulbuls were captured in Wenzhou City, Zhejiang, China. After 4 weeks of acclimation at a Ta of 10°C, the birds were divided into a control group (10°C) and an experimental group (30°C). The experimental group was further subdivided into four groups according to the duration of acclimation: 1 day, 3 days, 7 days, and 28 days. After each acclimation period, the body mass, basal metabolic rate (BMR), energy budget, organ mass, state 4 respiration (S4R), and the activity of cytochrome c oxidase (COX) in the liver and muscle were measured. Prolonged exposure to warm acclimation resulted in significant decreases in body mass, energy intake, BMR, tissue mass, S4R, and COX activity in both the liver and muscle, whereas the digestibility of food increased. Interestingly, the small intestine mass and liver metabolism rapidly decreased on the first day of transfer to a warm environment, while the muscle response to such thermal changes was relatively slow, suggesting tissuespecific differences in the physiological response rates to warm temperatures. Overall, these results revealed the influence of changing ambient temperature on the metabolism in Chinese bulbuls and offered valuable insights into the regulation of diverse biological processes that mediated the physiological adjustments of Chinese bulbuls during the seasonal temperature fluctuations.