High temperature induces the upward shift of the thermal neutral zone and decreases metabolic capacity in zebra finches.

The thermal neutral zone (TNZ) represents a fundamental concept in the thermal physiology of homeothermic organisms. TNZ is characterized as a specific range of environmental temperatures within which the metabolic rate remains at its basal level. The ambient temperature is regarded as a critical environmental factor that affects an animal's thermoregulation and propels the development of various morphological, physiological, and behavioral adaptations. In the present investigation, we assessed the influence of environmental temperature on various physiological parameters, including body mass, metabolic rate, thermoneutral zone (TNZ), state 4 respiration (S4R), cytochrome c oxidase (CCO) activity, body fat content, triglyceride content, free fatty acid content, β-hydroxyacyl Co-A dehydrogenase (HOAD) and citrate synthase (CS) activities, AMPK and PGC-1α mRNA levels, and triiodothyronine (T₃) and tetraiodothyronine (T₄) concentrations in zebra finches acclimated to 25 °C or 38 °C. zebra finches were found to have a TNZ of 32-42 °C when acclimated to 25 °C and a TNZ of 34-42 °C when acclimated to 38 °C. Acclimation to a high temperature led to an increase in the lower critical temperature (LCT), consequently resulting in a narrower TNZ. Zebra finches acclimated to 38 °C for a duration of four weeks exhibited a notable reduction in both body mass and basal metabolic rate as opposed to individuals maintained at 25 °C. Additionally, finches that were acclimatized to 38 °C exhibited a reduction in liver mass and a lower S4R level in both the liver and kidneys. Furthermore, these finches showed decreased CCO activity in the pectoral muscle and liver and lower avian uncoupling protein expression in the pectoral muscle compared with the birds acclimated to 25 °C. The T₃ level in the serum was lower in the 38 °C-acclimated finches than the 25 °C-acclimated finches. These findings suggested that the shift in the LCT of TNZ in zebra finches may possibly be associated with their metabolic capacity as well as their T₃ levels at a different ambient temperature. The changes in LCT of TNZ could be an important strategy in adapting to variations in ambient temperature in zebra finches.