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The period of time required by fasting fowls to adjust their metabolic rate following an increase in environmental temperature was investigated using respiration calorimetry.A significant decrease in the fasting metabolic rate of White Leghorn hens and Light Sussex cockerels was observed following increasing the environmental temperature from 22° C to 28° C. No significant decline in the fasting metabolic rate of the hens occurred during the first 3 d at the higher temperature. After 12 d no further decline occurred, indicating that the period of adaptation was between 3 and 12 d. In the cockerels no significant decline had occurred by the 7th day, but after 28 d the adaptation of metabolic rate was complete. The importance of these results in relation to the assessment of the thermoneutral zone of the fowl and the role of the thyroid secretion in acclimatisation in the fowl are discussed.

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1. Calorimetric experiments on the utilization of dietary energy by the laying White Leghorn hen are described.