Daily cycles in body temperature, metabolic rate, and substrate utilization in pigeons: influence of amount and timing of food consumption.
Pigeons lived in individual chambers where instantaneous metabolic rate (MR; indirect calorimetry), body temperature (Tb), and substrate utilization (RQ) were measured 24 times each hour throughout the 12h:12h light:dark cycle. The amount of food consumed influenced the amplitude of the MR and Tb cycles, primarily by affecting the dark-phase segment of the cycle: when food was consumed ad lib, low-amplitude daily cycles in MR and Tb occurred in which levels in the dark phase were lower than in the light; during reduced food intake in restricted feeding or in fasting, high-amplitude cycles occurred primarily because nocturnal hypometabolism and hypothermia developed; in restricted feeding, the level of MR and Tb during the dark-phase segment of the cycle was directly related to short-term variation in amount consumed. The timing of food consumption primarily affected the light-phase segment of the MR and Tb cycles: when feeding was restricted to a time late in the light phase, these measures became depressed early in the light phase, and then greatly elevated near the scheduled time of feeding. This distinctive light-phase pattern developed quickly after the restricted feeding schedule began and may reflect the influence of a circadian food-entrainable oscillator. RQ indicated carbohydrate utilization for most of the 24-h cycle during ad lib feeding and in restricted feeding. However, approximately 2 h before the first feeding bout of the day, the RQ cycle indicated a sizable shift towards lipid utilization, which terminated after the bout was completed. There was a smaller, more transient, decrease in RQ near the time of the light-dark transition, which may imply cessation of digestive activity in preparation for the nocturnal decrease in Tb. During fasting, RQ indicated lipid utilization throughout the entire cycle. Whole-day energy expenditure by pigeons in these laboratory circumstances was shown to be closely related to the changes in within-day cycles associated with variations in the amount and timing of food intake.