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 rat e (MR; indirect calorimetry), body temperature (T-b), and substrate ut ilization (RQ) were measured 24 times each hour throughout the 12h:12h light:dark cycle. The amount of food consumed influenced the amplitud e of the MR and T-b cycles, primarily by affecting the dark-phase segm ent of the cycle: when food was consumed ad lib, low-amplitude daily c ycles in MR and T-b occurred in which levels in the dark phase were lo wer than in the light; during reduced food intake in restricted feedin g or in fasting, high-amplitude cycles occurred primarily because noct urnal hypometabolism and hypothermia developed; in restricted feeding, the level of MR and T-b during the dark-phase segment of the cycle wa s directly related to short-term variation in amount consumed. The tim ing of food consumption primarily affected the light-phase segment of the MR and T-b cycles: when feeding was restricted to a time late in t he light phase, these measures became depressed early in the light pha se, and then greatly elevated near the scheduled time of feeding. This distinctive light-phase pattern developed quickly after the restricte d 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 feedin g. However, approximately 2 h before the first feeding bout of the day , the RQ cycle indicated a sizable shift towards lipid utilization, wh ich 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 th e nocturnal decrease in T-b. During fasting, RQ indicated lipid utiliz ation throughout the entire cycle. Whole-day energy expenditure by pig eons 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.