Photoperiod duration and energy balance in the pigeon.

Energy balance, and daily rhythms in feeding activity, body temperature (Tb), metabolic rate (O2 consumption), and RQ (CO2/O2) that affect that balance, were studied in pigeons when the duration of the photophase gradually lengthened (LP group) or shortened (SP group) from an initial starting point at LD 12:12. The end point of change for the LP group was LD 21:3, and for the SP group was LD 3:21. Standard laboratory conditions were in effect (moderate ambient temperature; ad lib food and water). On LD 12:12, energy balance was positive (the ratio of gross energy intake to energy expenditure approximated 1.25). In the light phase, a bimodal pattern of feeding was accompanied by elevated levels in Tb, O2 consumption, and RQ; in the dark phase, Tb and O2 consumption fell at lights-off, and prior to lights-on there were anticipatory rises in both measures and a drop in RQ. Energy balance was remarkably constant over a wide range of photoperiods, but at the shortest photoperiods energy balance became more positive (approximately 1.45) because energy intake increased without much change in energy expenditure. Changes in the daily rhythms of the various measures provided some bases for understanding the changes in energy balance. Analysis of the Tb rhythms indicated that the circadian system of the pigeon appears to be capable of adjusting to a wide range of photoperiods. It is suggested that the increase in energy balance at short photoperiods may occur because of inadequate feedback from nutritional and metabolic signals, or may reflect anticipatory winter seasonal adjustments triggered by photoperiod duration.

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.