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.