Evidence for a separate food-entrainable circadian oscillator in the pigeon

Abstract

In Experiment 1, four pigeons lived in a metabolic chamber on a 12h:12h LD cycle where they maintained a reduced body weight by consuming a daily ration of food presented at the eighth hour of the photophase. Body temperature ( T b) and oxygen consumption (V O 2 ) increased prior to the daily feeding. The possibility that a food-entrainable oscillator timed these anticipatory responses was tested by four manipulations, conducted in successive phases, each of which involved eliminating the regularly scheduled food presentation, which is the putative entraining stimulus for such an oscillator, while the 12h:12h LD cycle remained in effect. The manipulations, and their outcomes, were: when fasting was imposed for 3 days, the anticipatory responses continued to occur; when ad lib feeding was allowed for 11 days, the anticipatory responses were mostly eliminated; when fasting was reimposed for 5 days, there was evidence that the anticipatory responses reoccurred; and, when the time of the daily feeding was phase-shifted earlier in the photophase for 8 days, anticipatory responses persisted at the original feeding time and simultaneously developed at the new feeding time. In the first phase of Experiment 2, key pecking by two pigeons produced food only during hours 9–11 of the daily photophase (12h:12h LD). In this condition, T b increased and key pecking occurred in anticipation of the daily period of food availability. Evidence for a food-entrained oscillator was sought in a second phase when constant dim light (LL) was imposed without changing the hours of food availability. In one pigeon, a free-running rhythm in T b was observed while the anticipatory rise in T b remained entrained to food. The results of these experiments indicate support for a food-entrainable oscillator in pigeons, which appears to be functionally separate from photic-entrainable oscillators.