Long-term records of the number of food approaches per 30 min were obtained from rabbits fed ad libitum. In the initial experiments 13 rabbits were alternately kept free-running in constant illumination at 150-200 lux, or entrained to sawtooth regimes in which illumination at the same level was abruptly switched on at 24-hr intervals and decreased in a linear way to 0 lux over 24 hours, or was modulated in the opposite direction. Between the two entrained states there were specific differences in shape of the food intake pattern and in its phase relation with the entraining photic transition, suggesting that the rabbit's food intake pattern is determined by two weakly coupled oscillators with widely different sensitivities and types of response to the two transitions within 24-hr LD alternations. This was confirmed by the results of further experiments, performed with 10 of the original rabbits. These were now exposed to 24-hr photic schedules that were systematically varied in such a way that each of the two transitions occurred over many months at a fixed time of day or night, whereas during periods of many weeks the opposite transition was either left out (exposure to sawtooth illumination) or occurred after one out of five intervals (exposure to standard LD alterations). Analysis of the steady state patterns and of the transients in between pointed to the presence of two oscillators that could nearly independently be entrained by the light ON- and light OFF transitions of photoperiods with a length from 10 hr up to 19 hr. Although LD regimes with such photoperiods induced markedly different food intake patterns, the total number of food approaches per 24 hr was kept remarkably constant.
