Time of birth and daily activity mediated by feeding rhythms in the pregnant rat

The temporal relationship between time of birth and daily physical activity has been studied in rats submitted to different feeding rhythms. Animals, put under 14 h of light and 10 h of darkness (lights on from 6 to 20 h), were isolated at mating (day 1 of gestation) and assigned from day 8 to one of five groups. Group C was fed ad libitum, and groups 2PF, 9PF, 14PF and 21PF had food available for three hours per day from 2, 9, 14 and 21 h, respectively. In groups 9PF and 14PF, births were clustered in one single period during the night between days 22 and 23 of gestation. In groups C, 2PF and 21PF, births were distributed into two periods, during the afternoon of day 22 before darkness and after dawn of day 23. The birth rates of these groups were 55.3, 74.2 and 27.6%, respectively, on day 22 and 44.7, 25.8 and 72.4% on day 23. No births occurred during the times that food was available; they were noted in all groups at times when the animals were least active according to records kept throughout gestation. Thus, restricting food availability to periods of normal inactivity (groups 9PF and 14PF vs group C) led to a major shift in the time of delivery as well as to a distortion of the normal activity patterns. These results confirm that feeding rhythms are potent entrainers of birth time in rats and that they interact with the light regime. Pregnant rats seem to be organized so that birth either precedes the main daily physical activity or follows it, depending upon the environmental conditions.