Influence of photoperiod on the time of birth in the rat. IV. Effects of an imposed feeding rhythm.

In order to study the respective effects of photoperiod and feeding rhythm on the time of birth in rats, the onset of fetus expulsion was determined in six experimental groups. Under the standard light regime (14L-10D; lights-on at 6 a.m.), rats caged alone or in threes and fed ad libitum (groups C and 3C) gave birth over two preferential periods, the first during the late light phase of day 22 of gestation and the second during the early light phase of day 23. Under the same conditions, rats fed during a restricted period (from 9 a.m. to 11 a.m.) from day 8 of gestation (groups PF and 3PF) gave birth during a single period, starting just before and spreading over the first half of the night separating the last two days of gestation. The number of rats per cage affected birth distribution slightly, but an inverse effect was observed between the feeding regime and the social group constituted by mating (group 3C vs 3PF). Most births occurred after lights-on at day 23 of gestation when the rats were caged alone, fed ad libitum and submitted to a progressive shift in the light phase (group BC). This shift was realized by daily advancing dawn and dusk every day by 15 min from day 8 of gestation. Under these conditions, food restriction from 9 a.m. to 11 a.m. (group BC-PF) resulted in an intermediate distribution between groups BC and PF. The data show that time of birth in rats can be modulated by two different biological entrainers, photoperiod and feeding rhythm and that time of birth, in a given case, is determined by the interactions between these two factors.