Influence of photoperiod on the time of parturition in the rat. III. Comparison of different daily light lengths with changes in light timing or light pulse given during darkness.

Four groups of normal rats have been studied to elucidate the effect on the time of birth of four different light regimes applied throughout pregnancy (day 1: beginning of pregnancy). The results showed that the time of parturition (onset of expulsion) depended on the light treatment: the longer the duration of the daily illumination, the longer pregnancy lasted. With a 14 hrs light regime, the births occurred during two main periods: 50 p. 100 before 8 p.m. on day 22 of pregnancy and 36 p. 100 between 5 a.m. and 1 p.m. on day 23. With 8 h of light per day, 69 p. 100 of the rats delivered before 9 p.m. on day 22 and 85 p. 100 before 5 a.m. of day of pregnancy. On the other hand, only 22 and 49 p. 100 of the rats gave birth at the same periods under a 20 hrs light regime. Parturition under a completely dark photoperiod occurred mainly before day 23 of pregnancy and with a different rhythm than those of the preceding groups. The influence of maternal adrenalectomy on the time of parturition was determined using two light regimes (14 or 20 hrs of light per day). Under those conditions, the length of pregnancy was modified; parturition occurred earlier in adrenalectomized than in the normal rats submitted to the same light regime. These results suggest that the effect of light on the time of birth in the rat involves adrenal gland secretions.