The Relationship Between the Diurnal Light Cycle and the Time of Ovulation in Mice

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Abstract
The period required for the completion of ovulation in groups of mated females has been studied in 615 mice from three random-bred stocks (L, C and PCT) and in 137 mice of mixed origin. The mice were examined after having been kept for some time under one of four different diurnal light cycles, viz. the natural diurnal light cycle in Edinburgh in December (1) and in June (2) and artificial, reversed, light cycles of 10 hr. darkness/14 hr. light (3) and 4 hr. darkness/20 hr. light (4). The variation between mice in any one group was greatest when they were maintained under the cycles that had a relatively long dark phase: a period of 12-14 hr. was required for ovulation in groups of L and C stock mice under cycles with a dark phase of 10-15 hr. whereas groups of similar females maintained under cycles with a 4-6 hr. dark phase required only 8-9 hr. for ovulation. Probit analysis indicated that this difference was statistically significant. In changing the length of the dark phase of the diurnal cycle the beginning of each phase was altered by 3-4½ hr.; the mid-points of each phase were altered by less than 1 hr. The results suggest that the mid-point of the ovulation period was determined more by the mid-point of the dark phase than by its onset or completion. Differences noted between stocks of mice in the mid-point of the ovulation period and in the ability to adjust quickly to an altered light cycle gave indication that the neural mechanisms involved in the control of the time of ovulation in mice are modified according to the genetic constitution of the animal. The mean interval between coitus and ovulation in females of L and C stocks under natural light cycles was found to be approximately 5 hr. The average time required for the ovulation of three-quarters of the total number of eggs shed in any one mouse (mean 11-12 eggs) was estimated as 0.5 hr.