Light and Reproduction in the Rat: Effects of Photoperiod Length on Albino Rats from Two Different Breeders1

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Abstract
Many, if not most, successful matings in natural populations of rodents occur during postpartum estrus. The role of photoperiod in regulation of recurrent sterile estrous cycles is established, but its significance for postpartum estrous females is unknown. White-footed mice, recently mated, were maintained in long- or short-day photoperiods (14 or 10 h of light per day). During each succeeding postpartum estrus, females were mated with fecund males. Exposure to short days did not alter gestation length or litter size. However, a greater percentage of long-day mothers than short-day mothers produced more than one postpartum litter (77% vs 33%). Twenty-four percent of mice produced between three and five consecutive litters during exposure to short days. Short daylengths terminated reproduction in the majority of postpartum Peromyscus leucopus within about 13 weeks, but a significant minority of mice remained capable of breeding despite prolonged short-day exposure. It is suggested that, in the field, nonphotoperiodic environmental factors contribute to the arrest of reproduction in mice unresponsive to short daylengths.

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