Photoperiodic Regulation of Reproduction in Postpartum Peromyscus leucopus

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Abstract
Many 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/day). During each succeeding postpartum estrus, females were mated with fecund males. Exposure to short days did not alter gestation length or litter size. A greater percentage of long-day mothers than short-day mothers produced more than 1 postpartum litter (77% vs. 33%). Twenty-four percent of the mice produced between 3 and 5 consecutive litters during exposure to short days. Short daylengths terminated reproduction in the majority of postpartum P. leucopus within about 13 wk, but a significant minority of mice remained capable of breeding despite prolonged short-day exposure. Nonphotoperiodic environmental factors apparently contribute to the arrest reproduction in mice unresponsive to short daylengths.