Pregnancy blockage following multiple-male copulation or exposure at the time of mating in deer mice, Peromyscus maniculatus

Summary

Pregnancy blockage resulting from multiple-male copulation or exposure around the time of mating was studied in three experiments on deer mice, Peromyscus maniculatus. First, significantly more females were pregnant after copulating, either with or without disturbance and a delay, with one male than after receiving the same total number of ejaculations from two males. Second, when females received three ejaculations from one male and then were immediately exposed to either a blonde male or a wild-type male for 2 h, the pregnancy rate was lower than when they were exposed to an empty cage. Third, when immediately after mating females were placed for 2 h in the cage of a strange male they were less likely to deliver a litter than when placed in the familiar male's cage. This phenomenon bears at least a superficial resemblence to the Bruce effect but either communality or difference in underlying mechanism remains to be determined. The present multi-male pregnancy block may provide a means for females in unstable social situations to defer reproduction for 4 days or longer when more stable conditions may exist and a single male prevails.