Pregnancy-block in Microtus agrestis and induced ovulator

Abstract

A DECREASE in the incidence of pregnancy is caused by exposing recently inseminated female mice to the presence of strange males1. This "Bruce effect" has been demonstrated in the albino mouse, Mus musculus, and the deer mouse, Peromyscus maniculatus bairdii2, both of which are spontaneous ovulators3,4. The reduction in the number of pregnancies has been shown to be a consequence of the failure of the fertilized ova to implant and the effect of the strange male is mediated by olfactory stimuli5,6. Other environmental factors can also block pregnancy2. © 1968 Nature Publishing Group.