Induction of pseudopregnancy in pregnancy-blocked mice by re-exposure to stud males

Re-exposure to stud males beginning on day 4 post coitum induced pseudopregnancy in a large proportion of female mice whose pregnancies were terminated by a single injection of 0.5 mg of bromocriptine on day 1 post coitum. By contrast, exposure to alien or strange males was ineffective in inducing pseudopregnancy in pregnancy-blocked females. The results suggest that the newly inseminated female retains the memory of the stud male for some days after mating. Activation of the 'memory' by pheromones of the stud males exerts a luteotrophic effect in pregnancy-blocked females resulting in the incidence of pseudopregnancy. This may account for the inability of the stud male to block pregnancy in his coital partner and also for the inability of alien males to block implantation in newly inseminated females housed with stud males.