The Bruce effect is a form of pregnancy disruption in mammals in which exposure of a female to an unknown male results in pre- (Bruce 1959) or postimplantation failure.
Some form of pregnancy block or disruption has been reported in the laboratory for at least 12 species of rodents, including domestic mice, Mus musculus; deer mice, Peromyscus; and voles, Microtus.
The basic design of these experiments is that a recently inseminated female is exposed directly to an unfamiliar, nonsire male or to its urine or soiled bedding, which in turn causes her to prevent implantation or to abort or reabsorb her embryos. Pregnancy disruption may occur at any time from conception to 17 days postmating, depending on the species and experimental conditions.
Variables such as length of exposure, timing of exposure to a strange male, sexual experience, and behavior of strange males may all influence the degree of pregnancy failure. The overall implication is that some level of exposure to strange males disrupts normal pregnancy in female rodents. This response supposedly is adaptive for the male, in that termination of pregnancy results in the female coming into estrus within 1 to 4 days, providing the male with a mating opportunity.
The benefit to the female is less clear, but if the strange male were to commit infanticide and kill her offspring after parturition, a female could conserve reproductive effort by aborting her current litter and mating with the new male. Thus, pregnancy block, or termination of pregnancy, supposedly evolved as a female counterstrategy to infanticide by males.
The Bruce effect has not been demonstrated outside the laboratory, and does not occur in wild grey voles, so it might be a laboratory artifact.