Bruce effect

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 (e.g., Storey and Snow 1990).

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 (reviewed in Mahady and Wolff 2002).

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 (e.g., Stehn and Richmond 1975, Stehn and Jannett 1981, Storey 1994).

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 (e.g., Stehn and Richmond 1975, Kenney et al. 1977, Storey and Snow 1990). 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 (Schwagmeyer 1979). 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 (de la Maza et al 1999), so it might be a laboratory artifact.

Literature

 * Bruce HM.1959. An exteroceptive block to pregnancy in the mouse. Nature 184: 105.
 * Storey AE, Snow DT. 1990. Postimplantation pregnancy disruptions in meadow voles: Relationship to variation in male sexual and aggressive behavior. Physiology and Behaviour 47: 19–25.
 * Mahady S, Wolff JO. 2002.A field test of the Bruce effect in the monogamous prairie vole, Microtus ochrogaster. Behavioral Ecology and Sociobiology 52: 31–37.
 * Stehn RA, Richmond ME. 1975.Male-induced pregnancy termination in the prairie vole, Microtus ochrogaster. Science 187: 1211–1213.
 * Stehn RA, Jannett FJ Jr. 1981. Male-induced abortion in various microtine rodents. Journal of Mammalogy 62: 369–372.
 * Storey AE. 1994. Pre-implantation pregnancy disruptions in female meadow voles Microtus pennsylvanicus (Rodentia:Muridae): Male competition or female mate choice? Ethology 98: 89–100.
 * Kenney AM, Evans RL, Dewsbury DA. 1977. Postimplantation pregnancy disruption in Microtus ochrogaster, Microtus pennsylvanicus, and Peromyscus maniculatus. Journal of Reproduction and Fertility 49: 365–367.
 * Schwagmeyer PL. 1979. The Bruce effect: An evaluation of male/female advantages. The American Naturalist 114: 932-938.
 * de la Maza HM, Wolff JO, Lindsey A. 1999. Exposure to strange males does not cause pregnancy disruption or infanticide in the gray-tailed vole. Behavioral Ecology and Sociobiology 45: 107–113.
 * in parts from: Wolff, J. O. (2003) Laboratory Studies with Rodents: Facts or Artifacts? BioScience 53:421-7