A behavioral genetic analysis of pregnancy block in {\it Mus musculus}

Date of Completion

January 1997


Psychology, Psychobiology|Biology, Genetics




The Bruce effect is an olfactory block to pregnancy that occurs when a newly inseminated female mouse is exposed to the odors of a male mouse who is of a different strain than the original male. This androgen dependent effect induces implantation failure and the female returns to estrus to be reproductively available for the novel male. The present studies explore several aspects of this effect including maternal and paternal influences, early environmental influences, male sibling effects and behavioral components of the interactions between pregnant females and novel males.^ The baseline rates of pregnancy block were assessed for C57BL/6J females, DBA1/J females and females of each reciprocal cross. The mating condition of C57 stud x DBA stimulus was the most effective, out of the four conditions tested, at inducing the effect. The baseline data suggested that a paternal effect was operating in females who were sired by D8A males. These animals displayed the highest rates of pregnancy block in this condition. Since the reciprocal crosses differed in this condition, cross-fostering studies were also undertaken in order to explore possible maternal influences. These procedures indicated that maternal factors may influence the occurrence pregnancy block, possibly on the basis of histocompatibility odor types.^ A further set of studies assessed how strain differences between the behaviors of stimulus males may have contributed to the patterns of pregnancy block observed in the baseline data. Results illustrated that strain differences exist with regard to the levels of sexual behavior, vocalization and defecation associated with stimulus males in these encounters.^ The source of the paternal effect, observed in the baseline data, was explored through the use of males congenic for the Y-chromosome. These studies, in general, failed to illustrate a Y-chromosomal effect on either pregnancy block or behavior. However, they did further support the idea that behavioral differences do contribute the strain differences in the occurrence of pregnancy block.^ These results are discussed in terms of the evolutionary significance of the Bruce effect. ^