A field test of the Bruce effect in the monogamous prairie vole (Microtus ochrogaster)

Abstract.

The Bruce effect is a widely studied reproductive phenomenon in rodents in which exposure of pregnant females to unknown males causes termination of the current pregnancy. The Bruce effect has been reported from numerous studies in the laboratory, and one field study with the promiscuous gray-tailed vole, Microtus canicaudus, failed to support it. We conducted a field study with the monogamous prairie vole, M. ochrogaster, to determine if complete replacement of the male population every 10–14 days affected pregnancy and juvenile recruitment. The mean days to first parturition for control and treatment females were 36.8 and 44.4 days. Fifty-five percent of control females and 33% of treatment females conceived within the first 14 days of the study. All control females and 79% of treatment females successfully delivered at least one litter. These differences between treatment and control populations provide minimal support for the Bruce effect when compared with results from laboratory studies. Nulliparous females may have experienced some pregnancy disruption, but not parous females. Removal of mates, rather than exposure to strange males, may have contributed more to the lower reproductive success of treatment females than exposure to strange males. Treatment females, however, had lower juvenile recruitment than controls, which may have been due to infanticide from strange males. Our results are more similar to those of the field study of the gray-tailed vole than predicted, based on laboratory studies of prairie voles.