Pregnancy interruption in Microtus ochrogaster: laboratory artifact or field phenomenon?

Females of at least 8 species of rodent will terminate pregnancies after exposure to a "strange" male or his odors in the laboratory. Although only suggestive evidence exists for the occurrence of pregnancy interruption in the field, this laboratory phenomenon has been assigned important adaptive roles in the reproductive biology of rodents. In the present study, the extent of pregnancy interruption was investigated for prairie voles (Microtus ochrogaster) in semi-natural enclosures where continued close proximity to males or their stimuli was not necessarily enforced upon females. Heterosexual pairs of voles were housed together in large, out-of-doors enclosures. After 10 days either 1) the original pair remained together for 30 additional days, 2) the stud male was replaced with a strange male for 30 days during which time the stud male was absent, or 3) the stud male was allowed to remain with the female, but another male was also added. Other heterosexual pairs of voles were placed in small cages typical of previous laboratory studies; 10 days later, the original male was replaced by a strange male. Dates of parturition were recorded. The data indicate that pregnancy interruption can occur in situations where 1) the female can potentially avoid the strange male, 2) the female can repel the strange male via aggressive behavior, or 3) the stud male is present to defend his mate. The findings of this study are discussed in relation to the possible adaptive significance of pregnancy block for small rodents.