We used live-trapping data collected during a long-term study of the social organization of prairie vole Microtus ochrogaster, to investigate pair formation and break-up in this species. Most male-female pairs that formed in spring consisted of survivors of communal groups. Whether pairs formed from males and females from the same or different communal groups, the individuals were not family members. When new pairs formed during summer-autumn they typically consisted of unrelated individuals that had been wandering throughout the study site. Thus, our field data indicate that prairie voles avoided pairing with family members. We found no evidence that free-living females based their choice of mate on body mass, or that females preferred sexually experienced to sexually inexperienced males in the field or under semi-natural laboratory conditions. In our study population, pairs that separated were characterized by lower reproductive success, prior to separation, than were pairs that remained together. At any given time, the number of potential mates for males and females was limited. Thus, it seems likely that few individuals had the opportunity to compare simultaneously the characteristics of two or more potential mates. We suggest that pair formation in our study population most likely was opportunistic, with individuals pairing with the first available mate.published or submitted for publicationis peer reviewe
