Intraspecific social systems vary considerably as a function of environmental parameters (Lott 1984,Behaviour, 88, 266–325). For example, nonmonogamous species may engage in facultative partner preferences and parenting to offset the costs associated with harsher breeding conditions. Because no field or laboratory research has examined nonmonogamous meadow voles under suboptimal conditions (e.g. low-density summer populations or during colder months), it was not known whether meadow voles could form affiliative preferences for a specific partner. The aim of this experiment was to identify whether meadow voles develop selective partner preferences and if so, under what circumstances. We assessed partner preferences using a choice apparatus in which the test animal chose to spend time with a familiar mate or stranger. We paired and tested males and females (within photoperiod) under one of five different conditions: after 24h, 10 days, or 23 days of cohabitation with mating or after 24h or 10 days of cohabitation without mating. Male and female meadow voles rapidly formed selective partner preferences for a familiar mate when compared with controls in nearly every condition, regardless of photoperiod, cohabitation duration, or whether mating did or did not occur. Within 24h, males directed significantly more aggression towards unfamiliar animals, and mating enhanced this effect. For females, 24h of social cohabitation was sufficient to decrease aggression towards partners, but stranger-directed aggression appeared later, following delivery of the litter. These data suggest that meadow voles are capable of developing selective partner preferences and stranger-directed aggression and may have evolved these abilities to maximize reproductive success during the colder months of the year or under low population density during the summer breeding season.