Behavioral comparisons of male and female pups of prairie voles (Microtus ochrogaster) and meadow voles (M. pennsylvanicus).

Sexual dimorphism in mammals typically is reduced in monogamous species relative to polygynous species, with promiscuous species being intermediate. This pattern of dimorphism characterizes adult behavior and body mass of prairie voles, a monogamous species, when compared with meadow voles, a closely related polygynous or promiscuous species. We examined whether the pattern also applies to young of the two species by observing individual pups living in family groups in seminatural environments. Observations during the second week of life revealed no sex differences in pup behavior or body mass. However, we detected species differences in suckling behavior, jockeying for position within the huddle (especially among males), and body mass that replicate and extend our previous observations. These data indicate that patterns of sexual dimorphism associated with different mating systems may not be evident in juvenile mammals, but that species differences in behavior and body mass can be obvious at this stage.

Selectivity and Sociality: Aggression and Affiliation Shape Vole Social Relationships.

The formation of selective social relationships is not a requirement of group living; sociality can be supported by motivation for social interaction in the absence of preferences for specific individuals, and by tolerance in place of social motivation. For species that form selective social relationships, these can be maintained by preference for familiar partners, as well as by avoidance of or aggression toward individuals outside of the social bond. In this review, we explore the roles that aggression, motivation, and tolerance play in the maintenance of selective affiliation. We focus on prairie voles (Microtus ochrogaster) and meadow voles (Microtus pennsylvanicus) as rodent species that both exhibit the unusual tendency to form selective social relationships, but differ with regard to mating system. These species provide an opportunity to investigate the mechanisms that underlie social relationships, and to compare mechanisms supporting pair bonds with mates and same-sex peer relationships. We then relate this to the role of aggression in group composition in a comparative context.