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.
Keywords: Prairie vole (Microtus ochrogaster); juvenile behavior; meadow vole (Microtus pennsylvanicus; sexual dimorphism; suckling.
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