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Neuropeptides and Social Behavior Development 175sence of other partners, or when mate guarding is successful, social monogamycan promote sexual exclusivity.Although superficially confusing, the distinction between social systems andmating systems further highlights the overarching importance of social behavior.Even in the absence of absolute “sexual” or “genetic” monogamy, there is no doubtthat pair bonds and other forms of stable social groups do exist. These relationshipsmay endure beyond the periods of sexual interaction. In some species individualsmay remain together for a lifetime. For example, among the diverse speciesthat share the traits of social monogamy are wolves, several New World primatesincluding tamarins and marmosets, titi monkeys, and even a few rodents, includingprairie voles.Pair bonding and the capacity to form long-lasting social relationships can leadto the formation of extended family groups. Although male parental behavior isuncommon in polygynous species, among socially monogamous mammals bothparents tend to exhibit parental behavior. In addition, older reproductivelynaive offspring may remain with the family, helping to care for younger infants(alloparenting). The extended family groups which arise as a function of thesesocial bonds appear remarkably similar to those seen in some traditional humancultures.Animal Models for the Analysis of the Physiologyof Social BehaviorDifferences in sociality among species and individuals have proven especiallyuseful to understanding the biological basis of sociality and the role of hormonesin pair-bond formation. For example, much of the recent neuroendocrine workon social behavior has taken advantage of the fact that the genus Microtus includessocially monogamous prairie voles (M. ochrogaster) and related, nonmonogamousmeadow voles (M. pennsylvanicus) and montane voles (M. montanus).Descriptions of the proximate mechanisms responsible for different kinds ofsocial groups are most easily understood by analyzing the behavioral responsesof individual animals. The tendency toward selectivity in social responses probablyrelies on mechanisms (see below) that promote both general sociality, as wellas specific neurophysiological processes necessary to reinforce or reward selectivesociality (Insel, 2003). Animals in nonmonogamous species also may be capableof individual recognition, but may be less inclined to be selective in theirsocial or reproductive behaviors. In addition, species that are not socially monogamousmay lack or fail to activate neural mechanisms necessary to reinforce individualpreferences. Finally, after a pair bond is formed, individuals may begin toshow selective aggression toward strangers (Carter, DeVries, & Getz, 1995).