child bond resulting in the nuclear family. The precise nature of the nuclear family varies from culture to culture, ranging from strict monogamy reinforced by society and religion to polygyny or polyandry. Whether or not one considers human beings to be truly monogamous, it is clear that the selective bond between mates, manifested in our species as an emotion we call love, is extremely powerful, and is undoubtedly rooted in our biology and genetic heritage.

Controlled experiments on the neurobiological basis of sociosexual behaviors in humans are not possible. Thus, we must rely on animal models to provide principles that might generalize to humans. How might we find a suitable animal model for human bonding? Approximately 90 percent of bird species are considered monogamous, at least over one breeding season. In contrast, only approximately 5 percent of mammals exhibit a monogamous social structure (Kleiman, 1977). The term “monogamy” does not imply lifelong exclusive mating with a single individual. In fact, many birds form pair bonds over a season, raise their offspring together, and then select another partner the following season. For biologists, monogamy implies selective (not exclusive) mating, a shared nesting area, and biparental care. In recent years, genetic analyses of offspring have provided evidence for extra-pair copulations even among species thought to mate exclusively monogamously.

Biomedical research relies heavily on rodent models because rodents are small, breed well in the laboratory, and are suitable for many types of experimental manipulations. The use of behaviorally monogamous rodent species is efficient for investigating the biology of monogamy and social attachment. Several species of voles, genus Microtus, fit these criteria and have become rodent models for research on the neurobiological basis of pair bonding (Insel and Young, 2001). This chapter reviews the progress in understanding the molecular, cellular and neurobiological nature of pair bonding emerging from intensive studies of monogamous prairie voles, and discusses the implications of this research for human behavior.

Prairie voles (Microtus ochrogaster) are field mice found in the Mid-western prairie of the United States. Studies in the field indicate that prairie voles form long-term social bonds with their mates and produce multiple litters together (reviewed in Carter et al., 1995). In fact, one study reported that in pairs in which one individual disappears, fewer than 20 percent of the survivors took on a new mate. However, despite their social monogamy, not all prairie voles display exclusive mating, since females have been reported to carry mixed-paternity litters. The selective pressures leading to the evolution of monogamy in prairie voles are unclear. In theory, monogamous social structures are thought to be favored under conditions of low food availability, high nest predation, and low population density. Males in monogamous species typically display paternal care of their offspring by



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