Abstract
Genetic monogamy is rare-at least at the level of a species-and monogamy can exist in the absence of sexual fidelity. Rather than focusing on mating exclusivity, it has become common to use the term "social monogamy" to describe a cluster of social features, including the capacity for selective and lasting social bonds, central to what humans call "love." Socially monogamous mammals often exhibit selective aggression toward strangers and form extended families. These features of social monogamy in mammals are supported by patterns of hormonal function originating in the neurobiology of maternity, including oxytocin, as well as a more primitive vasopressin pathway. Another key feature of social monogamy is reduced sexual dimorphism. Processes associated with sexual differentiation offer clues to the mysteries surrounding the evolution of monogamy. Although there is consistency in the necessary ingredients, it is likely that there is no single recipe for social monogamy. As reviewed here, genes for steroids and peptides and their receptors are variable and are subject to epigenetic regulation across the lifespan permitting individual, gender and species variations and providing substrates for evolution. Reduced sensitivity to gonadal androgens, and a concurrent increased reliance on vasopressin (for selective defense) and oxytocin (for selective affiliation) may have offered pathways to the emergence of social monogamy.
Highlights
Reviewed by: Ben Dantzer, University of Michigan, United States Patricia Adair Gowaty, University of California, Los Angeles, United States Adam Smith, University of Kansas, United States
We further propose that alterations in mechanisms underlying the behavioral and anatomical traits of social monogamy, shifting from a reliance on androgens to a dependence on peptides would be adaptive, and would permit the emergence of the prosocial traits of social monogamy
Individual differences in social behavior are regulated by genetics, epigenetics, and patterns of short-term change in peptides and steroids
Summary
It was the desire to understand proximate mechanisms supporting lasting social attachments and parenting, that motivated one of us (CSC) to study monogamy from a neuroendocrine perspective (Carter et al, 1995; Carter, 1998). Male prairie voles typically exhibit one of two mating strategies—that of a resident male, maintaining a selective social bond with a female partner, or that of a wandering male, that does not live with a single partner, but instead engages in several acute mating interactions with females, similar to the pattern observed in non-monogamous vole species (Getz et al, 1993) This flexibility in sociality in males is reflected in variation in oxytocin receptor density in the nucleus accumbens. Arginine vasopressin receptor V1a activation in the lateral septum and ventral pallidum is critical for long-term bond formation in male prairie voles Both of these regions demonstrate divergent patterns of AVP V1a receptor density between socially monogamous and non-monogamous vole species (Insel et al, 1994; Pitkow et al, 2001; Lim et al, 2004), and this divergence in receptors results in predictable variations in expressions of behavior (Lim and Young, 2004; Ophir et al, 2009). Variation in SRD5A2 is another putative candidate for reducing sexual dimorphism
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