A number of studies have implicated the neurohypophyseal peptides oxytocin and vasopressin in the central mediation of complex social behaviors, including affiliation, parental care and territorial aggression. Research on a monogamous rodent, the prairie vole (Microtus ochrogaster), suggests that these neuropeptides are also involved in the control of several behaviors associated with monogamy, including pair bonding, paternal care and mate guarding. Comparative studies using several species of vole have identified species-specific patterns of oxytocin- and vasopressin-receptor expression in the brain that appear to be associated with a monogamous versus non-monogamous social structure. Molecular studies suggest that changes in the regulation of oxytocin- and vasopressin-receptor gene expression underlie these species differences in receptor distribution and might provide a mechanism for the evolution of monogamy in voles.