Cover legend: The prairie vole (Microtus ochrogaster) is a socially monogamous rodent that is used for studying the neurobiology of social attachment. Prairie vole mates form selective and enduring pair bonds that are characterized by sharing a nest, home territory, and parental duties. Pair-bonded voles are aggressive toward novel conspecifics, and this selective aggression is regulated by κ-opioid receptors within the brain's motivational circuitry. For more information, see the article by Resendez et al. (pages 6771–6784).