By: Zack Johnson

Imagine life without family, friends, romantic partners, neighbors, or co-workers. It’s difficult to picture because the social bonds between us are a deep part of human life; we are social animals by nature (Finkel and Eastwick 2015). In fact, we use complete isolation from social life as one of the most intense forms of punishment; and disrupted social function is a core symptom in what we consider to be some of the most devastating psychiatric disorders: depression, schizophrenia, and autism spectrum disorders. 
As social animals, understanding the biological basis of our social nature might allow us to better nurture it and develop more effective treatments for severe social impairments. Our social perceptions, decisions, and relationships are rooted in our brains, so one route is dissecting how our brains process social information, make social decisions, and form social attachments. Scientists are making progress on this front by imaging the human brain during social situations, but in some ways they are limited by technological constraints and well-justified ethical considerations. Fortunately, our evolutionary relatives can give us hints, too. Our brains are organized in much the same way as other animals; they use the same cell types and signaling systems, have many of the same connections, and evolved from the same ancestral nervous systems. 
Social life in the animal kingdom is phenomenally diverse, ranging from solitary animals like octopi to highly social animals that form complex societies, like bees. This is exciting for neuroscientists interested in social behavior, because they can compare species with similar or different behaviors, look for similarities or differences in their brains, and pinpoint the systems that regulate those behaviors. Down the road, these systems can be potential targets for therapeutic interventions in humans. 
This approach has shaped our understanding of how brains form social attachments. Different vole species vary in their capacity to form social bonds, from promiscuous meadow and montane voles that don’t bond with mates to monogamous prairie and pine voles that do. Scientists used these behavioral differences to study how the brain forms social bonds. They started by honing in on two molecules that are involved in social behaviors across many species, called “oxytocin” and “vasopressin.” These molecules are produced in certain kinds of brain cells and are released into specific brain areas where they are detected by receptor molecules—like radio waves being detected by antennae tuned in to the right frequency. The researchers noticed that both oxytocin and vasopressin receptor molecules were arranged completely differently in monogamous compared to promiscuous brains. In monogamous brains they were densely packed in reward and addiction brain pathways; in promiscuous brains they were not. When the scientists blocked the receptors in these regions, monogamous voles could no longer form social bonds with their partners; and when they artificially boosted the receptors in these regions, promiscuous voles formed social bonds with their mates. 
Oxytocin and vasopressin can be released in response to social contexts ranging from touch, social vocalizations, and eye contact to mating and territorial encounters. It may be that over the course of evolutionary history, rewiring these systems in the brain has allowed for vastly different responses in specific social contexts across species, as I recently reviewed here (Johnson and Young 2015). One form of rewiring is altering receptor levels in specific brain areas. In the case of the voles, it may be that environmental pressures favoring monogamous mating strategies drove reward and addiction regions to become giant antennae for oxytocin and vasopressin. During mating, release of these molecules could then modify activity in reward and addiction areas and contribute to bond formation between partners. 
What do voles have to do with us? Subsequent research has shown that these molecules are involved in affiliative behavior, bonding, and other aspects of sociality across fishes, birds, and mammals; including dogs, primates, and humans (Donaldson and Young 2008). In married heterosexual men, breathing oxytocin increases activity in reward areas when viewing their female partner’s face, but not an unfamiliar female’s face (Scheele, Wille et al. 2013). When we look at the genes in humans that produce oxytocin and vasopressin receptors, we find that specific mutations are associated with autism spectrum disorders (Ebstein, Knafo et al. 2012). We also know that breathing oxytocin increases human eye contact and the ability to read emotions from faces, and it improves these social functions in autistic patients, too (Domes, Heinrichs et al. 2007; Gordon, Vander Wyk et al. 2013; Auyeung, Lombardo et al. 2015). Could it be that, like voles, the evolutionary wiring of these systems in the human brain has contributed to our social behavior?
While these molecules certainly aren’t the whole story, they are providing important insights into how we construct and perceive our social world; one day, they might be used therapeutically for people suffering from severe psychiatric symptoms. Until then, it’s important to remember that even the most seemingly irrelevant branches of the evolutionary tree, like voles, can ignite profound progress in our understanding of cognition and behavior. 
Auyeung, B., M. V. Lombardo, et al. (2015). "Oxytocin increases eye contact during a real-time, naturalistic social interaction in males with and without autism." Transl Psychiatry 5: e507.
Domes, G., M. Heinrichs, et al. (2007). "Oxytocin improves "mind-reading" in humans." Biol Psychiatry 61(6): 731-733.
Donaldson, Z. R. and L. J. Young (2008). "Oxytocin, vasopressin, and the neurogenetics of sociality." Science 322(5903): 900-904.
Ebstein, R. P., A. Knafo, et al. (2012). "The contributions of oxytocin and vasopressin pathway genes to human behavior." Hormones and Behavior 61(3): 359-379.
Finkel, E. J. and P. W. Eastwick (2015). "Attachment and pairbonding." Current Opinion in Behavioral Sciences 3: 7-11.
Gordon, I., B. C. Vander Wyk, et al. (2013). "Oxytocin enhances brain function in children with autism." Proc Natl Acad Sci U S A 110(52): 20953-20958.
Johnson, Z. V. and L. J. Young (2015). "Neurobiological mechanisms of social attachment and pair bonding." Current Opinion in Behavioral Sciences 3: 38-44.
Scheele, D., A. Wille, et al. (2013). "Oxytocin enhances brain reward system responses in men viewing the face of their female partner." Proc Natl Acad Sci U S A 110(50): 20308-20313.