A recent study from the University of California, Berkeley, suggests that oxytocin, often called the "love hormone," plays a key role in forming friendships in addition to its established links with attachment and trust. Oxytocin is released during various social interactions as well as during sex, childbirth, and breastfeeding.

While previous research has shown that prairie voles without oxytocin receptors can still form mate bonds and display parenting behaviors, the UC Berkeley team found that these animals take longer to develop peer relationships. The study was led by Annaliese Beery, associate professor of integrative biology and neuroscience at UC Berkeley.

“Prairie voles are special because they allow us to get at the neurobiology of friendship and how it’s similar to and different from other types of relationships,” said Beery.

Beery and graduate student Alexis Black observed that prairie voles lacking oxytocin receptors were slower than typical voles to establish peer bonds. These animals did not show the usual preference for familiar peers over strangers after 24 hours together—a process that normally takes about a day in wild-type voles—and required up to a week to develop such preferences.

“Oxytocin seems to be particularly important in the early formation phase of relationships and especially in the selectivity of those relationships: ‘I prefer you to this stranger,’ for example,” Beery explained. “The animals that didn’t have intact oxytocin signaling took longer to form relationships. And then when we challenged those relationships by making new groups, they lost track of their original partners right away.”

Voles genetically modified at UC San Francisco by Dr. Devanand Manoli’s lab also displayed less effort in seeking out their friends and showed less avoidance or aggression toward unfamiliar individuals compared with normal voles.

“In other words, oxytocin is playing a crucial role not so much in how social they are, but more in who they are social with, their selectivity,” said Beery.

The researchers collaborated with Markita Landry’s lab at UC Berkeley using an innovative nanosensor technology made from carbon nanotubes paired with DNA sequences designed to bind specifically with oxytocin molecules. This allowed them to track real-time release patterns of oxytocin within brain regions linked to social reward. They found reduced amounts of oxytocin being released from fewer sites within these brain areas among receptor-deficient voles.

“That helped us understand the feedback consequences of lacking this receptor, and how oxytocin signaling was altered in the brain,” Beery noted.

The findings add evidence supporting a role for oxytocin in facilitating selective friendships among mammals like prairie voles—animals known for forming stable social bonds similar to humans. According to Beery's ongoing research comparing multiple rodent species both in laboratory settings and field studies across North America and South America, many vole species display preferences for specific peers even if only some practice monogamy as mates.

“While most rodents prefer to interact with unfamiliar individuals, it turns out that the majority of vole species we’ve tested in our early trials form peer-partner preferences, which is what we call these selective friendships. So there seems to be this widespread tendency to bond,” said Beery. “But only a couple of those species are also monogamous."

In experiments testing relationship strength by requiring animals to work (press levers) for access either to friends or strangers, wild-type female prairie voles demonstrated clear partner preferences while mutant animals did not do so for peer—but only mate—relationships.

“You can see contributions of oxytocin signaling to both sides of selectivity,” added Beery. “On the prosocial side, it’s involved in wanting to be with a known friend or peer, while on the antisocial side, it’s aiding in rejecting an unfamiliar animal."

Markita Landry contributed her expertise on nanosensor development for tracking molecular changes associated with behavioral shifts seen among mutant versus wild-type subjects.

Co-authors include Jiaxuan Zhao, Scarlet Taskey and Nicole Serrano from UC Berkeley; Ruchira Sharma from UCSF; Natsumi Komatsu (now assistant professor at University of Illinois); along with Black; Komatsu; Landry; Manoli; and Beery herself. Funding support came from grants awarded by agencies including the National Science Foundation (CAREER award 2239635) and National Institutes of Health (R01MH132908).

The full results appear online as an open-access article published August 8th by Current Biology.