A study by researchers at the University of California, Berkeley, has found that oxytocin, often called the “love hormone,” plays a key role in the formation of friendships. The findings were published on August 8 in the journal Current Biology.
Oxytocin is a hormone released in the brain during activities such as sex, childbirth, breastfeeding, and social interactions. It is known to contribute to feelings of attachment and trust. Although sometimes linked with aggression, it is commonly referred to as the “cuddle” or “happy” hormone.
Recent research using prairie voles has questioned whether oxytocin is essential for long-term mate bonding or parenting behavior. These studies showed that while oxytocin is not necessary for forming these bonds, its absence causes voles to take longer to form them.
Prairie voles are used in this type of research because they form stable relationships similar to humans. While much attention has been given to mate bonds, Annaliese Beery’s lab at UC Berkeley focuses on peer relationships—an animal model for human friendships—which could provide insights into psychiatric conditions like autism and schizophrenia that affect social bonding.
“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 Annaliese Beery, a UC Berkeley associate professor of integrative biology and neuroscience and senior author of the study.
Beery and graduate student Alexis Black found that prairie voles lacking oxytocin receptors needed more time than typical voles to form peer relationships. Normally, close vole friends huddle together or groom each other.
“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 said. “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.”
The genetically modified voles—developed in Dr. Devanand Manoli’s laboratory at UC San Francisco—also lacked normal social rewards from selective attachments; they did not work hard to stay close with their friends and were less avoidant or aggressive toward strangers.
“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,” she said.
Researchers also noted changes in how oxytocin was regulated and released within the brains of these animals using an innovative nanosensor developed by postdoctoral fellow Natsumi Komatsu and Markita Landry, a UC Berkeley professor of chemical and biomolecular engineering.
“That helped us understand the feedback consequences of lacking this receptor, and how oxytocin signaling was altered in the brain,” said Beery.
The research extends previous findings about oxytocin’s connection with monogamous mating behaviors among prairie voles but focuses specifically on peer or friendship bonds rather than mating pairs. In experiments comparing different species’ social behaviors—including South American rodents and North American Belding’s ground squirrels—Beery observed widespread tendencies toward forming selective friendships among various vole species.
“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,” Beery said. “But only a couple of those species are also monogamous. Someday, I hope to be able to tell you, ‘Do selective peer relationships precede the development of monogamy? Is that why monogamy has evolved so many times in this genus?’ I think this familiarity preference is deeply rooted.”
A previous study led by Manoli indicated that prairie voles unable to respond to oxytocin still displayed typical mating attachment and parenting behaviors; however, further research showed these animals took longer than normal voles—sometimes twice as long—to establish pair bonds.
To examine friendship formation further, Beery’s team ran several experiments: One measured how quickly partner preferences developed (normal voles took about 24 hours; receptor-deficient ones up to a week). Another assessed relationship strength by requiring lever presses for access; receptor-deficient animals did not show strong familiarity preferences compared with wild-type peers.
“Wild-type animals form this incredibly robust preference within one day of co-housing, but the null mutants have no sign of a relationship after 24 hours. After a week, they mostly get there… Our conclusion from that experiment is that oxytocin isn’t required to have a relationship but it’s really important in those early phases…to facilitate it happening quickly and efficiently,” Beery said.
Further tests placed bonded pairs into group environments resembling parties; wild-type animals stayed near familiar partners initially before mingling while mutants mixed indiscriminately regardless of prior connections.
“They can all separate…or hang out in any combinations…The wild-type animals keep track…It’s like if I went to a party with a friend…I would stand near that friend for the first part…then I might start mingling. The voles that lack oxytocin receptors just mixed. It was as if they didn’t even have a partner in there with them.”
Another experiment showed female wild-type voles pressed levers more frequently for partners over strangers (in both peer/mate contexts), whereas mutants only showed increased effort toward mates—not peers:
“Female wild-type voles typically press more…for their partner than…a stranger…in both peer/mate relationships…. The oxytocin receptor deficient mutants also press more…for their mating partner…but not for peer relationships,” Beery explained.
Lack of proper signaling thus delayed relationship building while causing deficits even after long-term association—and made affected animals less aggressive/avoidant towards strangers:
“You can see contributions…to both sides…On prosocial side…it’s involved wanting [to be] with [a] known friend/peer…[on] antisocial side…it’s aiding rejecting [an] unfamiliar animal…. We’ve seen effects…on both affiliation/aggression…and it parallels human findings on [the] role…in-group/out-group dynamics.”
Using carbon nanotube-based nanosensors designed by Landry’s lab—which fluoresce when binding specific DNA sequences matching oxytocin—the team tracked release patterns inside key brain areas responsible for social reward but found lower amounts being released from fewer sites among mutant animals’ brains compared with controls.
Co-authors included Jiaxuan Zhao, Scarlet Taskey & Nicole Serrano (UC Berkeley) plus Ruchira Sharma (UCSF). Funding came from National Science Foundation CAREER award 2239635 & NIH grant R01MH132908; Komatsu now serves as assistant professor at University of Illinois.
