"The 'love hormone' has been shown to be important for friendship as well

The work is published in the journal Current Biology.

Oxytocin is released in the brain during sexual intercourse, childbirth, breastfeeding and social interactions, enhancing feelings of closeness and emotional connection. It often appears under the names "hug hormone" or "happiness hormone." However, recent experiments on meadow voles (prairie voles) have shown that this hormone is not essential for long-term monogamous attachment or parenting behaviour, although it does accelerate their formation.

Meadow voles are interesting to researchers because, like humans, they form stable and selective bonds - both with a partner and with other individuals, reminiscent of human friendships. In experiments, they found: animals without receptors for oxytocin spend more time establishing friendships and lose them more quickly if the social group changes.

"Oxytocin is particularly important early in the formation of bonds and in their selectivity: conditionally speaking, in preferring a particular friend to a stranger," explained study author Annalise Biri, associate professor of biology and neuroscience.

The scientists conducted three series of tests. In the first case, common voles were enough to stay together for a day to choose a "friend" and stay close to him. In animals without receptors on this took a week. In the second experiment, in a "party" environment, common voles first stayed with a partner and then started to mingle with others, while "scarce" individuals mixed with everyone without preference. In a third test, normal females were more likely to choose access to a friend or partner than to a stranger, whereas voles without receptors valued only access to a partner.

The absence of oxytocin receptors also reduced aggression towards strangers and attenuated avoidance of them. This suggests that the hormone affects both the desire to be with "one's own" and aloofness from "strangers."

Using a new oxytocin nanosensor developed in Marquita Landry's lab, the researchers found that animals without receptors had lower than normal levels of hormone release in the brain region responsible for social reward.

The authors believe that understanding the role of oxytocin in friendship bonds may help in the study of disorders in which people have difficulty establishing or maintaining close relationships, such as autism or schizophrenia.