It's a tale as old as time. Meet someone you like, spend time together, get physical, settle down.
This is a normal pattern for the hopelessly romantic prairie vole, a rare monogamist among mammals, whose tiny brain has now yielded a sneak peak into the neurobiology of partnering up.
When choosing a mate for life, the decision-making part of the vole's brain -- the prefrontal cortex -- activates another area that controls how the creature experiences pleasure and reward, neuroscientists reported on Wednesday.
Not only does the cortex switch on the reward centre -- called the nucleus accumbens -- but also controls how active it becomes, which in turn determines how quickly the rodent falls in love.
Bonding in lab voles is measured by how soon they start cuddling with a new partner, which they will most likely stick with for life.
It is possible that the newly-discovered mechanism works by allowing a new partner's features, such as odour and sound, "to become stamped into the reward system (of the brain) so that the partner becomes rewarding," said study co-author Robert Liu of the Emory University in Atlanta, Georgia.
And the entire process is accelerated by sex.
"We showed that the change in (brain) activity around the first mating event was correlated with how quickly animals began huddling," Liu told AFP.
The scientists sought to recreate this effect by stimulating female voles' cortical neurons with light while in the company of a potential male partner, but without allowing them to copulate.
- Remote control -
The next day, light-stimulated voles showed a clear preference for their experimental partners than for strangers -- to a similar degree than if they had had intercourse.
"By simply recreating the control by the cortex of the reward area when the female was near the male, we influenced how affectionately she acted towards him," Li told AFP by email.
"It's like remote control of the brain circuitry to influence pair bonding."
The research has implications for the study of human social behaviour, and may aid treatment of conditions, such as autism or schizophrenia, which prevent sufferers from forming social bonds, said the team.
This could include stimulating communication between the two brain areas, for example, to allow afflicted people to experience the enjoyment normally gotten from spending time with loved ones.
"Pair bonding in voles likely shares many of the underlying neural mechanisms as falling in love in humans," said Liu.
It was already known that similar parts of the reward system studied in voles light up when humans look at pictures of their romantic partners, smell their scent or hear their voice.
"But, until now, we haven't known how the brain's reward system works to lead to those feelings," said Liu's colleague Elizabeth Amadei.