A SINGLE “love gene” that transforms Don Juans into loyal and attentive spouses has been identified by scientists, casting fresh light on the chemical cocktail in the brain that ignites romance.

When the brains of male meadow voles, usually the most promiscuous of lovers, are enhanced with a gene called the vasopressin receptor, they instantly reform their loose ways and form lasting pair bonds instead.

Because vasopressin is also active in the human brain, the remarkable experiment at Emory University in Atlanta, suggests that differing levels of the hormone could explain why some people find it harder to stay faithful than others.

“It is intriguing to consider that individual differences in vasopressin receptors in humans might play a role in how differently people form relationships,” Larry Young, who led the research, said.

Vasopressin and its receptors may also be implicated in drug addiction and social disorders such as autism, both of which affect the brain’s reward centre.

Scientists have long been intrigued by the dramatically different mating behaviours shown by the meadow vole, Microtus pennsylvanicus, and its close cousin the prairie vole, Microtus ochrogaster. Meadow voles, like most mammals, are highly promiscuous creatures that take multiple partners and do not form lasting relationships. Prairie voles, however, are among the tiny proportion of mammals — less than 5 per cent — that are habitually monogamous. Humans also fall into this category, though their moral standards are positively debauched in comparison with the prairie vole.

When boy prairie vole meets girl, they indulge in 24 to 36 hours of nearly continuous mating, which cements a bond that invariably lasts for life. When one partner dies, the survivor usually opts to remain celibate rather than find another mate.

Because the two species of vole are so similar in everything but sexuality, they have become a favourite of researchers investigating the chemistry of love. Previous studies have indicated that male prairie voles have much higher levels of vasopressin and its receptor in the ventral palladium, a part of the brain that processes rewards, than their cousins from the meadows. This suggests strongly that the hormone plays a key role in determining sexual fidelity.

The Emory experiment, details of which are reported today in the journal Nature, has now proved this theory beyond reasonable doubt. In the study Dr Young’s team took the gene for the vasopressin receptor from the prairie vole and bound it to a harmless virus.

This modified virus was then transferred into the ventral palladium of male meadow voles, to infect the brain cells in this region. As it did so the cells began to produce vasopressin receptors at levels typical of the prairie vole.

The meadow voles’ behaviour changed almost instantly. After they were infected with the virus, they became interested exclusively in a single female partner. Their mating preferences effectively changed overnight from promiscuity to monogamy.

Dr Young said that while more than one gene and hormone is probably involved in regulating human monogamy and pair-bonding, the results have powerful implications for understanding the biological and chemical origins of relationships. “Our study provides evidence in a comparatively simple animal model that changes in the activity of a single gene can profoundly change the fundamental social behaviour of an entire species,” he said.

Levels of vasopressin and its receptors in the brain may well influence whether somebody is likely to have eyes for nobody but his or her partner, while others play the field with no interest in settling down.

Vasopressin is released after sex, along with another brain signalling molecule known as oxytocin. Both appear to have important functions in building attachment between two individuals. Studies by Kathleen Light, of the University of North Carolina, have shown that oxytocin levels rise significantly both in prairie voles after sex and in couples after spending time hugging or watching romantic films.

Miranda Lim, another member of the team, said that the hormone also appears to be involved in drug addiction. “The process of bonding with one’s partner may be similar to drug addiction. Both activate reward circuits in the brain,” she said.

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