By ANNE MCILROY
Thursday, Jun 17, 2004 UPDATED AT 12:57 PM EDT
He says he loves you, but doesn't want to settle down. Science soon
may have an answer.
Researchers have found a way to turn naturally promiscuous animals into monogamous ones, a discovery that one day could lead to a "commitment pill" for human males.
Led by scientists at Emory University in Atlanta, the team worked with two species of voles. (Voles look like furry mice with short tails.) Male meadow voles are loners who like to play the field; male prairie voles tend to get attached to one female.
The researchers, in essence, were able to change the meadow vole's natural propensity to philander by inserting a single gene that changed the way the pleasure centre in their brains worked. After a single treatment, they became as monogamous as prairie voles.
Human males appear to have a similar system, which involves the hormone vasopressin, in their brains. Theoretically, the discovery opens the door to the possibility of medical treatment for men who have trouble committing to a relationship.
The vole model, however, offers no possibility of a pill that would stop men from having affairs outside a marriage or relationship.
Monogamy, as animal researchers define it, does not mean never having sex outside of a long-term relationship. Instead, they refer to social monogamy, which means settling down with one female, but not resisting temptation should opportunity present itself. Under this definition, former U.S. president Bill Clinton was monogamous.
Sexual monogamy, the more typical human definition, is spelled out in marriage vows: It's the bit about forsaking all others.
It is debatable how many humans qualify as sexually monogamous, but we are socially monogamous, says Emory University researcher Larry Young, whose findings are published today in the British journal Nature.
"I don't think we have a biological basis that makes us only want to mate with one female. It doesn't turn off. But there is a tendency to want to be with one partner, to live with that partner, to establish a life, so that makes us more like the prairie vole."
Very few mammals -- only 5 per cent -- meet even the loosey-goosey definition of social monogamy. Marmosets are monogamous; so are the titi monkey, the California mouse, the elephant shrew, and, of course, the prairie vole.
Dr. Young and other scientists wondered what's different about these animals. Because monogamous prairie voles are so closely related to the philandering meadow voles, they offered a unique opportunity to find out.
An earlier experiment established that prairie voles had more receptors for vasopressin in the part of their brains associated with pleasure and reward. The hormone is released after they have sex.
The scientists wanted to see if the extra receptors, which activate the part of the brain involved in experiencing pleasure, played a role in their monogamous nature. Dr. Young and his colleagues altered a single gene to create more vasopressin receptors in meadow voles. The animals became monogamous, mating for life.
Vasopressin is also present in the brains of human males, but scientists aren't sure what it does, Dr. Young says. "It is intriguing to consider that individual differences in vasopressin receptors in humans might play a role in how differently people form relationships."
It may be that men with fewer vasopressin receptors avoid long-term relationships and think more about themselves than their partners, he says.
The hormone plays much less of a role in females.
Like human behaviour, vole sexual behaviour shows huge variability. While most meadow voles go from partner to partner, some settle down. Most prairie voles form a strong attachment to one female, but some don't. The next stage in the research is to see whether biology -- the number of vasopressin receptors -- explains the difference.
Researchers tend to believe that something as complex as sexual behaviour depends on many genes, and on learning, too. What is striking about this work is that it shows that altering one gene can have a profound effect, at least in voles.
Could it be the same in humans?
"We do not yet know if a similar system helps explain male attachment in non-human primates, much less humans, but a medicine that might be offered to certain men is an interesting prospect," Melvin Konner, an anthropologist at Emory, writes in a second Nature article examining the implications of the discovery.
"We are a long way from a commitment pill, but perhaps closer to the neurology of romance."
Finding a mate:
A "commitment pill" may be in the future, but how faithful would it make someone? Only 5% of mammals are considered to be even socially monogamist - including the prairie vole. The following are common types of sexual commitment:
Forsakes all others
Who qualifies? Paul Newman
Settles down, but does not necessarily resist temptation
Who qualifies? Bill Clinton
Settles into a series of short relationships
Who qualifies? Jennifer Lopez
Marries more than one person at one time
Who qualifies? Brigham Young
Has casual and frequent sex with different people
Who qualifies? Wilt
Chamberlain (20, 000 women, 1.4/day, since age 15)
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