Everyone knows someone who is a compulsive womanizer; a man who simply can’t remain faithful to one woman.

Likewise, everyone knows someone who is a doting, faithful husband; for such a man the idea of sex with women other than his partner is unthinkable.

Ever wondered why men are so different?

Scientists working on one of the few other mammal species to form a pair bond have found the beginnings of an answer. Miranda Lim and colleagues at the Center for Behavioral Neuroscience, Emory University, Atlanta, Ga., have focused their research on prairie voles, one of the few mammal species that are monogamous and form lifelong pair bonds. They chose prairie voles as their study species because there is a closely related species that, although very similar genetically, exhibits very different social behavior.

While the male prairie vole is monogamous, the male meadow vole is polygamous. And by changing just one gene in the meadow vole, the biologists reversed its polygamy: They turned the “love rat” into a cuddly, loving little rodent.

The gene is for the vasopressin 1a receptor in the brain, and is denoted V1aR. These receptors pick up hormones that are released after sex, and prairie voles have more of these receptors in a particular region of their brains than do meadow voles.

The receptors are found in one of the brain’s principal reward regions, the ventral pallidum. This means that when a male prairie vole has sex with his partner, reward centers in his brain light up, and the vole feels good about being with his partner. Through a process that is not yet well understood, this leads to him maintaining the pair bond.

Formerly promiscuous

And, you guessed it, the meadow vole doesn’t have many of these receptors. So the scientists used a virus as a tool for transferring V1aR from prairie voles into the ventral pallidum of meadow voles. The result? The meadow vole got vasopressin receptors like the prairie vole. And the formerly promiscuous meadow voles then displayed a strong preference for their current partners rather than new females.

Naturally, this system is more complicated in humans. Emory researcher Larry Young said: “Our study, however, provides evidence, in a comparatively simple animal model, that changes in the activity of a single gene can profoundly change a fundamental social behavior of animals within a species.”

Previous research showed that vasopressin receptors might also play a role in disorders in which people lack the ability to form social bonds, such as in autism. “It is intriguing,” said Young, “to consider that individual differences in vasopressin receptors in humans might play a role in how differently people form relationships.”

And, Lim adds, past research in humans has shown that the same neural pathways involved in the formation of romantic relationships are also involved in drug addiction. “The brain process of bonding with one’s partner may be similar to becoming addicted to drugs: both activate reward circuits in the brain.”

This finding, which appeared in the June 17 issue of the journal Nature, will help in our understanding of disorders such as autism, where there is an inability to form social bonds. In addition, the finding supports previous research linking social-bond formation with drug addiction, which is also associated with the reward center of the brain.

The researchers’ next step is to determine why there is such variation in behaviors among individuals within a species.

Look back to the first paragraph. While there are men who are womanizers and men who are monogamous, most men (and women, for that matter) are probably somewhere in between.

Speaking in a commentary in Nature, Emory anthropologist Melvin Konner said: “We do not yet know if a similar system helps explain male attachment in nonhuman primates, much less humans, but a medicine that might someday be offered to certain men is an interesting prospect.”

However, he added, perhaps to the disappointment of some women: “We are a long way from a commitment pill.”

Nonetheless, this research lays good groundwork for understanding the evolution of social behavior and the neurobiology of romantic love. And we have also learned something about “genetic determinism.”

Opponents vindicated?

When it turned out that the human genome was made of “only” about 30,000 genes, opponents of genetic determinism felt vindicated. Geneticists had guessed we’d have around 100,000 genes. In comparison, the roundworm Caenorhabditis elegans has 18,000 genes, but only 959 cells. We have 100 trillion.

Craig Ventor, the owner of the American company that raced a publicly owned consortium of several laboratories to sequence the human genome, was surprised. “We simply do not have enough genes for this idea of biological determinism to be right,” Ventor declared. “The wonderful diversity of the human species is not hard-wired in our genetic code. Our environments are critical.”

In other words, Ventor and opponents of genetic determinism were happy to emphasize that nurture was apparently triumphant over nature. What the remarkable work on the meadow vole has shown is that the role of nature should not be underestimated.

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