You Dirty Vole

When Carter dosed male voles with a vasopressin-blocking chemical after mating, their feistiness disappeared. An extra jolt of vasopressin, on the other hand, boosted their territorial behavior and made them more protective of their mates.


Vasopressin is also present in humans. While scientists don't yet know the hormone's exact function in men, they speculate that it works similarly: It is secreted during sexual arousal and promotes bonding. It may even transform some men into jealous boyfriends and husbands. "The biochemistry (of attachment) is probably going to be similar in humans and in (monogamous) animals because it's quite a basic function," says Carter. Because oxytocin and vasopressin are secreted during sexual arousal and orgasm, she says, they are probably the key biochemical players that bond lovers to one another.


But monogamous animals aren't the only ones that have vasopressin and oxytocin in their brains. Philandering animals do too. So what separates faithful creatures from unfaithful ones? Conveniently for scientists, the generally monogamous prairie vole has a wandering counterpart: the montane vole. When Thomas Insel, a neuroscientist at Emory University, studied the two species' vasopressin receptors (appendages on a cell that catch specific biochemicals) he found them in different places. Prairie voles have receptors for the hormone in their brains' pleasure centers; montane voles have the receptors in other brain areas. In other words, male prairie voles stick with the same partner after mating because it feels good. For montane voles, mating is a listless but necessary affair, rather like scratching an itch.

A BRIEF INQUIRY INTO THE BIOLOGICAL EXPRESSIONS OF HUMAN LOVE

Of course, human love is much more complicated. The biochemistry of attachment isn't yet fully understood, and there's clearly much more to it than oxytocin and vasopressin. Humans experience different kinds of love. There's "compassionate love," associated with feelings of calm, security, social comfort, and emotional union. This kind of love, say scientists, is probably similar to what voles feel toward their partners and involves oxytocin and vasopressin. Romantic love-that crazy obsessive euphoria that people feel when they are "in love"-is very different, as human studies are showing.


Scientists at University College London led by Andreas Bartels recently peered inside the heads of love-obsessed college students. They took 17 young people who claimed to be in love, stuck each of them in an MRI machine, and showed them pictures of their lovers. Blood flow increased to very specific areas of the brain's pleasure center-including some of the same areas that are stimulated when people engage in addictive behaviors. Some of these same areas are also active during sexual arousal, though romantic love and sexual arousal are clearly different: Sex has more to do with hormones like testosterone, which, when given to both men and women, increases sex drive and sexual fantasies. Testosterone, however, doesn't necessarily make people fall in love with, or become attached to, the object of their attraction.