George is a typical Midwestern American male in the prime of his life, with an attractive spouse named Martha. George is a devoted husband, Martha an attentive wife. The couple has four young children, a typical home in a lovely valley full of corn and bean fields, and their future looks bright. But George is occasionally unfaithful. So, occasionally, is Martha. No big deal: That's just the way life is in this part of America.

This is a true story, though the names have been changed, and so, for that matter, has the species. George and Martha are prairie voles. They don't marry, of course, or think about being faithful. And a bright future for a vole is typically no more than 60 days of mating and pup-rearing that ends in a fatal encounter with a snake or some other prairie predator.

But if you want to understand more about the conflict in human relationships between faithfulness and philandering, have a peek inside the brain of this wee rodent. Researchers have been studying voles for more than 25 years, and they've learned that the mating behavior of these gregarious creatures uncannily resembles our own—including a familiar pattern of monogamous attachment: Male and female share a home and child care, the occasional dalliance notwithstanding. More important, researchers have discovered what drives the animals' monogamy: brain chemistry. And when it comes to the chemical soup that governs behavior associated with what we call love, prairie vole brains are a lot like ours.

Scientists are careful to refer to what voles engage in as "social monogamy," meaning that although voles prefer to nest and mate with a particular partner, when another vole comes courting, some will stray. And as many as 50 percent of male voles never find a permanent partner. Of course, there is no moral or religious significance to the vole's behavior—monogamous or not. Voles will be voles, because that's their nature.

Still, the parallels to humans are intriguing. "We're not an animal that finds it in our best interest to screw around," says Pepper Schwartz, a sociologist at the University of Washington, yet studies have shown that at least one-third of married people cheat. In many cases, married couples struggle with the simple fact that love and lust aren't always in sync, often tearing us in opposite directions. Vole physiology and behavior reinforce the idea that love and lust are biochemically separate systems, and that the emotional tug of war many of us feel between the two emotions is perfectly natural—a two-headed biological drive that's been hardwired into our brains through millions of years of evolution.

No one knew that voles were monogamous until Lowell Getz, a now-retired professor of ecology, ethology, and evolution at the University of Illinois, began studying them in 1972. At the time, Getz wanted to figure out why the vole population would boom during certain years and then slowly go bust. He set traps in the grassy plains of Illinois and checked them a few times a day, tagging the voles he caught. What surprised him was how often he'd find the same male and female sitting in a trap together.

Voles build soft nests about 8 inches below ground. A female comes of age when she is about 30 days old: Her need to mate is then switched on as soon as she encounters an unpartnered male and sniffs his urine. About 24 hours later, she's ready to breed—with the male she just met or another unattached one if he's gone. Then, hooked, the pair will stick together through thick and thin, mating and raising young.

Getz found vole mating behavior so curious that he wanted to bring the animals into the lab to study them more carefully. But he was a field biologist, not a lab scientist, so he called Sue Carter, a colleague and neuroendocrinologist. Carter had been studying how sex hormones influence behavior, and investigating monogamy in voles dovetailed nicely with her own research. The animals were small: They made the perfect lab rats.

The scientific literature was already rich with studies on a hormone called oxytocin that is made in mammalian brains and that in some species promotes bonding between males and females and between mothers and offspring. Might oxytocin, swirling around in tiny vole brains, be the catalyst for turning them into the lifelong partners that they are?

Sure enough, when Carter injected female voles with oxytocin, they were less finicky in choosing mates and practically glued themselves to their partners once they had paired. The oxytocin-dosed animals tended to lick and cuddle more than untreated animals, and they avoided strangers. What's more, when Carter injected females with oxytocin-blocking chemicals, the animals deserted their partners.

In people, not only is the hormone secreted by lactating women but studies have shown that oxytocin levels also increase during sexual arousal—and skyrocket during orgasm. In fact, the higher the level of oxytocin circulating in the blood during intercourse, the more intense the orgasm.

But there's more to vole mating than love; there's war too. Male voles are territorial. Once they bond with a female, they spend lots of time guarding her from other suitors, often sitting near the entrance of their burrow and aggressively baring their beaver-like teeth. Carter reasoned that other biochemicals must kick in after mating, chemicals that turn a once laid-back male into a territorial terror. Oxytocin, it turns out, is only part of the story. A related chemical, vasopressin, also occurs in both sexes. Males, however, have much more of it.

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.