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.
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.
Researchers weren’t particularly surprised by the parts of the lovers’ brains that were active. What astonished them was that two other brain areas were suppressed-the amygdala and the right prefrontal cortex. The amygdala is associated with negative emotions like fear and anger. The right prefrontal cortex appears to be overly active in people suffering from depression. The positive emotion of love, it seems, suppresses negative emotions. Might that be the scientific basis for why people who are madly in love fail to see the negative traits of their beloved? “Maybe,” says Bartels cautiously. “But we haven’t proven that yet.”
The idea that romantic love activates parts of the brain associated with addiction got Donatella Marazziti at Pisa University in Tuscany wondering if it might be related to obsessive compulsive disorder (OCD). Anyone who has ever been in love knows how consuming the feeling can be. You can think of nothing but your lover every waking moment. Some people with OCD have low levels of the brain chemical serotonin. Might love-obsessed people also have low serotonin levels? Sure enough, when Marazziti and her colleagues tested the blood of 20 students who were madly in love and 20 people with OCD, she found that both groups had low levels of a protein that shuttles serotonin between brain cells.
And what happens when the euphoria of “mad love” wears off? Marazziti tested the blood of a few of the lovers 12 to 18 months later and found that their serotonin levels had returned to normal. That doesn’t doom a couple, of course, but it suggests a biological explanation for the evolution of relationships. In many cases, romantic love turns into compassionate love, thanks to oxytocin and vasopressin swirling inside the lovers’ brains. This attachment is what keeps many couples together. But because attachment and romantic love involve different biochemical processes, attachment to one person does not suppress lust for another. “The problem is, they are not always well linked,” says anthropologist Helen Fisher, who has written several books on love, sex, and marriage.
TYING IT ALL TOGETHER: THE TRAVELING SALESMAN AND THE MARRIAGE VOW
In the wild, about half of male voles wander the fields, never settling down with one partner. These “traveling salesmen,” as Lowell Getz calls them, are always “trying to get with other females.” Most females prefer to mate with their partners. But if they get the chance, some will mate with other males too. And, according to Jerry Wolff, a biologist at the University of Memphis, female voles sometimes “divorce” their partners. In the lab, he restricts three males at a time in separate but connected chambers and gives a female free range. The female has already paired with one of the males and is pregnant with his pups. Wolff says about a third of the females pick up their nesting materials and move in with a different fellow. Another third actually solicit and successfully mate with one or both of the other males, and the last third remain faithful.
Why are some voles fickle, others faithful? Vole brains differ from one creature to the next. Larry Young, a neuroscientist at Emory University, has found that some animals have more receptors for oxytocin and vasopressin than others. In a recent experiment, he injected a gene into male prairie voles that permanently upped the number of vasopressin receptors in their brains. The animals paired with females even though the two hadn’t mated. “Normally they have to mate for at least 24 hours to establish a bond,” he says. So the number of receptors can mean the difference between sticking around and skipping out after sex. Might these differences in brain wiring influence human faithfulness? “It’s too soon to tell,” Young says. But it’s “definitely got us very curious.”
How does evolution account for the often-conflicting experiences of love and lust, which have caused no small amount of destruction in human history? Fisher speculates that the neural systems of romantic love and attachment evolved for different reasons. Romantic love, she says, evolved to allow people to distinguish between potential mating partners and “to pursue these partners until insemination has occurred.” Attachment, she says, “evolved to make you tolerate this individual long enough to raise a child.” Pepper Schwartz agrees: “We’re biologically wired to be socially monogamous, but it’s not a good evolutionary tactic to be sexually monogamous. There need to be ways to keep reproduction going if your mate dies.”
Many of our marriage customs, say sociologists, derive from the need to reconcile this tension. “As much as people love passion and romantic love,” Schwartz adds, “most people also want to have the bonding sense of loyalty and friendship love as well.” Marriage vows are a declaration about romantic love and binding attachment, but also about the role of rational thought and the primacy of mind and mores over impulses.
Scientists hope to do more than simply decode the biochemistry of the emotions associated with love and attachment. Some, like Insel, are searching for treatments for attachment disorders such as autism, as well as pathological behaviors like stalking and violent jealousy. It is not inconceivable that someday there might be sold an attachment drug, a monogamy pill; the mind reels at the marketing possibilities.
Lowell Getz, the grandfather of all this research, couldn’t be more thrilled. “I spent almost $1 million of taxpayer money trying to figure out stuff like why sisters don’t make it with their brothers,” he says. “I don’t want to go to my grave feeling like it was a waste.”
Gunjan Sinha is a freelance writer based in Frankfurt, Germany.