If you cheat on your spouse, you can't yet plead
biochemistry in divorce court. But rodent-brain research sheds
light on why some lovers stay, some stray.
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
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
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,
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."
by Gunjan Sinha
� Gunjan P. Sinha 2003/09 • GunjanSinha.com • Berlin - New York