Perhaps, researchers proposed, evolution had piggybacked on well-established neural circuitry. If the hormones responsible for maternal behavior in females and territoriality in males were released during sex, they could foster this novel male-female bond. Prairie vole sex, for instance, involves an unusual amount of vaginal-cervical stimulation—probably an adapted behavior that triggers the oxytocin release normally associated with childbirth. Instead of bonding with a baby, the female bonds with her partner.
Subsequent studies showed that unlike the bond-eschewing meadow voles, prairie voles have oxytocin and vasopressin receptors in areas of the brain associated with reward and addiction. The voles’ brains are rigged to associate the reward of sex with the presence of a particular partner, just like “an addict learns an association with drug paraphernalia when getting high, so even his crack pipe becomes pleasurable,” Young says. He thinks that humans’ oddball face-to-face mating style, which highlights a partner’s unique physical features at the moment of reward, probably also serves to cement a pleasurable connection with a single individual.
The vole-bonding studies of the early 1990s intrigued Young, who had discovered molecular biology in college. After graduating, he did research in a Texas lab studying gender-bending whiptail lizards, whose fluctuating hormones allow them to shift between male and female behavior. He found he could change their behavior dramatically by injecting them with one hormone or another. When he went to Yerkes, at Emory University, he took along various techniques that could also decipher gene activity. In the first experiment of its kind in these critters, Young’s team put a prairie vole gene that codes for a vasopressin receptor into a virus, and then injected the virus into the reward centers of the meadow vole brain. The point? To see whether the alien DNA would alter the meadow vole’s behavior. It did: As the animals grew up, they began exhibiting pair-bonding behaviors. “We transformed a meadow vole into a prairie vole, behaviorally,” he says.
Long before he began his vole work, Young understood the power of the pair-bond: He married his high-school sweetheart on his 18th birthday. Now Young understands that any pair-bond depends on a suite of genes and brain chemicals, probably working alongside oxytocin and vasopressin. His most recent foray is into transcriptomics, a field focused on messenger RNA, the genetic material responsible for shuttling information from a cell’s DNA to its protein-making machinery. While the DNA of every cell in the body stays the same, the level of proteins produced by the translation of that DNA changes from one minute to the next. Young’s lab is attempting to watch how messenger RNA fluctuates as the mysterious prairie vole pair-bond is forged. Researchers “sacrifice” the animals at various stages in the bonding process, then extract the mRNA. If the mRNA signal indicates that genes are active during mating in prairie voles but not meadow voles, those genes become candidates for study. “We can design experiments to manipulate those genes,” Young says, “and determine if they are involved.”
Likewise, he’s eager to look at his newly sequenced prairie vole genome alongside the meadow vole’s genome, to find differences worthy of further investigation. The challenge comes in getting computers to compare and contrast such a vast amount of genetic information.
“There’s 50 years of work ahead of us, and a whole lot that we don’t know,” he says.
Here’s a dirty little secret: Prairie voles are socially, but not sexually, monogamous. As with human romances, pair-bonding doesn’t preclude what researchers call opportunistic infidelity, as evidenced by Getz’s two-timing 12th couple. This infidelity means that many males sire young outside the nest—and can accidentally end up raising someone else’s babies. (About 10 percent of young are from a father that is not their mother’s main suitor.) And just like in the human dating pool, some males don’t pair-bond at all. These footloose individuals are known as “wanderers.”
One of Young’s claims to fame is pinpointing a genetic difference between the career bachelors and the devoted partners. It’s found in a portion of a vasopressin receptor gene called a microsatellite, repetitive genetic material that for a long time was called “junk DNA.” Males with a long version of the microsatellite are superior pair-bonders, because they have more receptors in certain brain areas, while males with a short version might remain unattached.
Similar variation may matter among people, too. Swedish researchers genotyped nearly 2,000 adults and asked them about relationships. Men with two copies of a specific version of a vasopressin receptor gene were twice as likely to report a crisis in their marriage in the last year as men with one or zero copies. Their partners also expressed less satisfaction. Young hasn’t had his own gene analyzed: “I don’t want to know,” he says.
What he does want to know is more about what makes prairie voles different from one another. Can early life experiences make a difference? And could that difference shed light on human behavior and social disorders?
Katie Barrett, a graduate student in Young’s lab, pulls on multiple pairs of gloves as she leads me into a room full of adult voles. “They’re biters,” she says, by way of an explanation. The male voles in the room, each roaming in chambered arenas instead of ordinary cages, are in the middle of a partner preference test, the foundation of much of vole research. Along with the male, females collared with plastic zip ties are tethered at opposite ends of each arena. One female is the male’s mate, and another is a complete stranger. Though he may mate with both, a well-bonded male should spend much more time huddling with his partner. A computer program analyzes the movements of his pudgy little body, adding up the minutes.
Barrett has found that baby voles isolated from the licking and grooming of parents, an interaction known to stimulate oxytocin production, have trouble bonding with future mates—but only if the isolated voles also have a relatively low density of oxytocin receptors in reward areas of the brain. She is conducting tests to find out whether an oxytocin-boosting drug can protect the neglected animals’ social futures. “Can you intervene early in life and protect against this outcome?” Barrett asks.
Previous work had shown that the effects of oxytocin were stronger in females than in males, but in today’s test, males treated as pups are bonding quite well. “I wasn’t expecting that,” Barrett says. Early-life oxytocin release appears to build a stronger social brain in both sexes.