The effect of alcohol on social behavior is no mystery: Go to any bar on a Saturday night to witness the posturing, giggling, and flirting that drinking encourages.

You may also notice differences in the way males and females relate to their partners while under the influence. Now a new study sheds light on the brain chemistry underlying the gender differences you're witnessing—especially if you've walked into a bar full of prairie voles.

The study, published online Monday in the journal PNAS, reveals that alcohol has opposite effects on the neural systems that regulate social bonding in male and female prairie voles.

After being paired for 24 hours and consuming alcohol during that time—yes, prairie voles will have a wee drink or two when given the chance—the males in the study often chose to spend time with a stranger rather than their partners in a subsequent three-hour "partner preference" test.

In contrast, the majority of females who met a male while tipsy wanted more "huddling time" with him, behavior that can signal commitment in these animals. The sexes also showed contrasting changes in the neural systems regulating social behaviors.

"It's the first time we've shown that alcohol drinking can directly affect social bonding and that these effects are paralleled by changes in neuropeptides," says Andrey Ryabinin of Oregon Health and Science University in Portland, who led the study. Neuropeptides are small molecules that brain cells use to communicate with each other.

Models of Monogamy

Prairie voles have long been a lab model for studies of human pair bonding; like people, the animals are socially monogamous, meaning they'll stick with a single partner long term.

Human and vole brains also process social encounters and drug-related altered states as similarly rewarding, and our bodies and minds handle stress in similar ways. Underlying these behaviors held in common are shared hormones and neuropeptides associated with all of these experiences.

"And it just happens that they drink alcohol readily," says Ryabinin.

In a previous study the authors showed that heavy-drinking or teetotaling prairie voles can even influence how much a "peer" drinks, sometimes encouraging and other times discouraging more consumption—another parallel between the rodents and humans.

In the current study, the animals preferred to lap up water laced with 10 percent ethanol rather than pure water. "It's very convenient—it eliminates the stress that comes with administering it to the animals," which would make it harder to interpret the results, he says.

Contrary to what you might expect between well-lubricated vole couples, the effects on pair bonding were not due to the alcohol's influence on mating behavior, aggression, or motor abilities.

In fact, alcohol had no significant influence on how much mating or aggression went on between the paired males and females. "The effects on bonding were happening independently," Ryabinin says.

While initially a surprise, the contrasting changes in the neuropeptides in males and females "could reflect the different ways the animals handle stress," he says. The neural systems that the alcohol affected in the voles are the same ones that regulate levels of anxiety in these animals.

The correlation between bonding and stress needs to be studied further, Ryabinin says, but he notes there's a certain logic to it: Males, very generally, deal with anxiety with a fight-or-flight response. While both fighting and fleeing are actions that are likely to disrupt social bonds, fleeing is in a sense what they're doing in leaving their partners. Females, in contrast, more often lean toward actions that "tend and befriend"—not a bad descriptor of their cuddly behavior after drinking.

Intoxicating Parallels

What motivates a vole under the influence is, of course, not exactly the same as the tangled interplay of biology, experience, and culture that affects the behavior of a similarly impaired human. That's one reason the rodents make better experimental subjects.

"In humans," says Ryabinin, "there are so many other factors to consider—for instance, the influence of another drinker or a history of drinking-related economic pressures—that might lead to broken marriages." Voles don't carry such baggage.

"This means that we can use prairie voles to model not just our alcohol-related behavior but [also] the underlying molecular influences on that behavior," he says. "More studies are required, but separating biological effects from purely cultural ones could lead to better treatments for both problem drinking and the resulting interpersonal conflicts."

Mark Egli, program director for the Division of Neuroscience and Behavior at the National Institute of Alcohol Abuse and Alcoholism, agrees that there's potential for broad benefit down the line.

"It's novel and exciting in alcohol research to bring together these perspectives—the social and the biological," he says. "And these neural systems are potentially some that we'd like to target for alcohol-dependence medications."