The details of what is going on—the vole story, as it were—is a fascinating one. When prairie voles have sex, two hormones called oxytocin and vasopressin are released. If the release of these hormones is blocked, prairie-voles' sex becomes a fleeting affair, like that normally enjoyed by their rakish montane cousins. Conversely, if prairie voles are given an injection of the hormones, but prevented from having sex, they will still form a preference for their chosen partner. In other words, researchers can make prairie voles fall in love—or whatever the vole equivalent of this is—with an injection.
A clue to what is happening—and how these results might bear on the human condition—was found when this magic juice was given to the montane vole: it made no difference. It turns out that the faithful prairie vole has receptors for oxytocin and vasopressin in brain regions associated with reward and reinforcement, whereas the montane vole does not. The question is, do humans (another species in the 3% of allegedly monogamous mammals) have brains similar to prairie voles?
To answer that question you need to dig a little deeper. As Larry Young, a researcher into social attachment at Emory University, in Atlanta, Georgia, explains, the brain has a reward system designed to make voles (and people and other animals) do what they ought to. Without it, they might forget to eat, drink and have sex—with disastrous results. That animals continue to do these things is because they make them feel good. And they feel good because of the release of a chemical called dopamine into the brain. Sure enough, when a female prairie vole mates, there is a 50% increase in the level of dopamine in the reward centre of her brain.
Similarly, when a male rat has sex it feels good to him because of the dopamine. He learns that sex is enjoyable, and seeks out more of it based on how it happened the first time. But, in contrast to the prairie vole, at no time do rats learn to associate sex with a particular female. Rats are not monogamous.
This is where the vasopressin and oxytocin come in. They are involved in parts of the brain that help to pick out the salient features used to identify individuals. If the gene for oxytocin is knocked out of a mouse before birth, that mouse will become a social amnesiac and have no memory of the other mice it meets. The same is true if the vasopressin gene is knocked out.
The salient feature in this case is odour. Rats, mice and voles recognise each other by smell. Christie Fowler and her colleagues at Florida State University have found that exposure to the opposite sex generates new nerve cells in the brains of prairie voles—in particular in areas important to olfactory memory. Could it be that prairie voles form an olfactory “image” of their partners—the rodent equivalent of remembering a personality—and this becomes linked with pleasure?
Dr Young and his colleagues suggest this idea in an article published last month in the Journal of Comparative Neurology. They argue that prairie voles become addicted to each other through a process of sexual imprinting mediated by odour. Furthermore, they suggest that the reward mechanism involved in this addiction has probably evolved in a similar way in other monogamous animals, humans included, to regulate pair-bonding in them as well.
