An Excerpt from “This is Your Brain on Love,” Emerald City Writers Conference 2011
By Val Roberts
Consider the prairie vole: The brownish-gray Microtus ochrogaster, four to six inches from whiskers to tail, is practically a poster child for monogamous life-long mating.
Contrast it to its cousin, the montane vole; Microtus montanus, unlike its flatlander cousin, is notably not monogamous, although they’re both cute in a furry, big-mouse kind of way. The secret behind this dichotomy lies in their brains.
Physiologically, the main difference between the two rodents is in a small (in their case very small) piece of brain called the nucleus accumbens, where dopamine does its thing. And that difference is oxytocin receptors.
Monogamous voles have lots of oxytocin receptors in their nucleus accumbens. Promiscuous voles, well, not so much.
Oxytocin has been called the hormone of connection, and the cuddle chemical. Men and women basking in the afterglow of good sex have high levels. New mothers holding their infants for the first time have high levels.
Prairie voles that have spent the last 24 hours mating continuously have high levels, as do montane voles, but the poor montane voles don’t have anywhere for the oxytocin to attach and work.
As a result, montane voles don’t associate the pleasure of a dopamine high or an endorphin rush from that marathon mating with a particular partner.
Vasopressin is a compound closely related to oxytocin that appears to be tailored for the testosterone-soaked ventral pallidum of the human (and vole) male brain, located just next to the nucleus accumbens.
Vasopressin not only promotes the sense of connection, but also triggers possessive and protective behaviors: “Stay away from my girl” behaviors.
The human brain, with its richly oxytocin- and vasopressin-receptive nucleus accumbens and ventral pallidum, behaves a lot more like the prairie vole brain than the montane vole brain.
However, there’s a dirty little secret of evolutionary biology—biologists have done paternity tests on prairie vole offspring, and it turns out that they’re only socially monogamous.
So when a cheating husband declares, “She means nothing to me,” he might be telling the truth; his ventral pallidum is already set for his wife, but he couldn’t resist the siren call of a dopamine high. Millions of crack and meth addicts can testify to power of dopamine.