What happens in Vegas...

There’s nothing more romantic than spending Valentine’s day in Las Vegas by yourself. Overdosed on glitz and relentlessly tempted by the ephemeral promises of pleasure, luxury and instant winnings, I seek solace in what is hopefully eternal: true love.

As a well-trained psychologist, I find comfort in defining a concept as slippery as true love in terms of behavior. And as a happily married woman, I gravitate towards monogamy as a hallmark of ever-lasting affection. Humans, as a model species, however, are annoyingly unpredictable and complicated. And while monogamy is the exception rather than the rule in the animal kingdom, there are a few choice species that have provided scientists with a wealth of data and bad jokes.

Take houseflies, for instance. Once the common housefly falls in love and gives up her virginity, she generally doesn’t stray. But this fidelity is not entirely a personal choice: there is something funny in the seminal fluid that males inject during mating that kills her libido. Furthermore, her mate does not share her level of commitment and has no problem seeking out other females. This one-sided romance is not particularly inspiring.

Birds, in contrast, are notoriously monogamous, with 90% of species demonstrating pair-bonding that is mutual and lasts anywhere from one breeding season to a lifetime. But the avian love nest is not without drama: baby birds are not always the direct genetic descendants of both parents. Females are known to ‘dump’ eggs into the nests of other birds, spreading the parenting costs around the neighborhood. And a few more recent genetic studies of bird families have unveiled the promiscuity of one or both parents.

But romantics need not lose heart, for inspiration can be found in the most famous loving mammal: the common prairie vole. Given that only about 3% of mammal species are monogamous, the prairie vole is in a class all by himself. The mating behavior of these cute little buggers has fascinated scientists for decades and has led to the discovery of hormones and genes involved in long-term attachment. Validating ‘love at first sight’, bonding prairie voles activate the brain pathways served by the hormone called oxytocin, which decreases stress and increases positive social behavior, on their first date. Oxytocin helps prairie voles, and humans, for that matter, remember positive social interactions. It’s the same hormone that bonds a baby to its mother. In females, oxytocin rules the day. Males, however, also rely on a second molecule called vasopressin, a close relative of oxytocin, but which is associated with territoriality and aggression. If things go well, the first date ends in a 24-hour marathon of love-making, after which the two voles get the same high from each other as addicts do from cocaine. Give a vole cocaine and dopamine, a powerful and far-reaching neurotransmitter floods the nucleus accumbens. Give a mated vole his girlfriend and dopamine floods the nucleus accumbens.

Prairie voles are not perfect, however. Despite the fact that both members of the pair share in parental duties and show many affectionate behaviors, such as grooming and spooning, they are not always sexually faithful. What happens in Vegas, stays in Vegas, and in the grassy meadows surrounding the strip*. Like their mousy counterparts, human males carry different variants of the gene that expresses vasopressin and differences in this gene can account for some of the variability in pair-bonding behavior.

Of course, we are more than just the products of our genes and Valentine’s day is about romance and promise, not cheating and losing. If nothing else, Vegas has taught me the power of celebrating wins and glossing over losses. Thankfully, a recent fMRI neuroimaging study comes to the rescue. According to Bianca Acevedo, Lucy Brown, Helen Fisher and other colleagues at Rutgers, long-term love can be just as rewarding as the initial romantic hit of a successful first date. But in addition to activating the nucleus accumbens and other dopamine-rich regions, pictures of the object of one’s affection also stimulate those regions involved in long-term attachment, such as the globus pallidus, thalamus, and anterior cingulate cortex. Just like the bright, noisy slot machines that keep the casinos in business, fMRI studies tempt us to draw conclusions that are not supported by the data. But today, surely the romantic notion that a pair bonded for life can keep the passion fresh and strong is worth considering. Eight years and counting, happy Valentine’s day, Adam.

*poetic license: it’s a desert out there.