If the male prairie vole were a human guy, we might call him "highly evolved." Among the world's 5,500 mammalian species, he's among the roughly 5% who, for the most part, mate for life, and who stick around to protect and raise offspring with his mate. He might even pick up a stick or twig around the burrow once in a while.
Ah, but evolution is a journey, not a destination. And it turns out that some male prairie voles actually have habits that, if they were human guys, we might not consider so "evolved" at all. They're chronic philanderers.
But the promiscuous forays of these voles may be adaptive behavior after all: Their wandering, and the genetic and behavioral diversity it fosters, probably has helped promote the rodent species' long-term survival, says a new study published Friday in Science.
The new research set out to explore a pattern of behavioral variance in male prairie voles: While most male prairie voles stay close to home, some tend to travel farther afield, almost as if they're lost. These meandering prairie voles are more likely to leave their vulnerable broods unattended and their mates available to other prairie voles who are also straying.
But rather than stop and ask someone for directions, these wandering voles have sex with females not their lifelong partners.
Researchers at the department of integrative biology at the University of Texas at Austin studied prairie voles living in experimental enclosures that mimic their natural environment. They looked for links between a male vole's tendency to wander and variations at three genetic sites that influence brain regions responsible for spatial memory.
Reporting their findings in Science on Friday, the researchers said the most common constellation of genetic settings at those sites was linked both to sexual fidelity and a well-functioning navigational sense.
But voles with a slightly different set of genetic variants at those sites seemed to get lost quite readily. The study's authors speculated these voles have fuzzier memories of where, in their past wanderings, they have intruded on other male voles' territory and gotten into a scrap. These voles were also the players, siring offspring with females not their life partners.
While scientists have long speculated that unfaithful prairie voles were more risk-taking or reward-seeking than their faithful peers, the new research suggests a far more prosaic explanation for their philandering: Some voles have inherited a mental GPS system that doesn't work so well.
One in four prairie vole babies is born "outside a pair bond," said the study's authors. Sure enough, they found, such offspring are more likely to have the genetic variances linked to poor spatial memory than did voles born to fathers who sired them and stayed close to home.
If there were really such a thing as being "fully evolved," we might expect vole populations to display a narrow range of variation in their spatial memories — and a narrow range of behavior that springs from that.
But our environments change constantly, putting new evolutionary pressures on us all, and there is no end to evolution. In species such as the prairie vole, which live in groups, a wide range of variance in brain function and behavior very likely improves the group's likelihood of survival, the authors of the new study suggest.
In the wild, vole colonies show a cyclical pattern of expanding and contracting. As colonies of voles live more densely in close quarters, lost (and philandering) voles appear more likely to wander into each others' burrows and mate, and genetic diversity reins. In the resulting baby boom, colonies spread out. With more space between neighboring burrows, the faithful voles may then gain a reproductive advantage, and their offspring are less prone to wandering.
"This brain variation isn't just there by chance. It isn't random," says Steven Phelps, associate professor of integrative biology and the study's lead invetigator. "It's actually something that selection has kept around for a very long time. When it comes to social behavior, maybe there isn't a normal brain."
Researchers have long studied voles to glean insights into a range of human behaviors, not least those related to nurturance and monogamy. That their tendencies toward monogamy vary widely — and that those variances have genetic underpinnings — might offer broader insights about brain and behavioral diversity in other species. Maybe even us.
"We may find this to be a common pattern in social behavior, including personality differences, in lots of species," said Phelps.