From Cheating Voles, Lessons on Evolution of Behavior


On account of their sheer abundance, prairie voles, a species of small Midwestern rodent, are what conservationists call a “species of least concern;” in other words, barring calamity in North America, there’s in no danger of prairie voles going extinct.

But for researchers examining the underlying biological mechanisms of social relationships, prairie voles are a species of great concern, even central import: Prairie voles form lifelong pair bonds. And, being small and adaptable, prairie voles can also be put through controlled lab experiments and studied relatively inexpensively. As such, decades of studies starring these furry fellows have greatly advanced what we know about selective bonds and social behavior among mammals—humans included.

There was a time when researchers believed the animals were monogamous in all senses of the word. But an avalanche of data now paints a less-romantic reality: While prairie voles do typically remain loyal lifelong companions to their mates (scientists call this social monogamy), it is also very common that they do indeed stray sexually, frequently siring or bearing pups with a vole outside the pair.

Now, UT scientists have found that the brains of prairie voles who stray from their partners are actually wired differently than those of their faithful counterparts. And their research sheds new light on our understanding of how social traits evolve.

The researchers looked at variations in a single gene, which coded for receptors for the brain hormone vasopressin. Vasopressin is known to be involved in social behavior, including sexual behavior. Vasopressin receptors can be found throughout the brain, but scientists looked in particular at their prevalence in the spatial reasoning areas of the brain.

They found that, on a population level, there were two versions of the gene that occurred often in prairie voles: one that coded for high numbers of vasopressin receptors, and one that coded for low numbers of receptors.

They also decked out a smaller number of voles with zip-tie necklaces embellished with radio tracking devices, then let them loose in an enclosed plot of simulated prairie, monitoring their daily comings and goings. They found that the male prairie voles with the “high” version of the gene were more likely to stay and only mate with their pair-bonded partner, whereas those with the “low” version were more likely to stray and sire offspring outside the pair.

“What we think is happening is that males that have a poor memory are cheating on their partners more, and they are either less able to keep track of their partners and they wander off as a result, or they are less good at remembering exactly where it is they’ve been beaten up by other males, so that keeps them coming back more often. The result is less time with their mate, [which] means more opportunity to mate with someone else,” said UT biologist Steven Phelps, PhD ’99, lead researcher on the paper, which was published in Science in December.

Phelps says his findings on prairie vole behavior and genetics at the individual and population levels suggests that both strategies—being sexually faithful, and being sexually unfaithful—were evolutionarily “fit,” reliably promoting survival of offspring to the next generation. Though he’s not sure yet of the mechanism by which memory influences male prairie vole behavior, Phelps says, the bigger-picture point of the study is that we now have evidence suggesting that brain diversity within a species that results in significant variation in social behaviors can be normal—possibly on account of being advantageous on a population level for a species as a whole.

Nearly a half-decade ago, when researchers C. Sue Carter and Stanley Getz first discovered that prairie voles were an ideal animal model for studying selective social bonds, the idea that studies of animal social behavior similar to ours might influence understanding of human social behavior ruffled feathers. But today researchers using prairie voles to study the biological—and in this case, genetic—underpinnings of social behaviors regularly spell out those possibilities.

“It doesn’t have one-to-one implications for humans, necessarily,” says Mariam Okhovat, the study’s lead author and an ecology and evolutionary biology graduate student at UT. Okhovat adds that the study gives valuable insight into the human brain as well.

“Even though [humans are] a different species, similar mechanisms can drive variation in our brain, too,” Okhovat says. “When we understand how variation in social behavior emerges from tiny changes in DNA in one species, it provides insight into what could be driving behavioral variation in another.”

Photo via Sensory Ecology


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