Sex can change a lot about a relationship. For male prairie voles, it can even change their brains.
Prairie voles, small, furry rodents native to North America, are one of the few mammals that form (mostly) monogamous partnerships. Many share homes and parenting duties, preferring to snuggle with their partners over any other vole. (Read more about how prairie voles form their bonds.)
So you might think that a little rodent romance would go into choosing these special life partners—but at least on the male’s end, this doesn’t seem to be true.
Before male prairie voles put a ring on it, they can't tell one single lady from another, according to a new study in the October issue of Animal Behavior.
But after forming a bond, the males show a significant preference for their partner, and somehow learn to recognize the distinct smells, appearances, and potentially behaviors of individual single females.
The skill may help them either be better partners and fathers—or cheaters, scientists say. (Related: "Of Voles and Men: Exploring the Genetics of Commitment.")
“What they’re really saying is that the mating effect is profound,” says Sue Carter, the director of the Kinsey Institute and an early pioneer in prairie vole research.
“When animals form pair bonds, they’re changed for life," says Carter, who wasn't involved in the research. “This may apply to humans as well.”
Have I Met You Before?
Because prairie voles fall in what can be anthropomorphized as love, researchers turn to them as tiny models of human love and attachment.
A few years ago, Alexander Ophir, a Cornell University professor of behavioral and evolutionary neuroscience, and team made a surprising discovery: Single male prairie voles could recognize other males, but it seemed like all single females looked and smelled alike to them.
“I remember when I read that original paper I thought well this doesn’t make any sense,” says Nancy Solomon, a biologist at Miami University in Ohio who was not a part of that research.
That's because males looking for long-lasting love would probably need to tell potential partners apart. (See National Geographic's pictures of animals in love.)
To confirm this odd finding, the team tested whether mating changes how male voles perceive females. They gathered 28 adult males who had never mated before from their breeding colony, and let half of them form relationships with females in the lab. The other half had to hang out with their single male siblings.
Then, the scientists allowed both partnered and single males to repeatedly interact with a single female they’d never met through a clear barrier, through which the rodents could see and smell.
Familiarity, in this experiment, was designed to breed boredom.
“Then, we provide them with a brand new female that they’ve never met,” explains Ophir. “And if they start to show an interest in this brand new female, it suggests they can tell the difference between the familiar one and this new one.” In rodents, interest means sidling up to the female and sniffing her, a lot.
The single males, though bored with the female they’ve been getting to sniff, didn’t show that spike of interest. But the paired males did.
“There’s something about forming a pair bond that changes these male prairie voles’ ability to recognize others. So there’s sort of change in cognitive capacity,” Ophir says.
He suspects that paired males experience changes in their brain hormones associated with forming bonds, such as oxytocin and vasopressin. For instance, the levels or the number of receptors for those hormones might shift, reconfiguring the rodents’ abilities to learn and remember individual females. (Read about the prairie vole love potion.)
Solomon, who was surprised by the early result, says she's now a believer.
“Now that I read the argument that [the researchers] make in this paper, it makes sense that it doesn’t matter who it is—just that it’s a female if you haven’t mated,” Solomon says.
From a human perspective, not being able to recognize one potential partner from another while dating could be a problem.
So why would prairie voles have evolved to be able to recognize individual females only after they’ve mated?
Young males are trying to find a mate—any mate. So Solomon speculates that the males might be able to recognize whether another vole is a sexually mature female prairie vole (preferably, apparently, a larger one)—even if they can’t tell apart individuals. (Also see "Owl Monkeys Shed Light on Evolution of Love.")
In addition, these young males may have a more pressing need to distinguish between other threatening males than between females.
After they’ve mated, the (mostly) monogamous male voles have to recognize whom they’ve had babies with in order to defend the nest and share parenting duties.
There’s also another, sleazier reason prairie voles may need to tell apart females after they’ve mated: To help them cheat on their partners.
“It’s possible that if you’re better at recognizing other members of your species, you might be more or less likely to cheat on your partner,” Ophir says.
“You could make an argument either way.”