Voles can fall in love without oxytocin
Although oxytocin is classically thought of as “the love hormone”, a new study shows that prairie voles can pair-bond without its involvement.
It’s serendipitous that “vole” should be an anagram of “love”. Most prairie vole nests are occupied by a pair-bonded male and female, who remain together for life. If an unfamiliar individual of the opposite sex gets too close to a happily committed vole, it’ll be met with aggression. Prairie voles get so attached to one another that even after death does them part, fewer than 20% of widowed voles will take on a new mate.
Researchers have long been fascinated by prairie voles and how their bonds with their mates seem so similar to our own. By understanding how voles form attachments to one another, we could gain insight into why and how human relationships develop.
So why do voles — or any creatures, for that matter — pair-bond? The practical explanation is that it’s easier to raise offspring with both parents around. Male voles contribute to nest building, huddle with their families for warmth and drag pups back to safety if they wander too far from the nest.
The physiological basis of pair-bonding is more mysterious. But if you had to pick one molecule to explain why couples get attached to one another, you’d pick oxytocin.
Oxytocin — the cuddle hormone
An abundance of research, much of it on the prairie vole, tells us that oxytocin plays an important role in the formation and maintenance of social bonds. The hormone is released through physical contact, most typically mating. Prairie voles form pair bonds after 24 hours of cohabitation — but if the partners sexually interact, 6 hours is sufficient. Leave two newly introduced single voles to their own devices, and you can expect 24 hours of intermittent mating.
Oxytocin seems to be linked to monogamy. Some of the prairie vole’s closest relatives, like the meadow vole, aren’t so keen to commit to one partner. These polygamous vole species have a lower density of oxytocin receptors in certain regions of the brain than their monogamous cousins.
Injecting female prairie voles with oxytocin causes them to develop a preference for the male that was with them at the time of administration over a stranger. We’re amenable to the hormone, too —research on humans suggests that an intranasal squirt of oxytocin can enhance trust and improve our ability to read one another’s emotions.
Some people have even wondered whether oxytocin could have practical applications. Psychiatry researchers have suggested that artificially administered oxytocin could enhance marriage satisfaction, though you’d be right to question whether sticking oxytocin up your nose is really the best solution to a dysfunctional relationship.
But couples’ therapy is far from the least ethical potential application of oxytocin. Take Enhanced Liquid Trust, a spray-on cocktail of oxytocin and pheromones that “promotes social bonding and enhances positive emotions in social and sexual interactions”. We can only hope it’s snake oil.
Loveless rodents
Previous research suggested that oxytocin was crucial to social bonding. In 1995, Thomas Insel and colleagues found that blocking oxytocin receptors prevented prairie voles from forming pair bonds.
Oxytocin isn’t only relevant to mated pairs — it’s also important for maternal bonding. In one study, 40% of mice without oxytocin receptors lost a high proportion of pups, compared to just 10% of normal mice. A possible explanation is that the oxytocin deficit impaired pup bonding. The weakening of maternal bonds could be even more devastating in the wild, where social competition causes mice to exhibit high levels of infanticide.
Reconsidering oxytocin
In a study published in Neuron this month, Berendzen and collaborators used the gene-editing technology CRISPR to generate three lines of prairie vole that lacked oxytocin receptors. Mutant voles were each co-housed with a normal vole of the opposite sex for a week.
To check whether the mutant voles had bonded with their partners, the researchers placed the mutant vole, their partner and an unfamiliar vole of the opposite sex in three interconnected chambers. Only the mutant vole had access to all three chambers.
Quite reasonably, the researchers expected the voles to show no preference for their mating partners. But the mutant voles rejected the unfamiliar voles and spent as much time huddling with their partners as did normal voles. The evidence was clear — prairie voles don’t need oxytocin to form pair-bonds.
The team also looked at the relationships between parents and their young. Here, results were more complicated. Since oxytocin is also involved in lactation, milk production was impaired in mutant mothers. As a consequence, their pups put on less weight and were less likely to survive. But the parental behaviour of both male and female voles remained fully intact.
With all the contradictory research that’s come before, you might wonder if these results are just a fluke. But social neuroscientist Larry Young reports that he and his colleagues have conducted a similar experiment with similar results, which have yet to be published.
“I said, ‘Oh no, what’s wrong here?’” Young recalls. “I didn’t believe it.”
How can we reconcile these new findings with those of previous studies, which so strongly suggested that pair-bonding could not occur without oxytocin?
Alternative pathways to love
It seems illogical that blocking voles’ oxytocin receptors should inhibit pair-bonding, while mutant voles with no receptors can form attachments just fine. But there’s a key difference — the mutant voles never had working oxytocin-signalling systems.
A vole that’s never known the influence of oxytocin could end up developing alternative mechanisms to facilitate pair-bonding. Conversely, an adult vole that suddenly loses its ability to respond to the hormone may not be able to compensate.
People who have been blind since birth are usually adept at finding their way around without visual information, but if an adult with good vision suddenly lost their eyesight, you’d expect them to struggle. Oxytocin could be similar. It’s not necessary for pair-bonding, but we’ve learned to rely on it.
“The wiring for attachment is so important,” says Devanand Manoli, a co-author of the new paper. “It would be a little surprising if there were a single point of failure.”
Exactly what alternative mechanisms may arise in the absence of oxytocin signalling remains unclear. The researchers wondered if vasopressin, another hormone involved in bonding, could compensate for the oxytocin deficit. But neither the production nor the binding of vasopressin increased in mutant mice.
Oxytocin is clearly important for social bonding. However, we now know that animals can form attachments without it. Bonding is a complex process that can’t be turned on and off by a single switch.
Love remains a mystery.
