The research found that voles bred to lack functioning receptors for oxytocin were still able to form strong pairs, produce young and nurse -- all behaviours previously believed to depend on the hormone.

Prairie voles are one of the few mammals that mate for life, and are often used to study social behaviours like pair-forming in animals.

In past studies, voles given drugs that stopped oxytocin being processed no longer formed pairs, and mothers failed to produce milk for their young.

Psychiatrist Devanand Manoli and neurobiologist Nirao Shah produced genetically altered prairie voles without working oxytocin receptors, and then observed how the mutant male and female voles behaved.

To their shock, the mutant voles appeared to have no difficulty pairing up with non-genetically altered partners, and mutant females could still deliver and nurse young, unlike those in the drug-driven studies.

"We were certainly surprised," said Manoli, an assistant professor at the University of California, San Francisco.

The results suggest that oxytocin is not the main, or only, driver of activities like partnering or nursing, he said.

"What the genetics reveals is that there isn't a 'single point of failure' for behaviours that are so critical to the survival of the species," he told AFP.

'Very complex behaviours'

That didn't mean there were no differences, however.

Some male mutant voles that paired with ordinary female partners didn't show the aggression towards interloping females that would normally be expected.

And while mutant females produced and nursed litters, some had fewer pups per litter than their counterparts, and fewer of their offspring survived to weaning, the paper published Friday in the journal Neuron explains.

Pups born to mutant mothers also tended to weigh less, suggesting that they were not able to nurse as effectively.

The study only involved pairing of mutant voles with "wild-type" partners, and the researchers said pairings with two mutant partners could produce different results.

Still, as a whole, the findings suggest a different picture of oxytocin's role in several important behaviours.

That could be because animals bred without the receptors developed "other compensatory pathways" that helped them pair up and nurse, said Shah, a professor at Stanford University.

But the researchers suggest it likely means oxytocin is only part of a set of genetic factors that control social behaviour.

"What I think our studies reveal is that there are multiple pathways that regulate these very complex behaviours," said Manoli.

Oxytocin has sometimes been suggested as a way to treat attachment disorders and other neuropsychiatric issues, but there is little settled science on how effective it is.

Now the researchers hope to investigate what other hormones and receptors may be involved in behaviours like pairing and nursing.

"These other pathways might serve as new therapeutic targets," Manoli said.