Out of sight makes heart grow closer

Anthropologists from the University of California, Santa Barbara, also found that the increase in oxytocin was greater for those men who were absent longer, and it positively correlated with changes in testosterone.

"Our goal was to look at the interaction between different hormones in motivating behaviour in a naturalistic context," said Adrian Jaeggi, the paper's co-lead author and a post-doctoral scholar.

The team conducted the study on Tsimane men who are an indigenous population of forager-farmers and hunters in the lowlands of Bolivia's Amazon basin.

"Hunting for subsistence and sharing meat is something people have done for hundreds of thousands of years," Jaeggi added in a paper that was published in the Royal Society journal Biology Letters.

The team found that high testosterone levels while hunting could be attributed to a "winner effect" experienced by men making a kill. The increased oxytocin could serve as a balance to make the hunters kinder, more generous and more willing to share their bounty.

"These men are coming home, they are finished with work for the day and they are about to eat and share food," Jaeggi continued.

So the need to be social coincides with the need to regenerate and it would make sense for the same hormones to facilitate both functions.

How can the findings among a group of indigenous hunter-gatherers in central Bolivia be applied to the modern world?

"I think the 'absence makes the heart grow fonder' effect could potentially be very widespread," Jaeggi noted.

Reconnecting with their families after a day of separation would have been a very common challenge for men throughout evolutionary history, and oxytocin could help with that.

"Another interesting correlation is that the average Tsimane hunt lasts eight and a half hours, roughly equivalent to a workday in the modern world," the authors wrote.

Oxytocin levels indicate how much you value another person. It is like a physiological measure of the value of the relationship. For example, fall in love with someone and your oxytocin level will skyrocket whenever your sweetheart is near, both literally and figuratively.

"Even talking to someone on the phone is enough to cause that oxytocin increase," Jaeggi continued.