Science

After testosterone rush of hunting, men's 'love hormone' surges upon return home

Absence makes the heart grow fonder: Perhaps a universal human experience?

From hunting grounds to athletic fields to trading floors, men moving together in packs, and sometimes alone, are typically engaged in what anthropologists term "male status competition." And their levels of testosterone--the hallmark hormone of maleness--tend to rise accordingly.

But a new study explores the nurturing, familial side of men who engage in such primal activities, often to support, feed or bring honor to their families. It finds that that side, too, is expressed hormonally, when a man arrives home to his family bearing dinner (or perhaps a paycheck or a trophy).

Indeed, the higher a man's testosterone has risen in the course of his engagement in traditionally male activity, the more the "love hormone" oxytocin tends to surge upon his arrival home, researchers have found. The longer his workday, researchers also found, the higher his oxytocin levels when he returns to his family.

That finding--one of the first to measure oxytocin release in a naturalistic setting--emerged after anthropologists from UC Santa Barbara followed male members of an Amerindian tribe in the Amazon Basin of Bolivia as they hunted for food.

The 31 men studied were members of the Tsimane hunter-gatherer tribe of Bolivia. UCSB anthropologist Benjamin C. Trumble collected salivary samples--spit--from the hunters, first as they departed on their daylong, and often solitary hunts; again after their first shot at a prey animal; and finally, about 10 minutes after they returned home.

Published in the Royal Society's Biology Letters, the resulting findings discovered--to their surprise--that men whose "day at the office" drove their testosterone highest experienced the highest levels of oxytocin when they came home.

Testosterone and oxytocin are not natural allies, said UCSB anthropologist Adrian V. Jaeggi: Though high levels of testosterone are usually associated with competitive, drive-for-dominance behavior, bursts of oxytocin are linked to sharing, cooperation, trust and tenderness.

Here, the needs of the social group may demand that a man who has driven hard all day find a way to reintegrate with his kin and share his food. Release of the hormone oxytocin may serve precisely to aid in that transition, or to signal that the returned hunter has successfully shifted roles, said Jaeggi.

 “Testosterone, whatever the reason for the increase, is liable to make you more asocial, and that might not be a good thing when you’re coming home to your family and community,” Jaeggi said. “Oxytocin on the other hand makes you more empathetic, which would be useful in a social context."

Both hormones may play another role for returning hunters: After the physical exertions of the hunt, testosterone and oxytocin also have been shown to assist in rebuilding muscle. That tonic effect may be a happy coincidence, or it may have been the hormonal influence that helped men in early human societies to form bonds with their partners and children.

The latest study on oxytocin is a sharp contrast to a welter of new research on the hormone.  This study measured oxytocin in circulating blood (not in the brain, where it may do its heaviest lifting) in a naturalistic setting. But many other studies have puffed solutions of oxytocin up the nose in an effort to see how an increase would affect behavior.

Both methods seem to converge on the conclusion that oxytocin has probably played a mighty role in the evolution of humans' powerful social bonds.

Jaeggi said the finding that a lengthy absence from home was linked to higher oxytocin levels upon return suggested the notion that absence makes the heart grow fonder could be a universal human experience.

“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.”

Human behavior intrigues me. Follow me on Twitter @LATMelissaHealy and "like" Los Angeles Times Science & Health on Facebook.

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