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Chopping boosts testosterone more than sport

Manly boost Chopping wood to clear land and feed the family produces more testosterone in men than competitive activities like sport, a new study has found.

The discovery is changing scientists' understanding of testosterone's role in the human body - and it isn't all about competition and aggression.

Dr Ben Trumble from the Institute of Social, Behavioural and Economic Research at the University of California, Santa Barbara and colleagues studied a group of Tsimane forager-farmers in the Bolivian Amazon.

Their saliva was sampled after playing soccer, and after chopping down trees while clearing jungle to grow crops.

The study, which was published in Evolution and Human Behaviour, found that one hour of tree chopping resulted in a 48 per cent increase in salivary testosterone levels in all men, regardless of age or state of health. By contrast levels increased by only 30.1 per cent during a soccer game.

Trade-off for survival

Trumble says that in males there is a trade-off between survival and reproduction.

"Previous studies showed that in men, testosterone drops very rapidly when men fast, or get sick," he says. "There are only so many calories available for the body to use as energy, and so we face a trade-off between investing in survival-related tasks like immune function, or in less critical functions like [maintaining or growing] muscle mass."

"Muscle mass is expensive to maintain; it takes about 20 per cent of the male resting energy budget just to maintain muscle mass, and even more to grow additional muscle."

However, acute increases in testosterone enhance muscle performance, he adds.

"This can be advantageous for men who are using their muscles for sports competition, but also for men who are using their muscles for food production.

"Larger spikes in testosterone enhance muscle performance, and increase men's ability to chop trees, resulting in more food production."

Beyond the battlefield

Trumble says that testosterone research in humans has often focused on male-male competition, but in fact testosterone is a vital metabolic hormone that plays a number of roles in the body.

"By focusing so much on the role of testosterone in aggression and competition, we have missed out on the importance of testosterone in a variety of other tasks," he explains.

"In animals like birds, where males compete to control a territory, and then mate with all of the female birds in that territory, links between testosterone and aggressive competition make perfect sense."

"In humans, men tend to compete for the attention of women via economic productivity as opposed to fighting other men in the street, so examining changes in testosterone during food production is important."

"Never in human history have we lived in an environment where we have such an abundance of food, and where we so rarely face illness; this is very different than the hunter-gatherer lifestyle experienced by our ancestors."

The lifestyle of subsistence populations like the Tsimane is more representative of the conditions throughout human evolution than our own current living situation, he says.

People have to hunt, fish, or grow all of their food, and the environment contains higher levels of parasites and pathogens, and their bodies have to trade-off between investing in immune function or growing muscle mass.

Trumble says, "By examining how the environment affects hormone-behaviour interactions in populations like the Tsimane, we can get a better idea of how these systems were calibrated in environments that are more relevant to our species history."

He says the research has one final interesting insight.

"Tsimane men, who have a far more active life, have lower levels of testosterone when compared to age-matched US men, but also appear to have less of a decline in testosterone with age.

"Even late in life, these men can express the same spikes in testosterone as younger men."