Testosterone spikes in hunters after a hunt

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While small-scale horticulture is a relatively recent addition to the human repertoire of food provisioning, hunting has deep evolutionary roots. In practically every society, hunting ability correlates with reproductive success the better the hunter, the more children he is likely to father.

Studying physiological data including testosterone and cortisol levels collected from members of the Tsimane, an indigenous group of forager-farmers and hunters in central Bolivia, UC Santa Barbara anthropologists Ben Trumble and Michael Gurven and colleagues at the University of Washington and the University of New Mexico have found evidence suggesting that testosterone and cortisol levels increase when men successfully hunt an animal, and remain high as they return home with meat. Their research appears online today in the Proceedings of the Royal Society B.

The research Trumble and colleagues conducted on short-term testosterone spikes among Tsimane hunters follows previous studies examining similar increases resulting from chopping down trees another physically demanding task that is critical to successful farming and food production.

Until recently, the testosterone literature has been largely focused on mating and male-male competition, said Trumble, the papers lead author. But thats really oversimplifying the male life history strategy. Its a lot more nuanced than simple male-male competition. Offspring provisioning is also an important aspect of male behavior. In mammals, competing against other males is the key to gaining access to females and increasing reproductive success. But humans are different female choice plays an important role, and in most subsistence societies, women prefer men who are better providers.

The acute spikes in testosterone increase the muscles ability to take in blood sugar, which, in turn, enhance performance during physically active production strategies such as tree chopping for farming, or hunting. If youre better able to pull blood sugar into your muscle tissue, and better able to use that energy, youll perform better, Trumble explained.

Cortisol can play a similar role. Short-term increases in that particular hormone are beneficial for energy mobilization in the body. If you come across a predator and need to be able to mobilize your energy reserves quickly, you would definitely benefit from a spike in cortisol, he continued. While associated with the acute stress response, this can be a good thing. But organisms face serious trade-offs, and often times whats good in the short term can have deleterious effects long term.

Regarding the role of testosterone in male provisioning strategies, little research has been conducted to date. There was a study a couple of years ago that looked at day traders in the stock market, said Trumble. The researchers found that on days the traders had a higher than average profit-to-loss margin, meaning they made more money that day, they showed greater increases in testosterone and cortisol.

Day trading isnt physically vigorous, but it is a high-variance activity, he continued. Hunting is high variance, but also very physical. On 60 percent of hunts youre successful, and on the rest you arent. So thats a really high variance rate. Sometimes you come back with a couple hundred pounds of meat, and sometimes you come back with nothing.

Most studies examining testosterone variability are conducted on college campuses with young men who have high, potentially novel levels of testosterone compared to what would have been experienced during human evolution. Our industrial lifestyle is quite different from the experiences faced by hunter gatherers throughout most of human existence, Trumble noted.

Testosterone levels are highly reactive to environmental factors, including pathogens, parasites and food scarcity. If you get sick at all, you see a decrease in testosterone, said Trumble. If you have low energy coming into the system, you see decreased testosterone. If you dont have enough calories coming into the system, you see decreased testosterone. If youre expending too much energy, you see decreased testosterone.