Ten minutes after they play in a competitive soccer match with an audience of friends and family cheering them on, men from the Tsimane people in lowland Bolivia have testosterone levels 30% higher than they were before the game. If the players were athletes in the United States, this number wouldn't be surprising. But Tsimane men have much lower levels of testosterone throughout their lives than do men in developed countries. The findings may provide clues to how the body regulates short-term versus long-term increases in the hormone.
The Tsimane, a population of 15,000 spread among small villages in the Amazon, rely on farming, hunting, and gathering to survive. With only recent exposure to immunizations and modern sanitation methods, the people are plagued by infections, pathogens, respiratory illnesses, and gastrointestinal diseases. This disease burden suggested to anthropologist Benjamin Trumble of the University of Washington, Seattle, that the men would likely have low testosterone levels. "Testosterone is quite energetically expensive and is also thought to interfere with immune functioning," he notes. "So if you're part of a population that faces lots of parasites and pathogens, generally you've adapted to have less testosterone."
To confirm his hypothesis, Trumble and colleagues recruited 88 Tsimane men who were playing in a competitive inter-village soccer tournament. Despite their hunter-gatherer lifestyle, the Tsimane have had increasing contact with other populations over the past few decades and have become avid fans of soccer. Men who were participating in the tournament play, on average, three times a week.
The researchers measured the men's testosterone levels using saliva samples taken 15 minutes before the game began. Their baseline levels were about 70% of those of similarly aged U.S. men , confirming the impact of their environment or genetics on testosterone levels. Ten minutes after the game, however, the men's testosterone was up by about 30%. In studies of U.S. men engaging in similar physical competition, testosterone spiked an average of 37%.
The similar increases in testosterone suggest that varied baseline hormone levels don't affect the competition-associated spike in testosterone. That an increase still occurs makes sense, says Trumble, because of the physical role that testosterone plays in the body. "That sudden boost in testosterone during competition, or during the 'fight or flight' response to fear, is quite beneficial for the body," says Trumble. "It allows muscle tissues to uptake glucose faster and allows you to react to your environment faster." How much of an increase is needed to set these physiological changes into motion isn't known.
Trumble and his colleagues also questioned the men about their performances in the game. Those who had scored goals and who rated their own performance the highest had the largest increase in testosterone. These findings echo correlations that have been seen in studies of men in the United States, the team reports online today in the Proceedings of the Royal Society B. The results confirm that there's more to the hormone spike than simply participating in a competition, says Trumble.
Knowing that baseline testosterone levels don't change the magnitude and nature of hormone spikes doesn't explain how testosterone is regulated, however, says biological anthropologist Peter Gray of the University of Nevada, Las Vegas. "This work extends the ecological scope of work on male testosterone and competition," he says. But it doesn't get at mechanism. How are spikes in testosterone generated if overall production has been dampened? Understanding these mechanisms could lead to new ways of treating low testosterone in older men.
Soccer isn't the only activity that affects testosterone. Trumble and his colleagues are now trying to sort out the different effects of physical activity versus competition on the testosterone spikes of the Tsimane. They're testing hormone levels after the men chop trees, hunt, and participate in a soccer shootout. "We hope this will help answer the question of whether it's just physical competition that causes the increase, or whether it's also non-physical competition or non-competitive exercise," says Trumble.