Researchers studying an isolated group of forager-farmers in central Bolivia, the Tsimane, found that their men had relatively low baseline testosterone levels — one-third lower than American men — but that their testosterone spiked during physical competition, like a soccer game.
Actually, their testosterone levels spike with any physically demanding activity:
According to Trumble, whose research lies at the intersection of hormones, behavior, and the environment, testosterone levels are closely related to the availability of food energy. When young men skip even a single meal, their testosterone levels can drop as much as 10 percent. Fast for a couple of days, and they decrease to castrate levels.
“The same is true for infection,” he added. “An infection from a pathogen or parasite — even injuries, burns, or surgery — all cause an immediate decrease in testosterone.”
The body uses food energy for a number of critical processes. Among them are building muscle mass and maintaining proper immune function. When food energy is limited, the body has to choose between one and the other. For populations in industrialized countries like the United States, there isn’t much of a tradeoff,” Trumble said. “I can go to the grocery store and gather 20,000 calories in 10 minutes without breaking a sweat. I don’t have to worry about a deficit.”
However, for a group such as the Tsimane, who are more physically active than most Americans — and use a lot more food energy — but also have to grow, hunt, or fish for the vast majority of the calories they consume, the tradeoff is much greater. In addition, the Tsimane’s regular exposure to pathogens and parasites requires additional calories for maintaining necessary immune function.
Previous studies by Trumble, Gurven, and others have demonstrated that competitive activity — such as soccer — causes a short-term spike in testosterone. “Past research has mostly focused on the role of testosterone in aggressive competition,” Trumble said. “Given the important of testosterone in supplying energy to muscles, we wanted to look at how testosterone changes during another vital part of Tsimane life — food production.” He and the research team collected saliva specimens from Tsimane men before and after an hour of tree chopping, just as previous studies had examined saliva specimens taken from Tsimane men immediately before and after an hour of soccer. “With soccer, we saw a 30.1 percent increase in testosterone,” Trumble said. “With chopping, we saw a 46.8 percent increase. It was significantly greater.”
The acute spike in testosterone increases the muscle’s ability to take in blood sugar, which, in turn, enhances soccer performance and reaction times. It turns out the same is true for tree chopping. “If you’re better able to pull blood sugar into your muscle tissue, and better able to use that energy, you’ll be able to chop more trees,” Trumble explained.
While Tsimane men have a relatively low baseline testosterone level — 33 percent lower than that of men living in the United States, where life is less physically demanding — they appear to maintain their testosterone levels over the course of their lives. This is contrary to the United States and other industrialized populations, where men generally experience decreases in testosterone as they age.