A recent study conducted by anthropologists from UCSB and the University of Washington found that Tsimane men of Bolivia have a baseline testosterone level 33 percent lower than that of American men. Their findings appeared in last month’s issue of The Proceedings of the Royal Society B.

The Tsimanes are an indigenous people in central Bolivia who spend much of their time hunting, fishing, foraging and farming. Despite the high level of daily physical activity of these men, their testosterone levels are curiously lower than that of their less active American counterparts.

According to UCSB anthropology professor Michael Gurven, having high testosterone levels may present some adverse effects, which could explain the Tsimanes’ testosterone levels as a result of biological adaptation.

“In the best of all worlds, we’d all have high testosterone all the time. But that is not the case, because it is costly to have high testosterone.” Gurven said. “[High levels of testosterone are] potentially energetically costly, but also costly because it tends to have some antagonistic effects on the immune system. It affects it in a way that under many conditions can lead to a reduced ability to fight an infection.”

The Tsimane live in an environment where infections, pathogens and parasites are common, so maintaining high testosterone all of the time could be unhealthy because of the way testosterone may compromise the immune system, Ben Trumble — a former UCSB anthropology student and current graduate student at the University of Washington — said in a recent press release.

In the more urban areas of the U.S., however, higher testosterone levels are more feasible.

“In populations that are fairly well nourished and where there are not that many pathogens in the environment, the testosterone level tends to be higher,” Gurven said. “U.S. men of the same age tend to have testosterone levels that are about 30 percent higher, and that is baseline level.”

A key inquiry of the study involved researching the variability of testosterone levels of American and Tsiname men in different circumstances. Researchers observed that, despite the baseline differences in testosterone, Tsimane and American men both experience short-term testosterone spikes during competition.

To collect data, the researchers organized a soccer tournament for Tsimane teams. In order to measure the testosterone levels of their subjects, the researchers analyzed spit samples of both groups of men before and after competing.

They observed that the men experienced a 30 percent increase in testosterone immediately after a game. An hour after each game, testosterone was still 15 percent higher than under normal conditions.

According to a press release, similar increases have been shown in men living in the U.S. or other industrialized countries following sports competitions.

“What we found that was surprising was that the percent increase in testosterone was similar,” Gurven said. “If you were American with high testosterone or if you were Tsimane with fairly low testosterone, the percent increase was the same. In other words, reproductive competition, or competition of any sort, is important enough that even if it is costly to elevate testosterone, you are going to do it, regardless of the environment you are in.”

Although higher levels of the hormone could lead to an advantage in sports, Trumble said in a press release that, since U.S. men tend to be taller and heavier than Tsimane men and tend to be exposed to fewer parasites and pathogens, they would likely have a competitive advantage anyway, regardless of baseline testosterone levels.

The study suggests that testosterone spikes during competition occur in radically different populations and even in environments where increased testosterone levels could be hazardous.

“The system is designed in such a way that it is going to pay that cost of potentially impacting immune function, because the stakes of competition among males can be quite high,” Gurven said.

According to Gurven, studying hunter-gatherer societies like the Tsimane allow scientists to construct a more accurate conception of what human life, biology and behavior might have been like before modern amenities. Particular behaviors evident in both societies, such as competition-linked increases in testosterone, imply that this phenomenon may be a primitive and universal aspect of human behavior.