And unlike men in the U.S., the Bolivian foragers-farmers do not show declines in testosterone with age.
"Maintaining high levels of testosterone compromises the immune system, so it makes sense to keep it low in environments where parasites and pathogens are rampant, as they are where the Tsimane live," said Ben Trumble, an anthropology graduate student at the University of Washington.
That men living in the U.S. have greater circulating levels of testosterone represents an "evolutionarily novel spike," Trumble said. The spike reflects how low levels of pathogens and parasites in the U.S. and other industrialized countries allow men to maintain higher testosterone without risking infection.
Trumble also pointed out that whereas men in the U.S. show a decline in testosterone as they age, and testosterone drops serve as a sentinel for age-related disease, Tsimane men maintain a stable amount of testosterone across their lifespans and show little incidence of obesity, heart disease and other illnesses linked with older age.