Anthropologists just upended our understanding of “normal” testosterone levels

A study published in the American Journal of Human Biology suggests that Western medical standards for male testosterone might not reflect the natural variation found in different environments. Researchers found that among Indigenous Shuar males in the Ecuadorian Amazon, testosterone levels change across the lifespan in ways that differ from patterns seen in high-income nations. These findings imply that what doctors consider a typical hormonal profile is actually a physiological response to specific lifestyle and environmental factors.

Testosterone is a hormone produced primarily in the testes that supports male physical development, muscle growth, and reproductive function. It is part of a biological feedback system involving the brain and the pituitary gland. The hypothalamus releases a hormone that triggers the pituitary to produce substances that tell the testes to make testosterone. When levels of the hormone rise, the brain senses the increase and slows down the production cycle.

This biological process is sensitive to the environment and the energy available to the body. Building muscle and maintaining high hormone levels requires a lot of calories. In environments where food is scarce or where the body must fight off many parasites, the body may lower its hormone production to save energy for survival. This trade-off is a central idea in the study of how organisms allocate their limited resources.

Theresa E. Gildner, an anthropologist at Washington University in St. Louis, led the study to understand how these factors work in a non-Western setting. Gildner and her colleagues focused on the Shuar people because they live in a resource-constrained environment in Amazonian Ecuador. Many Shuar individuals continue to rely on traditional gardening, hunting, and foraging while facing high rates of infectious diseases.

Most research on male hormones has focused on men in wealthy, industrialized nations who are relatively sedentary and have easy access to calorie-rich food. In those populations, testosterone typically peaks in early adulthood and then steadily declines as a man ages or gains body fat. This decline is so common in the West that it is often viewed as a universal part of male aging. Gildner wanted to see if this same pattern would appear in a population living under different ecological pressures.

The research team worked with 104 Shuar males between the ages of 12 and 67 from 11 different communities. To measure hormone levels, the researchers collected saliva samples from each participant twice a day for three consecutive days. They took one sample in the morning before nine o’clock and another in the evening after four o’clock. This allowed the team to see how hormone levels fluctuated from the time a person woke up until the end of the day.

The researchers also measured the height, weight, and body fat percentage of the participants. They used these measurements to calculate the Body Mass Index of each person. While this index is not a perfect measure of fat, it helps researchers estimate the energy reserves an individual has stored in their body. The team used statistical models to see how age and body fat related to the daily changes in testosterone.

The analysis revealed that Shuar males generally have lower testosterone levels than men in the United States. On average, the concentration of the hormone at the time of waking was about 401.77 picomoles per liter. The levels then dropped by about 2.1 percent for every hour that passed throughout the day. This daily decline is a standard biological rhythm that helps the body mobilize energy in the morning and rest in the evening.

Age played a notable role in how high these morning levels were for each participant. Young men in their twenties showed the highest levels of the hormone at the start of the day. Participants in their teens and those over the age of fifty had the lowest waking concentrations. This suggests that the hormone peaks during the years when men are most active in seeking partners and starting families.

The study also looked at the ratio between morning and evening hormone levels to see how the daily rhythm changed over time. As men got older, the difference between their morning peak and evening low became less pronounced. This specific finding suggests that some aspects of hormonal aging might be consistent across different cultures. It may happen because the brain becomes less sensitive to energetic signals as the body gets older.

A clear interaction was found between age and body fat that challenged common Western assumptions. In men with lower levels of body fat, the researchers observed an inverted U-shaped pattern for testosterone over the lifespan. These lean men had lower hormone levels in their youth, which then rose to a peak in middle age before declining slightly in their later years. This differs from the Western pattern where levels tend to drop continuously after early adulthood.

For participants with higher levels of body fat, the pattern looked more like what is typically seen in the United States. In these individuals, the highest hormone levels were seen in younger men, followed by a steady decrease as they aged. This suggests that having more stored energy in the form of fat allows the body to maintain higher hormone levels earlier in life. When energy is limited, the body may delay its peak hormone production until middle age.

The researchers also looked at how social factors influenced these biological markers. Men who were living with a partner or wife had notably lower morning testosterone levels than single men. This association suggests that the body may lower hormone production once a man has successfully found a partner. Shifting energy away from the pursuit of mates may allow a man to invest more in his current family and household.

While the number of children a man had was not a statistically significant factor in the model using Body Mass Index, it did show an association in the model using body fat percentage. In that specific analysis, men with more children tended to have lower waking hormone levels. This finding aligns with the idea that the body adjusts its physiology based on a man’s social role and parental responsibilities. These biological shifts may encourage cooperative behaviors rather than competitive ones.

The study provides evidence that there is no single “normal” level of testosterone for all men worldwide. Instead, the body seems to calibrate its hormone production to match the specific challenges of its environment. In the Amazon, where physical labor is intense and pathogens are common, a lower level of testosterone may be a healthy and adaptive response. This contrasts with Western clinical views that often treat lower levels as a medical deficiency in need of treatment.

There are some limitations to this research that should be considered. The study was cross-sectional, meaning it looked at a group of people at one point in time rather than following the same individuals for years. This makes it hard to say for sure how an individual’s hormone levels will change as they specifically grow older. Following the same group of Shuar men over a decade would provide even clearer data on these transitions.

The number of older participants in the study was also relatively small. Because many Shuar communities have a younger population, there were fewer men over the age of 50 available to participate. Future studies could benefit from including more elders to better map out the end of the male reproductive lifespan. Gathering more detailed information on physical activities like hunting or soccer would also help clarify short-term hormone spikes.

Using Body Mass Index as a measure of fat also has its drawbacks in this context. This index cannot tell the difference between muscle mass and fat mass, which is a point of contention among some biologists. In a population where men perform a lot of manual labor, a high index score might reflect muscle rather than body fat. Future research could use more precise tools, such as skinfold measurements or isotope tests, to get a better picture of body composition.

The study, “Salivary Testosterone, Age, and Adiposity Associations Among Shuar Males in Amazonian Ecuador Challenge Assumptions of “Normal” Testosterone Patterns,” was authored by Theresa E. Gildner, Melissa A. Liebert, Joshua M. Schrock, Samuel S. Urlacher, Dorsa Amir, Christopher J. Harrington, Felicia C. Madimenos, Tara J. Cepon-Robins, Richard G. Bribiescas, Lawrence S. Sugiyama, and J. Josh Snodgrass.