New research detected an intriguing change in the average body temperature of the Tsimane people, an indigenous foraging and farming group in the Bolivian Amazon that has recently started to interact more with industrialized communities. Over 16 years, the Tsimane have experienced a slight but rapid drop in body temperature—mirroring a similar decline seen among Americans over the past century and a half.
This January, a study found evidence in well-maintained medical records that the body temperature of Americans has dropped roughly 0.05 Fahrenheit every decade since the 1860s. The findings weren’t the first of their kind but seemed to provide the clearest confirmation that these downward trends in body temperature were real, at least in places comparable to the U.S.
One proposed explanation for the drop has been changes in sanitation and public health that have lowered the risk and incidence of infectious diseases in modern life. With fewer infections, the theory goes, the immune system doesn’t need to cause as much inflammation to ward off germs, leading to a chillier body on average.
Michael Gurven, an anthropologist at the University of California, Santa Barbara, and his team decided to study trends in body temperature in a unique way, by turning to their own long-standing work with the Tsimane people in Bolivia.
“Reports about body temperatures lower than 98.6 Fahrenheit have largely come from high-income countries like the U.S. and the UK,” Gurven said in an email. “I saw an opportunity to first assess what body temperatures looked like in a very different context—the rural tropics—and whether they have also been declining over time.”
The Tsimane are one of the few communities on Earth that still largely subsist on the farming of small crops as well as foraging and hunting, much as people did prior to the industrial era. In recent years, however, the Tsimane have started to intermingle more with their industrialized neighbors, even sending their children to schools nearby. In other words, their journey could be seen as a sped-up version of how humankind in general has changed in the past several centuries.
“Though the Tsimane environment looks similar today as it did when we first started working there two decades ago, access to medicines, markets, and other amenities has improved,” Gurven said. “So this gives us an opportunity to test whether body temperatures might be declining in rural Bolivia as well.”
Looking at medical records collected from 5,000 indigenous Tsimane people between 2002 and 2018, Gurven’s team saw the average body temperature of the Tsimane drop from about 98.6 degrees Fahrenheit (37 degrees Celsius) in 2002 to 97.7 degrees Fahrenheit (36.5 degrees Celsius) in 2018. The study’s findings were published in Science Advances.
While these changes echo what’s happened in places like the U.S., Gurven and his team think the explanations behind it are more complex than simply having fewer infections. For one, the Tsimane still experience a more frequent and wider range of diseases than other populations, even as their life expectancy has risen by over a decade in recent times.
“Our findings suggest that it is not just a lower likelihood of having a particular infection, but that our body’s experience with infection itself may be different now than in the past,” he said. “That experience could be because we now have easily accessible over-the-counter anti-inflammatory medicines like ibuprofen or that we are in better shape now than in the past.”
The reasons why body temperatures have lowered for one part of the world likely differ from why they’ve lowered elsewhere, according to the researchers. It’s possible that a reduction in daily physical activity could have led to lower body temperatures in some places, but it wouldn’t explain the trends among the Tsimane people, since they remain as active as ever. And not all of the changes introduced to the Tsimane recently have been positive. Other research from Gurven’s team has shown that with the introduction of cooking oil to their communities, rates of obesity have started to climb among the Tsimane (an average higher body weight has been proposed as a possible factor for colder bodies, but they found no evidence of that here).
On the whole, it does seem possible that lower body temperature can be a reliable indicator of better population health over time, just like life expectancy, Gurven said, though more research will need to be done to be sure.
“I would love to see others explore whether similar patterns of body temperature decline coincide with improved conditions, and if these generalize to many other environmental contexts,” Gurven said. “It might be the case, too, that if conditions worsen, we might see body temperatures shift upward.”