Why is human body temperature around 36–37°C?
No one knows for sure, but one theory says it’s warm enough to fend off fungi without burning too much fuel.
The thermometer beeps: 36.5°C (97.7°F). Same as usual, you think, and relax. We check our temperature all the time, but have you ever wondered why the human body runs at 36 to 37°C in the first place? Why not 40, or 30? Why does it settle right around there?
This simple question still has no definitive answer. But trace the history of that famous “37°C,” and compare us with other animals, and some intriguing clues start to emerge.
The German doctor behind “37°C”
The man who turned body temperature into a diagnostic tool was the German physician Carl Wunderlich. In a book published in 1868, he gave the average temperature of a healthy person as about 37°C and popularized the rule of thumb that anything above 38°C counts as a fever. He saw fever not as a disease in itself but as a symptom of disease, and he is also credited with bringing charts that tracked patients’ temperatures day by day into hospitals.
His figure rested on an enormous body of data: underarm readings from about 25,000 patients, said to number more than a million measurements in all. The thermometers of the day are said to have been about 30 centimeters (a foot) long and to have taken 20 minutes to give a reading.
In Japan, a team led by Sadataka Tasaka of the University of Tokyo measured the underarm temperatures of about 3,000 healthy men and women aged from their teens to their fifties in 1957, and reported an average of 36.89°C. That figure is still widely quoted as “normal” for Japanese people.
Is normal slowly falling?
Then, in 2020, a research team at Stanford University in the United States published a surprising analysis. They examined more than 670,000 temperature readings spanning about 160 years, from the records of soldiers who served in the American Civil War, through national health surveys from the 1970s, to hospital data from the 2000s onward.
They found that Americans’ body temperature had fallen by about 0.03°C with each successive birth decade. By their calculation, men born in the early 19th century ran 0.59°C warmer than men today. The modern average, measured by mouth, was about 36.6°C.
The researchers pointed to improvements in hygiene and medicine. With fewer lingering infections such as tuberculosis, and less inflammation from tooth decay and gum disease, there are fewer sources of low-level heat smoldering in the body. Heating and air conditioning, which mean we spend less energy keeping warm or cool, may also play a part. That said, some point to differences in the accuracy of old thermometers and in how temperatures were taken, and how much of the change is real is still debated.
A shield against fungi?
Among animals that keep a constant body temperature, “normal” varies a lot. Many mammals run at around 36 to 39°C, and birds run hotter still, typically above 40°C. Staying that warm means burning a great deal of food, nonstop. So why do we pay that fuel bill at all?
One candidate answer that has drawn attention involves fungi. In 2009, Arturo Casadevall and colleagues at the Albert Einstein College of Medicine in the United States tested more than 4,800 strains of fungi (molds, yeasts and their relatives) to see how warm they could grow. Above 30°C, each one-degree rise cut the share of strains able to grow by roughly 6 percent. At mammalian body temperatures, most fungi struggle to multiply.
The following year, the team set the benefit of fending off fungi against the fuel cost of staying warm in a mathematical model, and calculated that the best balance came at about 36.7°C. That is almost exactly our own normal temperature.
The idea also chimes with the plight of amphibians such as frogs, which have low body temperatures and are declining worldwide from an infection caused by chytrid fungi. Some researchers have even proposed a bold hypothesis linking the rise of mammals after the dinosaurs’ extinction to their resistance to fungi. Still, other factors may have shaped how warm we run, and the fungus theory remains just one strong hypothesis among others.
Normal isn’t a single number
Finally, it’s worth remembering that “normal” is a range. Body temperature rises and falls by about 0.5°C over the course of a day, usually lowest in the early morning and highest in the evening. It also depends on where you measure: readings are normally highest in the rectum, then the mouth, then the armpit. One person’s 36.2°C under the arm and another’s 36.9°C can each be perfectly normal for them.