Curly Hair Keeps the Head Coolest

Scientists have long wondered why humans' scalps are covered in hair even though we are far less hairy elsewhere. A new study published this month in Proceedings of the National Academy of Sciences suggests these strands coming out of our heads may have evolved to stop our ancestors’ large brains from overheating, with curly hair cooling more. 

Shielding the head from heat could have been crucial for early hominid ancestors living in Africa under the equatorial sun. “The brain is a large and very heat-sensitive organ that also generates a lot of heat,“ says Penn State anthropologist Tina Lasisi to National Geographic’s Tom Metcalfe. “So we figured, evolutionarily, this could be important—especially in a period of time when we see the brain size of our species growing.”

To better understand how hair affected the temperature of the head, Lasisi and her colleagues placed three different wigs or no wig on a research manikin, called a “thermal manikin.” The manikin was heated to a body temperature of 95 degrees, according to National Geographic, and placed under hot lights in a climate-controlled wind tunnel. The scientists measured the temperature on the manikin’s head when covered with no wig and human hair wigs that had straight strands, loose curls or tight coils—which were similar in thickness and color. 

Under a simulated sun beaming down at 86 degrees Fahrenheit, the starkest difference in heat was between no hair and hair. The manikin head always became hotter, but adding the straight-haired wig cut that heat gain by more than half compared to a bare head. The moderately-curled wig made the scalp area less hot than the straight-haired wig, and the tightly-coiled wig led to the coolest head. 

4/8 We ran a series of experiments involving a thermal manikin that looked more like a prop from a sci-fi movie than a research tool (plugs that go into the eye sockets were certainly a *choice* ). This is Newton: pic.twitter.com/CCGHB4KTJG

— Tina Lasisi, PhD (@TinaLasisi) June 9, 2023

These results suggest that all kinds of hair protected the scalp from the sun in hot, humid conditions. Curls might further reduce heating by creating a layer of air between the surface and the scalp, the researchers speculate. 

Another experiment aimed to simulate sweat on the head by wetting the manikins. In this case, bald heads cooled the most through evaporation of water. But cooling with sweat might not be that helpful overall. Having hair lowered the amount of sweat required to balance the sun’s heat, according to New Scientist. 

“Any mechanism that could help cool the body, and at the same time save precious water, would definitely have been acted on strongly by natural selection,” says Joseph Graves, an evolutionary biologist at North Carolina Agricultural & Technical State University who was not involved in the study, to Michael Le Page at New Scientist.

The study is “a great leap forward about thinking why we have so much hair on our heads,” Elizabeth Tapanes, evolutionary biologist at the University of California San Diego, tells National Geographic.

Lead author Lasisi acknowledges that the manikins don’t match human physiology perfectly but hopes that more research will be done to understand when and why different hair types evolved, New Scientist reports.

 

Victoria Sayo Turner | READ MORE

Victoria Sayo Turner is Smithsonian magazine's 2023 AAAS Mass Media Fellow.