Human scalp hair as a thermoregulatory adaptation

Abstract

Humans are unique among mammals in having a functionally naked body with a hair-covered scalp. Scalp hair is exceptionally variable across populations within Homo sapiens. Neither the function of human scalp hair nor the consequences of variation in its morphology have been studied within an evolutionary framework. A thermoregulatory role for human scalp hair has been previously suggested. Here, we present experimental evidence on the potential evolutionary function of human scalp hair and variation in its morphology. Using a thermal manikin and human hair wigs at different wind speeds in a temperature and humidity-controlled environment, with and without simulated solar radiation, we collected data on the convective, radiative, and evaporative heat fluxes to and from the scalp in relation to properties of a range of hair morphologies, as well as a naked scalp. We find evidence for a significant reduction in solar radiation influx to the scalp in the presence of hair. Maximal evaporative heat loss potential from the scalp is reduced by the presence of hair, but the amount of sweat required on the scalp to balance the incoming solar heat (i.e., zero heat gain) is reduced in the presence of hair. Particularly, we find that hair that is more tightly curled offers increased protection against heat gain from solar radiation.

Keywords: Homo; hair; human evolution; thermoregulation.

Fig. 1.

Diagram of the experimental setup and conditions. Panel (A) shows the physical setup and the four scalp-hair conditions simulated: none, straight, moderately curled, and tightly curled. Panel (B) shows the experimental input variables: dry or wet scalp, three wind speeds, and radiation (light) on or off.

Fig. 2.

Regression coefficients for dry heat loss (A) and evaporative heat loss (B). Our results show that wind increases heat loss in both wet and dry scenarios with and without solar radiation. With dry heat loss, we see significant differences between heat loss in conditions with solar radiation compared to those without solar radiation, and there are significant differences between different wigs. With evaporative heat loss, we see that heat loss is not significantly affected by the type of wig, with all wigs showing a decrease in evaporative heat loss compared to the baseline “nude” condition.

Fig. 3.

Net dry heat exchange while exposed to a solar load with dry skin (yellow) and net wet/evaporative heat loss while exposed to solar load for a fully wet skin (blue) for all wig conditions and wind speeds were calculated for an ambient temperature of 30 °C with 60% relative humidity. Note that negative numbers indicate a heat gain; positive a heat loss.

Fig. 4.

Net solar influx in the dry skin (A; radiant and convective heat exchanges) and wet skin (B; radiant, convective and evaporative heat exchanges) condition in a 30 °C, 60% relative humidity condition.

Fig. 5.

Estimated required sweat rate to achieve maximum evaporative heat loss potential from fully wet scalp skin in the climate (A) and sweat rate required to achieve zero heat gain, i.e., heat balance, (B) across different wig conditions and wind speeds at an ambient temperature of 30 °C and relative humidity of 60%.