A most peculiar parasol: Exploring thermoregulation through human hair curl

Comment on: Lasisi T, Smallcombe JW, Kenney WL, et al. Human scalp hair as a thermoregulatory adaptation. Proc Natl Acad Sci U S A. 2023;120:e2301760120.

In our recent study [1], we investigated the possible thermoregulatory function of scalp hair through a series of experiments on thermal manikins, with human hair wigs ranging from straight to tightly curled (Figure 1). As anticipated based on prior studies on the thermoregulatory role of hair in mammals, we found that the presence of scalp hair significantly reduces solar influx, thereby protecting the head from overheating. However, we also uncovered a novel insight: the morphology of hair, specifically the tightness of the curl, serves to maximize the reduction in solar heat gain without affecting dry heat loss from the head. This unique property offers a “peculiar parasol” that circumvents the typical trade-off between thermal insulation and solar protection.

Figure 1. Experimental setup illustrating the methodology used to study the impact of hair texture on solar radiation heat protection. The figure demonstrates the simulated solar radiation using lamps, the placement of wigs with varying textures on a thermal manikin, and the factors considered during testing such as wind speed and scalp moisture. This graphical abstract was designed by M. Morales Garcia.

The seeming contradiction of dense hair coverage on the scalp with sparse coverage on the body in humans elucidates a complex thermoregulatory strategy. The active process of sweating effectively regulates core body temperature through evaporative cooling. This is a physiologically costly solution that may struggle with the overwhelming solar influx. By contrast, the presence of scalp hair serves as a passive barrier, specifically minimizing heat gain from solar radiation without sacrificing water or electrolytes as is the case through sweating.

Among mammals, the depth of the coat paradoxically serves to reduce solar heat gain; while a thick coat might be expected to trap heat, it actually minimizes the amount of solar heat gained, thereby improving net heat loss [2]. This function is seemingly mirrored in human scalp hair. In actuality, human hair curl introduces a unique mechanism, achieving this protection without the tightly packed fibers found in other mammals. The helical structure of human curl creates volume and air pockets, offering protection from solar heat without excessive insulation. The term “wooly” to describe human curl is a misnomer, as it inaccurately compares the hair’s helical structure to the crimped nature of wool. This comparison overlooks the distinctive thermal properties of human curl, which functions to minimize solar heat gain while simultaneously maximizing heat loss from the head.

Humans’ evolutionary path, marked by bipedalism and the development of a larger brain, presents a distinctive scenario where tightly curled scalp hair may have evolved to meet specific challenges. As the early hominins transitioned to an upright stance, the top of the head became more exposed to the sun, necessitating a specialized solution to prevent overheating [3]. Coupled with the evolution of a larger brain that has heightened metabolic demands, the necessity to effectively mitigate solar heat gain became more evident. Sweating remains the primary mechanism for heat dissipation in humans and provides advantages such as enabling prolonged physical activity in hot conditions [4–6]. The evolution of tightly curled hair can be seen as a complementary adaptation that mitigates the need for evaporative cooling, thereby reducing fluid loss without sacrificing thermoregulatory efficiency.

Physiological studies, such as those conducted by Cabanac & Brinnel [7], provide insights into the complex interplay between hairlessness and scalp hair in thermoregulation. While the initial observations seemed to favor hairlessness for heat loss, subsequent research has revealed a nuanced understanding, showing that the absence of hair necessitates higher sweat rates [8]. This points to a net disadvantage of hairlessness and resonates with our findings on the reduced need for evaporative cooling in tightly curled hair [1].

In the unforgiving environment of early human evolution, where conservation of water was paramount, tightly curled scalp hair appears to have offered a novel solution to efficient and passive thermoregulation. These insights not only deepen our understanding of human evolutionary adaptations but also open new avenues for exploration. From a physiological standpoint, the head’s protection may be of particular interest. Despite being a small surface of the body, the head’s influence on physiological heat responses can be disproportionately large.

These findings not only illuminate our evolutionary journey but also underscore the marvelous complexity hidden within the simplest aspects of our biology. Tightly curled hair, something so visible yet often overlooked, demonstrates a remarkable adaptation for thermoregulation. By studying these unique characteristics, scholars in physiology can enhance our comprehension of heat responses, and engineers could harness these insights to inform biomimetic designs, bridging the gap between nature’s adaptations and human innovation. This unique insight into the ingenuity of hair curl provides a fresh perspective on thermoregulation, enriching our comprehension of human biology and the clever solutions nature crafts.

Disclosure statement

No potential conflict of interest was reported by the author.