The reasons we have hair, and the functions of its growth patterns, are not completely understood. Our pre-historic ancestors were much hairier than we are today; the reason for the decreased hairiness of modern man is unknown, although it is reasonable to assume that it parallels the use of clothing for warmth and protection from the sun and physical trauma. Hair serves as insulation from the cold; however, this does not explain why different human groups have distinct patterns of hair growth. Most people of Asian descent have very sparse body and facial hair, but some of these peoples such as the Inuit, Tibetans and Mongols people, inhabit some of the coldest regions on earth.
Hair has the additional function of extending the sensory capability of the skin beyond its surface. Although human hair lacks the wealth of sensory nerve fibers found at the root of whiskers of some animals, each hair has a nerve fiber going to the bulb of the hair follicle. Mechanical displacement of each hair causes a sensation that translates into an awareness movement on the skin’s surface. For example, when an ant or fly walks on one’s arm, one feels the displacement of hairs caused by the insect.
Hair also plays a role in the defense mechanisms of most fur-bearing animals. When an animal confronts a potential enemy, its fur bristles; standing on end to make the animal appear to be larger and more threatening. In dogs, this response is most visible in the neck area where the neck hairs, called hackles, rise. In cats, the most visible response is in the tail. An extreme example of the use of hair for self-defense occurs in porcupines: their quills, which are modified hairs, stand out from the body when the animal feels threatened. Porcupines have converted a reflex (that in most animals is purely defensive) into a formidable weapon. In modern man, with relatively sparse body hair, only vestigial traces of these reactions remain. A tiny muscle, called the erector pili, connects the lower portion of each hair shaft with the underside of the skin. When you are frightened, cold or angry, these small muscles contract, causing your hair to stand on end.
Each hair shaft also contains a small gland called the sebaceous gland, located next to the hair shaft. Sebaceous glands make a yellow, fatty substance called sebum that lubricates the hair. Each time the erector pili muscle contracts, the gland is squeezed, and a small amount of lubricant is applied to the surface of the hair.
Hair, along with skin pigmentation, is the major natural protection that we have against the sun’s harmful ultra-violet rays. Scalp hair also plays an important role in preventing mechanical trauma to the skull. Hair acts as a “dry lubricant” in areas that rub, such as under the arms and in the groin, and serves to disperse pheromones (body secretions that are involved in sexual attraction).
Hair is integral to our body image and can have a profound influence on our self-esteem and self-confidence. There is no other part of the human anatomy that can be changed or manipulated so easily. Hair can be groomed, styled, waved, straightened, dyed, braided, or cut, and, unlike tattoos or body piercing, changes made to our hair can be completely reversed. Hair serves as an important means of self-expression, and the loss of this form of self-expression in those going bald may account, at least in part, for the despair that they may experience.