Hair

Cross section of a hair

Hair is a filamentous biomaterial, that grows from follicles found in the dermis. The human body, apart from its glabrous skin, is covered in follicles which produce thick terminal and fine vellus hair. Most common interest in hair is focused on hair growth, hair types and hair care, but hair is also an important biomaterial primarily composed of protein, notably keratin.

Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class.^[1] Although other non-mammals, especially insects, show filamentous outgrowths, these are not considered "hair" in the scientific sense. So-called "hairs" (trichomes) are also found on plants. The projections on arthropods such as insects and spiders are actually insect bristles, composed of a polysaccharide called chitin. There are varieties of dogs, cats, and mice bred to have little or no visible fur. In some species, hair is absent at certain stages of life.

Hair often refers to two distinct structures: 1) the part beneath the skin, called the hair follicle or when pulled from the skin, called the bulb. This organ is located in the dermis and maintains stem cells which not only re-grow the hair after it falls out but also are recruited to regrow skin after a wound;^[2] and 2) the shaft, which is the hard filamentous part that extends above the skin surface. A cross section of the hair shaft can be divided roughly into three zones. Starting from the outside: 1) the cuticle which consists of several layers of flat, thin cells laid out like roof shingles, 2) the cortex which contain the keratin bundles in cell structures that remain roughly rod like and in some cases, 3) the medulla, a disorganized and open area at the fiber's center.^[3]

Description

Construction of the root and strand

Hair Follicle Structure

Hair is mainly composed of the protein keratin. Keratin assembles into rope-like intermediate filaments. The structure of these filaments provides strength to the hair shaft.

Hair growth begins under the skin in a hair follicle. The only "living" portion of the hair is found in the follicle. The hair that is visible is the hair shaft, which exhibits no biochemical activity and is considered dead.^[4] The base of the root is called the bulb, which contains the cells that produce the hair shaft.^[5] Other structures of the hair follicle include the oil producing sebaceous gland which lubricates the hair and the erector pili muscles, which are responsible for causing goose bumps.^[6]

Strand of human hair

Each strand of hair is made up of the medulla, cortex, and cuticle.^[7] The innermost region, the medulla, is not always present and is an open, unstructured region.^[4][8] The highly structured and organized cortex, or middle layer of the hair, is the primary source of mechanical strength, water uptake. The cortex contains melanin, which colors the fiber based on the number, distribution and types of melanin granules. The shape of the follicle determines the shape of the cortex, and the shape of the fiber is related to how straight or curly the hair is. Asian hair typically has a round fiber and is quite straight. Oval and irregularly shaped fibers are generally more wavy or even curly.^[4] The cuticle is the outer covering. Its complex structure slides as the hair swells and is covered with a single molecular layer of lipid that makes the hair repel water.^[7] The diameter of human hair varies from 17 to 180 µm (0.00067 to 0.0071 in).^[9]

Hair pigment

All natural hair colors are the result of two types of hair pigment. Both these pigments are types of melanin produced inside the hair follicle and packed into granules found in the fibres. Eumelanin is the dominant pigment in dark-blond, brown and black hair, while pheomelanin is dominant in red hair.^[4] Blond hair is the result of having little pigmentation in the hair strand.^[4]. Gray hair occurs when melanin decreases or disappears.

Human hair growth

Hair grows everywhere on the body except for glabrous skin, such as that found on the soles of the hands and feet, lips, etc.

Hair follows a specific growth cycle with three distinct and concurrent phases: anagen, catagen, and telogen phases. Each phase has specific characteristics that determine the length of the hair. All three phases occur simultaneously; one strand of hair may be in the anagen phase, while another is in the telogen phase.

The body has different types of hair, including vellus hair and androgenic hair, each with its own type of cellular construction. The different construction gives the hair unique characteristics, serving specific purposes, mainly warmth and protection.

Purpose

Many animals have fur and other hairs that serve different purposes. Many animals use hair for thermal regulation and for camouflage; others use it to warn or signal other animals, and some use hair for defensive and, rarely, even offensive protection.

Warmth

Polar bears use their fur for warmth, camouflage while hunting and for protection by hiding cubs in the snow.

While humans have developed clothing and other means of keeping warm, the hairs found on the head serve as primary sources of heat insulation; elsewhere their function is debated. Hats and coats are still required while doing outdoor activities in cold weather to prevent frost bite and hypothermia, but the hair on the human body does help to keep the internal temperature regulated. When the body is too cold, the erector pili muscles found attached to hair follicles stand up, causing the hair in these follicles to do the same. These hairs thus form a heat-trapping layer above the epidermis. This is called piloerection, which is a Latin word where 'Pilo' means hair and 'erect' means to stiffen. This process is more commonly known as having goosebumps.^[10] The opposite happens when the body is too warm. The erector muscles make the hair lay flat on the skin which allows heat to leave.

Protection

Human hair may not compete with the painful spines of the porcupine, but much of the hair on the human body is suited to protect it. This natural armor cannot directly protect humans from potential predators, but it does help to keep the sense organs, like the eyes, working properly.

Eyebrows and eyelashes

Eyelashes and eyebrows help to protect the eyes from dust, dirt, and sweat

The eyebrows are situated above the eyelids on the forehead. While they have little significance to the survival of humans, they remain a part of the body's physiological makeup because of the role they play in protecting the eyes from dirt, sweat, and rain, as well as communication.^[11] People have developed other means, such as hats, umbrellas, and goggles to serve the purpose of eyebrows more effectively, but the presence of eyebrows may suggest that they were once important to earlier species' survival.^[11]

The eyelash grows at the edges of the eyelid and protects the eye from any dirt that may enter the eye. Camels have particularly long and thick eyelashes. The eyelash is to humans, camels, horses, ostriches etc., what whiskers are to cats; they are used to sense when dirt, dust, or any other potentially harmful object is too close to the eye.^[12] The eye reflexively closes as a result of this sensation.

Evolution

A 2008 study by scientists from the Medical University of Vienna traced the origins of hair to the common ancestor of mammals, birds and lizards that lived 310 million years ago. The study found chickens, lizards and humans all possessed a similar set of genes that was involved in the production of alpha keratin. In chickens and lizards, the α-keratin produced was found in their claws, but in mammals it was used to produce hair. The scientists involved continued searching for the mechanisms that allowed mammals to use the keratins of animal claws to produce hair.^[13][14]

Human "hairlessness"

Human hair is barely visible as it is thinner, shorter and more translucent than the hair of other mammals. Humans are part of a trend toward sparser hair in larger animals (fewer follicles per square inch of exposed skin). There are several African mammals that have very sparse fur, including the elephant and the hippopotamus, both at the upper end of this trend. The density of human hair follicles on the skin is actually about the average for an animal of equivalent size.^[15] It is still not clear why so much of human hair is short, underpigmented vellus hair, rather than terminal hair, and the effect of testosterone on the hair follicles in both human and other mammalian species. Because most human hair is less apparent vellus hair, this gives the appearance of being hairless, especially in females.

Human hair under 200-times magnification

Most mammals have light skin that is covered by fur, and biologists believe that human ancestors started out this way also. Dark skin probably evolved after humans lost their body fur, because the naked skin was vulnerable to the strong UV radiation as would be experienced in Africa. Therefore, evidence of when human skin darkened has been used to date the loss of human body hair, assuming that the dark skin was needed after the fur was gone.

Dr. Alan R. Rogers, an evolutionary geneticist at the University of Utah, used mutations in the MC1R gene to estimate when human skin darkened. He said humans may have gone through several genetic "clean sweeps" with light-skinned individuals dying off and dark-skinned individuals surviving. He estimates the last of these clean sweeps took place 1.2 million years ago.^[16] Therefore, apparent bareness in humans, likely has existed at least since that time.

Balding, where terminal hair switches to vellus hair, usually occurs at around 30 – 40 years of age. In prehistoric times, most individuals were not as likely to live past 30, and thus it wasn't likely that this trait was subject to selection.

The soft, fine hair found on many nonhuman mammals is typically called fur.

Most species evolved as the climate in Africa changed, to adjust their thermoregulation to the intense UV and sunlight at the equator, mostly by panting. Early hominids likely possessed fur similar to other large apes, but about 2.5 million years ago they developed sweat glands  that enabled them to perspire. It is not clear whether the change in body hair appearance occurred before or after the development of sweat glands. Humans have eccrine sweat glands all over our body ^[17]. Most mammals just have Eccrine sweat glands on their hands and feet. The rest of their body is covered in apocrine glands, which are not used to cool off. There is a trend in primates to have increased eccrine sweat glands over the general surface of the body ^[17]. It is unclear to what degree other primates sweat in response to heat, however.

The sweat glands in humans could have evolved to spread from the hands and feet as the body hair changed, or the hair change could have occurred to facilitate sweating. Horses and humans are two of the few animals capable of sweating, yet horses are larger and still have fully developed fur. In humans, the skin hairs lie flat in hot conditions, as the erector pili muscles relax, preventing heat from being trapped by a layer of still air between the hairs, and increasing heat loss by convection. Thus the apparent reduction in hair is not necessary for sweating to be enabled.

Historically, some ideas have been advanced to explain the apparent hairlessness of humans, as compared to other species.

The thermoregulatory hypothesis (developed by Dr. Peter Wheeler, 1984, 1985) suggests that when human ancestors started living on the savanna, humans began sweating more to stay cool ^[18]. Thick hair got in the way of the sweat evaporating, so humans evolved a sparser coat of fur. Although hair provides protection against harmful UV radiation, since our hominin ancestor was bipedal, only our heads were exposed to the noonday sun ^[19]. Humans kept the hair on our head which reflects harmful UV rays, but our body hair was reduced. Wheeler's (1984, 1985) hypothesis neglects to account for the other side of thermoregulation: staying warm. Without body hair, human infants risk hypothermia at night when temperatures drop. It may not have been until humans controlled fire that it was possible to reduce our coat of fur. Although humans do sweat a lot, other animals, such as horses, sweat too, but have retained their body hair.^[20]. It remains to be tested whether hair interferes significantly with sweat evaporation. Although humans sweat copiously, it may be a response to lacking hair, rather than the cause of it. Nonetheless, the thermoregulatory hypothesis remains the most accepted explanation for the human hair phenotype.

The aquatic ape hypothesis posits that sparsity of hair is an adaptation to a semi-aquatic environment, but the theory has yet to gain support among scientists.^[21]

Another hypothesis for the thin body hair on humans proposes that Fisherian runaway sexual selection played a role (as well as in the selection of long head hair), (see types of hair and vellus hair), as well as a much smaller role of testosterone in women. Sexual selection is the only theory thus far that explains the sexual dimorphism seen in the hair patterns of men and women. On average, men have more body hair than women. Males have more terminal hair, especially on the face, chest, abdomen and back, and females have more vellus hair, which is less visible. The halting of hair development at a juvenile stage, vellus hair, would also be consistent with the neoteny evident in humans, especially in females, and thus they could have occurred at the same time.

A final hypothesis is that human hair was reduced in response to ectoparasites ^[22][23]. Getting rid of our hair might have reduced the risk of fleas, ticks, lice and other biting insects.

Evolutionary variation

Evolutionary biologists suggest that the genus Homo arose in East Africa approximately 2.5 million years ago.^[24] They devised new hunting techniques.^[24]. The higher protein diet led to the evolution of larger body and brain sizes.^[24] Jablonski^[24] postulates that increasing body size, in conjunction with intensified hunting during the day at the equator, gave rise to a greater need to rapidly expel heat. As a result, humans developed the ability to sweat and thus lost body hair to facilitate this process.^[24] A major problem with this theory, however, is that it does not explain why males are larger, hairier and were more active in hunting than females.

Other primates have armpits that sweat like those of humans, and thus it is probable that human sweat glands evolved from a similar distribution, spreading to more areas of the body, rather than occur through evolution of a new trait. It is not known whether the increased distribution of sweat glands occurred before, during, or after, the change in body hair, or even whether the two are related developments. Horses also sweat, and they are larger, hairier and expend more energy running than human males, so there may not be any connection between the ability to sweat and the apparent hairlessness of humans.

Another factor in human evolution that also occurred in the prehistoric past was a preferential selection for neoteny, particularly in females. The idea that adult humans exhibit certain neotenous (juvenile) features, not evinced in the great apes, is about a century old. Louis Bolk made a long list of such traits,^[25] and Stephen Jay Gould published a short list in Ontogeny and Phylogeny.^[26] In addition, paedomorphic characteristics in women are widely acknowledged as desirable by men. For instance, vellus hair is a juvenile characteristic. However, while men develop longer, coarser, thicker, and darker terminal hair through sexual differentiation, women do not, leaving their vellus hair visible.

Texture

Curly hair

Jablonski^[24] agrees that it was evolutionarily advantageous for pre-humans (Homo erectus) to retain the hair on their heads in order to protect the scalp as they walked upright in the intense African (equatorial) UV light. While some might argue that, by this logic, humans should also express hairy shoulders given that these body parts would putatively be exposed to similar conditions, the protection of the head, the seat of the brain that enabled humanity to become one of the most successful species on the planet (and which is also very vulnerable at birth), was arguably a more urgent issue (axillary hair in the underarms and groin were also retained as signs of sexual maturity). During the gradual process by which Homo erectus transitioned from furry to naked skin, their hair texture putatively changed gradually from straight (the condition of most mammals, including humanity's closest cousins—chimpanzees), to Afro-like or 'kinky' (i.e. tightly coiled). In this sense, during the period in which humans were gradually losing their straight body hair and thereby exposing initially the pale skin underneath their fur to the sun, straight hair would have been an adaptive liability. Hence, tightly coiled or 'kinky' Afro-hair may have evolved to prevent the entry of UV light into the body during the transition towards dark, UV-protected skin.

Alternatively, some intuit that tightly coiled hair that grows into a typical Afro-like formation would have greatly reduced the ability of the head and brain to cool. They reason that although hair density in African peoples is much less than their European counterparts, in the intense sun the effective 'woolly hat' produced would have been a disadvantage. However, anthropologists such as Nina Jablonski make the opposite argument with regards to this hair texture. Specifically, Jablonski's assertions^[24] suggest that the adjective "woolly" in reference to Afro-hair is a misnomer to the extent that it connotes the high heat insulation derivable from the true wool of sheep. Instead, the relatively sparse density of Afro-hair, combined with its springy coils actually results in an airy, almost sponge-like effect. This, in turn, Jablonski argues^[24], actually facilitates an increase in the circulation of cool air onto the scalp. Further, Afro-hair does not respond as easily to moisture and/or sweat as straight hair. Thus it does not stick to the neck and/or scalp when wet. Rather, unless totally drenched, it tends to retain its basic springy puffiness. In this sense, the trait may enhance comfort levels in intense equatorial climates compared to straight hair (which, on the other hand, tends to naturally fall over the ears and neck to a degree that provides slightly enhanced comfort levels in cold climates relative to tightly coiled hair).

Further, some interpret the ideas of Charles Darwin as suggesting that some traits, such as hair texture, were too trivial for natural selection to have played a role. They argue that Darwin's explanation was that sexual selection may be responsible for such traits.^[27] However, the concept of "triviality" is a human value judgment. It has nothing to do with whether physical traits are/were actually adaptive. In fact, while the sexual selection hypothesis cannot be totally ruled out, the asymmetrical distribution of this trait does not indicate that this was the primary causal factor. Specifically, if hair texture were simply the result of arbitrary human aesthetic preferences, one would expect that the global distribution of the various hair textures would be fairly random. Instead, the distribution of Afro-hair is strongly skewed towards the equator. Further, it is notable that the most pervasive expression of this hair texture can be found in sub-Saharan Africa; a region of the world that abundant genetic and paleo-anthropological evidence suggests was the relatively recent (~200,000 year old) point of origin for modern humanity. In fact, although genetic findings (Tishkoff, 2009) suggest that sub-Saharan Africans are the most genetically diverse continental group on Earth, Afro-textured hair (along with a small cluster of other physical features) approaches ubiquity is this region. This points to a strong, long-term selective pressure that, in stark contrast to most other regions of the genomes of sub-Saharan groups, left little room for genetic variation at the determining loci. Such a pattern is, again, not indicative of the relatively variable trends associated with human sexual aesthetics.

Straight hair

According to the recent single origin hypothesis, anatomically modern humans (Homo sapiens) arose in East Africa approximately 200,000 years ago (Tishkoff, 1996). Then, ~150,000 years later, modern humans began to expand their range to regions outside of (and within) this continent (Tishkoff, 1996). The skin of those in the group that left the African continent had developed the ability to manufacture vitamin D (which was essential for bone development) upon exposure to UV light.^[24] However, the UV light of northern regions was too weak to penetrate the highly pigmented skin of the initial migrants in order to provide enough vitamin D for healthy bone development.^[24] Malformed bones in the pelvic area were especially deadly for women because they interfered with the successful delivery of babies; leading to the death of both the mother and the infant during labor. Hence, those with lighter skin survived and had children at higher rates because their skin allowed more UV light for the production of vitamin D.^[24]

Woman with straight hair

In this sense, the evidence with regard to the evolution of straight hair texture seems to support Jablonski's suggestions^[24] that the need for vitamin D triggered the transition from dark to light skin. Specifically, the distribution of this trait suggests that this need may have grown so intense at certain points that Northerners with mutations for straighter hair survived and had children at higher rates. This argument is made based on the principle that straight fibers better facilitate the passage of UV light into the body relative to curly hair. It is substantiated by Iyengar's (1998) findings that UV light can pass through straight human hair roots in a manner similar to the way that light passes through fiber optic tubes (Iyengar, 1998).

Nonetheless, some argue against this stance because straighter hair ends tend to point downward while fiber optics requires that light be transmitted at a high angle to the normal of the inner reflective surface. In light of this, they suggest that only light reflected from the ground could successfully enter the hair follicle and be transmitted down the shaft. Even this process, they argue, is hindered by the curvature at the base of the hair. Therefore, coupled with the amount of skin covered by long head hair, these factors seem to mitigate against the adaptive usefulness of straight hair at Northern latitudes. They further argue that UV light is also poorly reflected from soil and dull surfaces. These ideas can be countered by the fact that during the winter, the time of year in which UV light is most scarce at Northern latitudes, the ground is often covered with white snow. Given that white is the most effective color in terms of facilitating the reflection of ground light, the hypothesis that straight hair could have been adaptively favorable cannot be fully discounted in this regard. In addition, as mentioned in the previous section, straight hair may have also contributed to enhanced comfort levels in the North. This is evident in the extent to which, relative to curly hair, it tends to provide a layer of protection for ears and necks against the cold.

The EDAR Locus

A group of studies have recently shown that genetic patterns at the EDAR locus, a region of the modern human genome that contributes to hair texture variation among most individuals of East Asian descent, support the hypothesis that (East Asian) straight hair likely developed in this branch of the modern human lineage subsequent to the original expression of tightly coiled natural afro-hair^[28][29][30]. Specifically, the relevant findings indicate that the EDAR mutation coding for the predominant East Asian 'coarse' or thick, straight hair texture arose within the past ~65,000 years, which is a time frame that covers from the earliest of the 'Out of Africa' migrations up to now.

Removal practices

Though growing hair is an inevitable part of being human, many believe that it is unsightly and should be removed. Hair removal is almost always done for cosmetic reasons.

Depilation is the removal of hair from the surface of the skin. This can be achieved through methods like shaving. Epilation is the removal of the entire hair strand, including the part of the hair that has not yet left the follicle. A popular way to epilate hair is through waxing it.

Shaving

Many razors have multiple blades purportedly to ensure a close shave. While shaving will initially leave skin feeling smooth and hair free, stubble will inevitably appear a few hours after hair removal.

Shaving is done with bladed instruments, such as razors. The blade is brought close to the skin and stroked over the hair in the desired area to cut the terminal hairs and leave the skin feeling smooth. Depending on the rate of growth, one can begin to feel the hair growing back within hours of shaving. This is especially evident in men who develop a five o'clock shadow after having shaved their faces. This new growth is called stubble. Stubble typically grows back thicker than the original hair in the area because the very ends of terminal hairs are the thinnest parts of the hair shaft.

Waxing

Waxing involves using a sticky wax and strip of paper or cloth to pull hair from the root. Waxing is the ideal hair removal technique to keep an area hair-free for long periods of time. It can take five to nine weeks for waxed hair to begin to resurface again.^[31] Hair in areas that have previously been waxed is also known to grow back finer and thinner, especially compared to hair that has been shaved with a razor.^[31]

Cutting and trimming

Because the hair on the head is normally longer than other types of body hair, it is cut with scissors or clippers. People with longer hair will most often use scissors to cut their hair, whereas shorter hair is maintained using a trimmer. Depending on the desired length and overall health of the hair, periods without cutting or trimming the hair can vary.

Many people will confuse what a haircut is versus what a trim is. A haircut is usually performed in order to change one's hairstyle, while a trim helps to keep away split ends and keep the hair well-groomed and healthy. Cutting hair tends to take off more hair than trimming hair does. When hair is trimmed, only the first few centimeters need to be removed, whereas haircuts can sometimes result in the loss of many inches of hair.

Social role

Hair has great social significance for human beings. It can grow on most areas of the human body, except on the palms of the hands and the soles of the feet (among other areas), but hair is most noticeable in most people in a small number of areas, which are also the ones that are most commonly trimmed, plucked, or shaved. These include the face, nose, ears, head, eyebrows, eyelashes, legs and armpits, as well as the pubic region. The highly visible differences between male and female body and facial hair are a notable secondary sex characteristic.

Indication of status

Healthy hair indicates health and youth (important in evolutionary biology). Hair colour and texture can be a sign of ethnic ancestry. Facial hair is a sign of puberty in men. White hair is a sign of age or genetics, which can be concealed with hair dye. Male pattern baldness is a sign of age, which can be concealed with a toupee, hats or religious/cultural adornments. Although drugs and medical procedures exist for the treatment of baldness, many balding men simply shave their heads. Hair whorls have been discovered to be associated with brain development.

Hairstyle can be an indicator of group membership. During the English Civil War, the followers of Oliver Cromwell decided to crop their hair close to their head, as an act of defiance to the curls and ringlets of the king's men.^[32] This led to the Parliamentary faction being nicknamed Roundheads. Having bobbed hair was popular among the flappers in the 1920s as a sign of rebellion against traditional roles for women. Female art students known as the "cropheads" also adopted the style, notably at the Slade School in London, England. Regional variations in hirsutism cause practices regarding hair on the arms and legs to differ. Some religious groups may follow certain rules regarding hair as part of religious observance. The rules often differ for men and women.

Many hippies, metalheads and Indian sadhus have long hair; many punks wear a hairstyle known as a Mohawk or other spiked and/or dyed hairstyles; skinheads have short-cropped or completely shaved heads; Mullet hairstyles have stereotypically been associated with rednecks; the Deathhawk, a larger, fuller, back-combed version of a Mohawk is popular with goths and deathrock fans; an undercut in which the sides and back of the head are shaved short or bald, and the top hair is allowed to grow long, is common among cybergoths and fans of Industrial and heavy electronic music; hair that is usually short with a long side fringe — often dyed black or vibrant and contrasting colours — is associated with emo fans.

Heads were shaved in concentration camps, and head-shaving has been used as punishment, especially for women with long hair. The shaven head is common in military haircuts, while Western monks are known for the tonsure. By contrast, among some Indian holy men, the hair is worn extremely long.

The ethnically Manchu Qing Dynasty, beginning in the late 17th century China, ordered all Chinese citizens to adopt Manchurian hairstyles by shaving the front of their head and adopting a queue.

Regular hairdressing in some cultures is considered a sign of wealth or status. The dreadlocks of the Rastafari movement were despised early in the movement's history. In some cultures, having one's hair cut can symbolize a liberation from one's past, usually after a trying time in one's life. Cutting the hair can be a sign of mourning. Yoko Ono famously cut her very long hair after the assassination of her husband John Lennon, saying "John loved my long hair, so I gave it to him".

Tightly coiled hair in its natural state can be worn in an Afro. This hairstyle was once worn among African Americans as a symbol of racial pride. Given that the coiled texture is the natural state of most African Americans' hair, this simple style is now often seen as a sign of self-acceptance and an affirmation that the beauty norms of dominant (northern/European) culture are not absolute.

The film Easy Rider (1969) includes the assumption that the two main characters could have their long hairs forcibly shaved with a rusty razor when jailed, symbolizing the intolerance of some conservative groups toward members of the counterculture.

At the conclusion of the Oz obscenity trials in the UK, the defendants had their heads shaved by the police, causing public outcry. During the appeal trial, they appeared in the dock wearing wigs.

Religious practices

Women's hair may be hidden using headscarves, a common part of the hijab in Islam and a symbol of modesty required for certain religious rituals in Orthodox Christianity. Orthodox Judaism endorses the use of wigs, scarves and other headcoverings for women for modesty reasons as in Islam. Sikhs generally keep their hair uncut and men keep it tied in a bun on the head, which is then covered appropriately using a turban.

See also

• Hypotrichosis, the state of having a less than normal amount of hair on the head or body
• Natural afro-hair
• Hair follicle
• Cowlick

References

Notes

Bibliography

• Iyengar, B. (1998). The hair follicle is a specialized UV receptor in human skin? Bio Signals Recep, 7(3), 188-194.
• Jablonski, N.G. (2006). Skin: a natural history. Berkley, CA: University of Califiornia Press.
• Pagel, M. & Bodmer, W. (2003). A naked ape would have fewer parasites. Proceedings of the Royal Society of London. (http://www.anthro.utah.edu/~rogers/pubs/Pagel-BL-270-S117.pdf)
• Rogers, Alan R.; Iltis, David & Wooding, Stephen (2004), “Genetic variation at the MC1R locus and the time since loss of human body hair”, Current Anthropology 45 (1): 105-108.
• Tishkoff, S.A. (1996). Global patterns of linkage disequilibrium at the CD4 locus and modern human origins. Science. 271(5254), 1380-1387.

External links

• Discussion about shaving and cultures
• Answers to several questions related to hair from curious kids
• How to measure the diameter of your own hair using a laser pointer
• Instant insight outlining the chemistry of hair from the Royal Society of Chemistry

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