hair - Human hair, Cultural attitudes, Animal hair

A thread-like structure consisting of dead keratinized cells produced by the epidermis in mammalian skin. The root of the hair below the skin surface is contained in a hair follicle, which is responsible for producing the hair. The covering of hair in mammals helps to maintain constant body temperature by insulating the body. Some hairs, such as whiskers, have a specialized sensory function.

Human hair

Body hair

Historically, several ideas have been advanced to explain the reduction of human body hair. Savanna Theory suggests that nature selected humans for shorter and thinner body hair as part of a set of adaptations to the warm plains of the savanna, including bipedal locomotion and an upright posture.

Another theory for the thin body hair on humans proposes that Fisherian runaway sexual selection played a role here (as well as in the selection of long head hair). see types of hair and vellus hair.

The Aquatic Ape Hypothesis posits that sparsity of hair is an adaptation to an aquatic environment, but it has little support amongst scientists and very few aquatic mammals are, in fact, hairless. The outstanding question is why so much of human hair is short, underpigmented vellus hair rather than terminal hair.

Head hair

The most noticeable part of human hair is the hair on the head, which can grow longer than on most mammals and is more dense than most hair found elsewhere on the body. The average human head has about 100,000 hair follicles. Anthropologists speculate that the functional significance of long head hair may be adornment, a byproduct of secondary natural selection once other somatic hair had been lost. Another possibility is that long head hair is a result of Fisherian runaway sexual selection, where long lustrous hair is a visible marker for a healthy individual (with good nutrition, waist length hair—approximately 1 meter or 39 inches long—would take around 80 months, or just under 7 years, to grow). This would explain why long head hair (in both sexes) is viewed as attractive even now.

Types of hair

Humans have three different types of hair:

Lanugo, the fine hair that covers nearly the entire body of fetuses Vellus hair, the short, fine, "peach fuzz" body hair that grows in most places on the human body in both sexes Terminal hair, the fully developed hair, which is generally longer, coarser, thicker, and darker than vellus hair

Growth

Different parts of the human body feature different types of hair. The density of the hairs (in hair follicles per square centimeter) varies from one person to another.

The rising level of male hormones (androgens) during puberty causes a transformation process of vellus hair into terminal hair on several parts of the male body. The rate of hair growth and the weight of the hairs increase.

Areas on the human body that develop terminal hair growth due to rising androgens in both sexes, men and women, are the underarms and the pubic area. There is a sexual dimorphism in the amount and distribution of androgenic hair, with males having more terminal hair (particularly facial hair, chest hair, abdominal hair and hair on legs and arms) and females having more vellus hair, which is less visible.

Increased body hair on women following the male pattern can be referred to as hirsutism. An excessive and abnormal hair growth on the body of males and females is defined as hypertrichosis. Considering an individual occurrence of body hair as abnormal does not implicitly depend on medical indications but also on cultural and social attitudes.

Individual hairs alternate periods of growth and dormancy. During the growth portion of the cycle, hair follicles are long and bulbous, and the hair advances outward at about a third of a millimeter per day. After three to six months, body hair growth stops (the pubic and armpit areas having the longest growth period). Head hair, by comparison, grows for a long duration and to a great length before being shed.

Texture

Hair texture is measured by the degree of which one's hair is either fine or coarse, which in turn varies according to the diameter of each individual hair. Hair can also be textured if straightening irons, crimpers, curlers, etc are used to style hair.

Aging

Older people tend to develop grey hair because the pigment in the hair is lost and the hair becomes colourless. The age at which this occurs varies from person to person, but in general nearly everyone 75 years or older has grey hair, and in general men tend to become grey at younger ages than women.

It should be noted however, that grey hair in itself is not actually grey - the grey head of hair is a result of the contrast between the dark and white/colorless hair forming an overall 'grey' appearance to the observer. As such, people starting out with very pale blond hair usually develop white hair instead of grey hair when aging. In fact, the gray or white appearance of individual hair fibers is a result of light scattering from air bubbles in the central medula of the hair fiber. Some degree of scalp hair loss or thinning generally accompanies aging in both males and females, and it's estimated that half of all men are affected by male pattern baldness by the time they are 50.

It is commonly claimed that hair and nails will continue growing for several days after death.

Pathological impacts on hair

Drugs used in cancer chemotherapy frequently cause a temporary loss of hair, noticeable on the head and eyebrows, because they kill all rapidly dividing cells, not just the cancerous ones.

The hair shafts may also store certain poisons for years, even decades, after death.

Width

According to The Physics Factbook, the diameter of human hair ranges from 17 to 181 µm.

Cultural attitudes

Head hair

The remarkable head hair of humans has gained an important significance in nearly all present societies as well as any given historical period throughout the world.

In ancient Egypt head hair was often shaved, especially amongst children, as long hair was uncomfortable in the heat. Children were often left with a long lock of hair growing from one part of their heads, the practice being so common that it became the standard in Egyptian art for artists to depict children as always wearing this "sidelock". The head hair of women was long and pulled back into a chignon.

The traditional hair styling in some parts of Africa also gives interesting examples of how people dealt with their head hair. The Maasai warriors tied the front hair into sections of tiny braids while the back hair was allowed to grow to waist length.

Contemporary social and cultural conditions have constantly influenced popular hair styles. Famous long-haired men include Oliver Cromwell, George Washington and during his younger years Napoleon Bonaparte had a long and flamboyant head of hair.

However it has also been advanced that short hair on men has been enforced as a means of control, as shown in the military and police and other forces that require obedience and discipline.

Growing and wearing long hair is a lifestyle practiced by millions worldwide. Many women in conservative Pentecostal groups abstain from trimming their hair after conversion (and some have never had their hair trimmed or cut at all since birth). Hair length is measured (in inches or centimeters) from the front scalp line on the forehead up over the top of the head and down the back to the floor. Standard milestones in this process of hair growing are classic length (midpoint on the body, where the buttocks meet the thighs), waist length, hip length, knee length, ankle/floor length and even beyond.

Hair style can also surpass personal expression and enter the realm of artist expression. Using the medium of human and synthetic hair, elaborate fantastical head pieces, such as spider webs, flowers and flying "hair-y copters", have been made by participants.

Body hair

The attitudes towards hair on the human body also vary between different cultures and times. An upper class Egyptian woman took great pains to ensure that she did not have a single hair on her body, except for the top of her head (and even this was often replaced with a wig). Islam stipulates many tenets with respect to hair, such as the covering of hair by women and the removal of armpit and pubic hair (see five physical characteristics traits of fitrah).

In Western societies it became a public trend during the late twentieth century, particularly for women, to reduce or to remove their body hair. It is also beginning to gain currency among men, among whom shaving or trimming one's body hair is sometimes jokingly called "manscaping".

Hair as business factor

Hair care for humans is a major world industry with specialized tools, chemicals and techniques.

Animal hair

The hair of non-human mammals is also known as pelage, fur or wool. Some of the various forms of hair that make up the pelage are guard hairs (such as the defensive spines of porcupines) and bristles (which make up the manes of lions). Many aquatic mammals, such as whales and dolphins, have little or no body hair (though life in the water does not necessarily lead to hair loss, as the pinnipeds and otters demonstrate). A general inverse trend exists between body size and the thickness of body hair (measured in hair follicles per square centimeter). Large animals such as elephants and rhinoceros that have a higher ratio of body mass to surface area (and that need to keep from overheating) also have little or no body hair.