Hair serves a number of different functions. It provides insulation from cold weather and, in some species, from particularly hot weather. Because hair is often pigmented, it provides coloration. This might serve to camouflage an individual; in some mammals, the pigmentation changes with the seasons, becoming white during the snowy winter, for example.
The hair of non-human animal species is commonly referred to as fur.
Among humans, nature selected for little body hair as part of a set of adaptations including bipedal locomotion and an upright posture. Bipedal locomotion is extremely inefficient, and many animals can outrun human beings for short periods of time; such animals, however, are inefficient radiators of heat, and cannot run for long periods of time. Thus, human hunters must be able to chase animals for long periods of time, and must therefore have an efficient mechanism for radiating body heat. Upright posture, which exposes less surface area of the body to direct solar radiation, and subcutaneous sweat glands, which operate best with an absence of hair.
Typically, humans have more hair on the top of the head (perhaps to protect against too much radiation from the head), where extremities meet the torso (axillary (arm-pit) hair and pubic hair), on the eyelids and above them (eyebrows). In most societies people shave, style or adorn their hair for aesthetic reasons.
Sometimes, the term body hair is used, to distinguish hair on the body from hair on the head. The difference between body hair and scalp hair (and, in males, chin and moustache hair) is that head hair for practical purposes grows continuously, whereas body hair alternates regular periods of growth and dormancy. During the growth portion of the cycle, body hair follicles are long and bulbous, and the hair advances outward at about a third of a millimeter per day. After a few weeks, body hair growth stops. The follicle shrinks and the root of the hair rigidifies. Following a period of dormancy, another growth cycle starts, and eventually a new hair pushes the old one out of the follicle from beneath. Head hair, by comparison, grows to great length, whereas body hair does not. Anthropologists speculate that the functional significance of long head hair is almost certainly adornment, a by-product of natural selection.
Unlike other animals, human beings often have their hair cut or remove it by shaving or other means.
Hair is a biological polymer; over 90% of its dry weight is made up of proteins called keratins. Under normal conditions, human hair contains around 10% water, which modifies its mechanical properties considerably. Hair proteins are held togther by disulfide bonds, from the amino acid cysteine. These links are very robust; for example, virtually intact hair has been recovered from ancient Egyptian tombs. Different parts of the hair have different cysteine levels, leading to harder or softer material.
Structurally, hair consists of an inner cortex, comprising spindle-shaped cells, and an outer sheath, called the cuticle. Within each cortical cell are many fibrils, running parallel to the fibre axis, and between the fibrils is a softer material called the matrix.
The cuticle is responsible for much of the mechanical strength of the hair fibre. It consists of scale-shaped layers. Human hair typically has 6-8 layers of cuticle. Wool has only one, and other animal hair may have many more layers.
Hair responds to its environment, and to its mechanical and chemical history. For example, hair which is wetted, styled and then dried, acquires a temporary 'set', which can hold it in style. This style is lost when the hair gets wet again. For more permanent styling, chemical treatments (perms) break and re-form the disulphide links within the hair structure.
The diameter of a human hair ranges from about 18 µm to 180 µm. In people of European descent, blond hair and black hair are at the thinner end of the scale, while red hair is the thickest. The hair of people of African descent is typically thicker than the hair of other groups.
The speed of growth is roughly 10 cm/yr = 0.3 mm/day = 3 nm/s.
Notable variations in physical appearance of the top and back of the head are:
See also facial hair, pubic hair, hirsutism, social role of hair, blonde joke.
Humans
Hair spray, gel, etc. may be used for fixation of the arrangement and may also make it shiny.