Hair (and a coat of hairs, called fur or pelage) is uniquely mammalian. No other creature possesses true hair, and at least some hair is found on all mammals at some time during their lives.
Hairs grow out of pits in the skin called follicles. The base of the hair, sunk in the skin, is called the root, and the part that emerges to the exterior is the shaft. Follicles often lie next to a skin gland called a sebaceous gland. These glands secrete an oily substance, which lubricates the hair and conditions it. Next to the follicle may be a small, involuntary arrector pili muscle. Hairs normally lie nearly parallel to the skin or at an acute angle to it. Contraction of the arrector pili muscles causes the hair to erect (become more perpendicular to the skin), and at the same time, pulls down on the skin and causes the bumps and pits known as "gooseflesh."
The shaft of a hair is made up primarily of a protein called keratin and consists of three distinct and easily recognizable parts. The outer layer is made up of dead, transparent cells called cuticular scales. These are arranged in distinctive, often overlapping patterns. Beneath the cuticular scales is the cortex, an often-thick layer that appears featureless except that it may be pigmented. At the center of most hairs is the medulla, made up of large, cuboidal cells, often distinctively colored and interspersed with air pockets. The arrangement of scales, thickness and color of the cortex, and distribution and color of medulla cells are often diagnostic of species. Using a microscope, a researcher can easily examine the cortex and medulla of individual hairs. Cuticular scales are more difficult to visualize; to see them, we usually imbed a hair in substances such as ethyl acetate, then remove it and examine its impression. An excellent set of instructions for examining hairs can be found in Teerink (1991). Numerous keys for identifying individual hairs have been published (e.g., Miles, 1965; Mayer, 1952; Teerink, 1991).
Hairs also differ in cross-sectional shape. Round hairs tend to be straight; oval or flattened hairs are curly.
The color of hair is due to a group of proteins called melanins. Eumelanin is very dark, pheomelanin is paler. Most individual hairs contain at least a few alternating bands of eumelanin and pheomelanin (human hairs are exceptions); this pattern is called agouti. White hair results from the lack of pigment; black is due to dominance of eumelanin. The overall color of the pelage comes from (1) the color of the individual bands, (2) the relative size of individual bands, and (3) the distribution of hairs characterized by different banding patterns.
The pelage of most mammals consists of more than one kind of hair. The most conspicuous hairs on most mammals are the guard hairs, which overlay the fur and serve to protect it. The guard hairs are sometimes modified to form defensive spines (as in porcupines, in which the cuticular scales elongate to form barbs that make it difficult to remove imbedded spines), bristles (long, firm hairs that grow continuously, such as those that make up the mane of a lion), and awns (hairs that do not grow continuously, and which have an expanded tip on a narrower, weaker shaft). Beneath the guard hairs is usually a layer called the underfur, made up of wool (ever-growing hairs), fur (relatively short hairs with definitive growth), and/or velli (down or fuzz). Mammalian embryos (including humans) also are often covered with a pelage, called lanugo, which is a kind of velli.
An especially important class of hairs is the vibrissae or whiskers. These hairs are usually long, straight, and stiff. Their bases are well-supplied with nerves, and vibrissae are very sensitive to touch. They provide the animal with information about its immediate surroundings. Vibrissae are usually restricted to certain parts of the body and are often found in tufts or clumps. We are all familiar with the whiskers on the face of a dog, but perhaps less aware that vibrissae may occur on other parts of the body as well, for example, on the ankles of a squirrel or the rump of some crevice-roosting bats.
Most hair is shed periodically in a process called molt. Molt may take place continuously, with a few hairs being replaced at any time, as in humans. More commonly, molt is restricted to certain seasons of the year or certain times of an animal's life, at which times all hairs are replaced. Seasonal molts sometimes involve dramatic alterations in color, as in the case of many weasels or snowshoe hares, which change from a brownish (agouti) summer pattern to a mostly-white winter pattern. Molts that take place during development are also very common. Most species have distinctive juvenile pelage (e.g., the spotted coat of a fawn or the gray pelage of a young white-footed mouse); many also go through an intermediate (subadult) pelage before attaining their adult form.
What is the function of hair? In modern mammals, hair serves to insulate, to conceal, to signal, to protect, and to sense the immediate surroundings. Insulation serves to conserve heat, but also, as in the case of diurnal desert animals such as the camel, to protect against excessive heat. The color of most species is probably cryptic, matching the animal's background. In some cases, such as the dramatic stripes of zebras or tigers, cryptic coloration can only be properly evaluated when the animal is seen against its natural background. Many mammals are dark dorsally and relatively pale ventrally, a pattern called countercoloration. This makes sense in the case of aquatic or arboreal species (predators above look down on a dark dorsum, matching the depths or forest floor below, while predators below see the pale venter, against light streaming down from above). Its role is less clear in the case of the many countercolored terrestrial and nocturnal rodents. Hair also provides by its color a means of signaling other members of one's own species (e.g., the white tail of the white-tailed deer, flashed by a fleeing animal to signal danger) or members of other species (e.g., the contrasting pattern of striped skunks, a warning to predators). The pelage also serves to protect the skin from abrasion and from excessive UV radiation. And, through specialized vibrissae, it provides a tactile sense, used, for example, to locate prey or to navigate in total darkness.
We do not know when hair evolved, as it is usually not preserved in fossils. Pits in the fossilized rostrum of the some Cretaceous therapsids may be an indirect indication of the presence of vibrissae (McLoughlin, 1980); if so, these provide the first direct evidence of hair. The small body size (i.e., high surface area to mass ratio and therefore propensity to loose heat) and possible endothermy of the earliest mammals and their immediate ancestors suggest that they could not have existed without a good covering of insulation. So it is likely that these animals had hair, and most reconstructions of them show their bodies covered with fur (e.g., Savage and Long, 1986). But was insulation the primitive function of hair, the reason why it originally evolved? Insulation is the most conspicuous and perhaps the most universal function of hair in modern mammals, but hair doesn't serve as effective insulation until it is fairly well developed as pelage. It may be that the first hairs evolved as sensory outgrowths between the scales of some ancestral therapsid, and only later took on the function of insulation.
Phil Myers (author).
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McLoughlin, J. C. 1980. Synapsida. Viking Press, N.Y. xii+148 pp.
Miles, W. B. 1965. Studies of the cuticular structure of the hairs of Kansas bats. Search, Univ. Kansas Publications, 5:48-50.
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Teerink, B. J. 1991. Hair of West-European Mammals. Cambridge Univ. Press, Cambridge. vii+224 pp.
Vaughan, T. A. 1986. Mammalogy. Third Edition. Harcourt Brace Jovanovich, Publishers, Orlando Fl. vii+576 pp.