Fur and hair

The term "fur" is used in several ways, all related to its definition as the hairy covering of the skin of a mammal. In its most common usage, fur refers to a coat of dense, often soft hair that covers the majority of the body. However, even a human's relatively sparse body hair may be viewed as fur, since this, too, qualifies as the hairy covering of a mammal's skin.

Each individual hair in a mammal's fur is a threadlike epidermis outgrowth. Collectively, these hairs form the body coverings, or pelages, of all mammals. Each hair is made mostly of a fibrous, sulfur-containing protein called keratin. A pelage is most correctly called fur when its individual hairs are fine and spaced closely together. If the hair is soft, kinky, and matted together, the pelage is called wool, which grows in a fleece. Coarse, stiff hairs, such as those found on pigs, are called bristles; when pointed, as in the case of porcupines and hedgehogs, they are spines or quills.

Hair grows on most parts of the bodies of mammals. As their body covering, its primary roles are protection, insulation, and ornamentation, similar to bird feathers and reptile scales.

The Structure of a Hair

Any hair consists of two main parts, the root and the shaft. The shaft is the part of the hair that is outside of the skin. Hair shaft cross-sections range in shape from round to flattened; rounder cross-sections correspond to straighter hair, while flatter cross-sections result in curly hair.

A hair shaft consists of dead epithelial cells arranged in three separate layers. The central layer, called the medulla, is a disorganized, open core that may not always be present. It is surrounded by the cortex, which in turn is covered by overlapping flat scales that make up the cuticle. The free ends of the cuticle scales face upward, away from the hair’s root.

The cortex makes up the majority of each hair. It consists of longitudinally arranged, keratin-rich, spindle-shaped dead cells that are tightly attached to each other. Hair color results from pigments in cortex cells and light reflected from the medulla. The medulla is less dense than the cortex, and its cells are only loosely attached to one another.

At every hair’s base is a saclike hair follicle. The hair grows from the bottom of the follicle, nourished by blood vessels in a structure called a papilla. The papilla extends into the follicle and into the hair’s root. A tiny muscle is attached to each hair follicle. Action of the nervous system can cause the muscle to contract to make hair “stand on end.”

Hair Growth and Replacement

A hair forms from cells that grow from the surface of the papilla, which means that it grows from the root, not the free end. As new cells develop, they push forward old ones, which become part of the shaft. Hair growth continues as long as the follicle and papilla are functional. The lifetime of a hair from start of growth until it is shed depends on the organism that produces it. When an old hair falls out, a new one takes its place.

Hair follicles produce hairs in cycles of hair growth, in which the hair follicle and the shaft pass through a complex series of morphological changes. During hair growth, the follicle penetrates into the dermis, and cells of the shaft are joined together. In addition, the follicle’s melanocyte cells deposit pigment into shaft cells. Once a hair shaft attains its characteristic length, the follicle contracts and a “dead” hair protrudes from it. The growth period of a single hair ranges from three years in humans to around two weeks in rodents.

Hairs are continually replaced, or shed, throughout the life of a mammal. However, their development and loss occur asynchronously, so mammals are never completely naked of pelage. In rodents, replacement occurs in waves across the body. In primates, each follicle passes through the growth cycle, independent of those around it. Hormones control hair growth; however, there are other, as yet unknown components that must also affect the process, because hormones are carried in the blood, and if they acted alone they would simultaneously affect all follicles in the body. This would be disastrous, because if all follicles grew in together, at the same rate, all of a mammal's hairs would be shed at the same time, and it would have naked periods where it was deprived of hair’s protection and insulation.

Continuous growth of hairs can sometimes be hazardous. For example, in merino sheep, long growth phases produce long-stranded wool. If these sheep strayed, were not minded well, or were sheared irregularly, their long wool might cause them to become entangled in underbrush, which could lead them to starve to death.

Hair Origin and Function

Hair in the pelage of contemporary mammals acts mainly to insulate against temperature variation. It has been proposed that mammal hair evolved into pelage from “prehair” that had the same functions. One theory of hair origin is that it evolved from epidermal mechanoreceptors, a concept supported to some extent by the existence of sinus hairs in mammals. These hairs, whiskers (vibrissae) in mammals such as felines and rodents, have blood-filled sinuses in the skin around the follicle. This tissue, together with associated nerve fibers, engenders mechanoreception, which facilitates nocturnal movement. However, most mammal body hairs lack nerves and only insulate and protect. The basis for hair evolution from vibrissae is therefore unclear.

Certainly mammalian hair has other functions today. It can serve the unusual protective function of the quills of porcupines, and perhaps it may more generally serve to attract mates. It seems possible that the change of prehairs into attractive pelage drove development into these main contemporary forms.

Protecting Endangered Furbearers

The desire on the part of humans for fur garments has led to atrocities committed on many mammal species that have gorgeous pelages. One of many examples is clubbing young fur seals to death. Beyond that, many species have been hounded to near extinction by hunters. Classic examples of such endangered species are the big cats, such as tigers and leopards, and rodents, such as the beaver. Many other mammals are threatened species, likely to become endangered in the foreseeable future.

Fortunately, several organizations have sought to protect these fur-bearing mammals. Efforts of organizations such as the World Wildlife Fund have focused public opinion and led to animal conservation legislation. Preeminent among these are the US Endangered Species Act of 1973 and the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), first signed in 1973 and taking effect in 1975, which as of 2016 had 183 member countries. These actions have led to agreement that furs will not move interstate or between signatory countries without proof that the species from which they were harvested is not threatened or endangered.

Principal Terms

cortex: the main part of a hair, made of pigment-containing cells, surrounding a central medulla

cuticle: the outermost layer of a hair, made of scales

epidermis: the dead, outermost portion of the skin

follicle: the saclike organ from which a hair grows, containing blood vessels that nourish the hair

keratin: a tough fibrous protein, seen in large quantities in epidermal structures such as hair

medulla: the innermost layer of a hair

shaft: the main hair part, made of dead cells arranged in a complex fashion

Bibliography

Deems, Eugene F., Jr., and Duane Pursley, editors. North American Furbearers: A Contemporary Reference. International Association of Fish and Wildlife Agencies / Maryland Department of Natural Resources–Wildlife Administration, 1983.

Harding, Harry, and George Rogers. "Physiology and Growth of Human Hair." Forensic Examination of Hair, edited by James Robertson, Taylor & Francis, 1999, pp. 1–77.

Robbins, Clarence R. Chemical and Physical Behavior of Human Hair. 5th ed., Springer, 2012.

Robertson, James. "Forensic and Microscopic Examination of Human Hair." Forensic Examination of Hair, edited by Robertson, Taylor & Francis, 1999, pp. 79–154.

Spearman, Richard Ian Campbell, and Patrick Anthony Riley, editors. The Skin of Vertebrates. Academic Press, 1980.

Stanford Environmental Law Society. The Endangered Species Act. Stanford UP, 2001.