From a scientific outlook, hair and fur are comprised of the exact same material, known as keratin. This versatile fibrous structural protein is actually responsible for the generation of hair, fur, hooves, feathers, horns, nails, wool, claws, teeth, eyelashes, whiskers, quills, and skin. The various texture formations are primarily determined by the amount of cysteine, a sulfur-containing amino acid, found within the structure. This sulfur is what causes burning hair or flesh to have such a pungent odor.

In biological terms, there is no difference between hair and fur. Each consists of no more than three layers. The inner layer to thick hairs is the medulla. The mid-layer is the cortex, which provides the strength, color, and texture of hair. The cuticle is the outermost layer, and is designed to protect the cortex. Additionally, hair and fur act the same way below the skin. Underneath the surface is a hair root located inside a hair follicle. At the base of the hair follicle is the dermal papilla, which absorbs nutrients from the bloodstream to produce new growth.

When discussing fur, it most frequently refers to its fixed growth length. Head hair on a human typically has a broader range of growth from most other animals. This distinction resonates as a unique trait and as a result, it is called hair. However, reflect a moment on arm and leg hair. This hair grows to a genetically fixed length, which is the characteristic distinguishable to fur. Some insist that coarseness determines whether it should classify as fur or hair. However, there is no rationale behind such a distinction, as there are animals with soft, silky tufts that are considered to have fur. Still, there is another perspective that speaks to density. The philosophy is that if the fibers are thicker but less dense, like on a human or a horse, the term hair is typically applied. Thinner fibers are customarily considered as fur. Then again, there are people with very thin fibers atop their heads, and it is still considered hair. When a coat is extremely dense but very fine, like those found on sheep or alpaca, it is generally referred to as wool. However, there are people sporting thin, dense locks – of hair. Therefore, aside from discussing genetically fixed lengths to animal strands, other distinctions are splitting hairs.