When examining a sheep’s fleece, the question of whether it is fur or wool touches on a long-standing biological and conventional distinction. The material is technically a highly specialized form of hair, placing it in the same broad category as the hair that makes up fur. This specialized fiber, which we call wool, developed its unique characteristics through natural evolution and human intervention. Understanding the difference requires looking beyond common names to the microscopic structure and developmental history of the sheep’s coat.
Wool Is Technically Specialized Hair
From a biological standpoint, all mammalian body coverings are composed of keratin protein fibers that grow from hair follicles in the skin. Wool is therefore an example of hair, just as fur is defined as a dense coat of shorter, finer hair found on many non-human animals. The conventional separation of “hair,” “fur,” and “wool” is based on properties like fiber density, texture, and growth pattern, rather than a fundamental difference in composition.
Wool distinguishes itself by growing continuously, unlike the typical fur of animals like the fox or rabbit, which grows to a maximum length before stopping. This continuous growth pattern is a consequence of breeding and is a major functional difference that makes sheep valuable for textiles. Furthermore, the sheer density of the fibers in a sheep’s coat, particularly in breeds like the Merino, far exceeds the density of typical hair. This dense, continuous growth is the primary technical reason why a sheep’s covering is labeled as wool, differentiating it from the general term of fur.
Physical Differences Between Wool and Fur
The true distinction between wool and other mammalian hair or fur lies in the microscopic architecture of the fiber. Each strand of wool is encased in a layer of overlapping cuticle cells, often described as resembling roof tiles. These scales are responsible for the unique ability of wool to interlock and cling together when agitated. This interlocking process is integral to both the natural shedding of dirt and the industrial process of felting.
Another defining characteristic is the natural waviness, known as crimp, which is almost entirely absent in most fur or hair. This crimp is structurally determined by the cortex, the internal backbone of the fiber. The cortex is composed of two distinct cell types, the ortho-cortex and the para-cortex, arranged bilaterally. Because these two halves absorb moisture at different rates, the fiber is forced to bend and curl, creating the permanent crimp that gives wool its bulk, elasticity, and insulating properties. Fine wool, such as Merino, can exhibit up to 40 crimps per centimeter, which significantly contributes to its ability to trap air and provide warmth.
The third unique feature is the presence of lanolin, a natural grease secreted by the sheep’s sebaceous glands during fiber growth. This waxy substance coats the wool, providing water resistance and protection against the elements while the fiber is still on the animal. Lanolin is a byproduct of the wool industry and is later extracted during the washing process. This unique chemical composition sets wool apart from the coats of most other mammals.
The Impact of Domestication on Sheep Coats
The specialized nature of modern wool is a direct result of thousands of years of human-directed selective breeding. Wild sheep ancestors, such as the Mouflon, naturally possessed a double coat that was much closer to typical fur. This primitive coat consisted of a layer of coarse, long guard hairs covering a shorter, fine, insulating undercoat.
Both layers of the wild sheep’s coat would shed seasonally, much like the fur of other wild mammals. Early agriculturalists began intentionally selecting sheep that produced more fine undercoat fibers and fewer coarse guard hairs. This sustained selection pressure gradually transformed the sheep’s covering, leading to the near-total elimination of shedding guard hairs in most modern wool breeds. The final result is the dense, continuously growing fleece of homogeneous fine fibers recognized today as wool, a product of human needs rather than natural selection.