Hair, a protein filament emerging from follicles in the skin, is a distinguishing characteristic of mammals. Understanding hair’s unique features allows for its identification in various fields, from forensic investigations to archaeological discoveries. Examining specific features of a hair strand can reveal whether it originated from a human or an animal.
Basic Hair Structure
Each hair strand has three primary layers: the cuticle, cortex, and medulla. The outermost cuticle consists of flattened, overlapping cells resembling scales. This protective layer helps shield the inner hair shaft from damage and regulate its water content.
Beneath the cuticle lies the cortex, which forms the bulk of the hair shaft. The cortex is made up of elongated cells containing keratin, a protein that provides hair with its strength and elasticity. This layer also houses melanin, the pigment responsible for hair color.
The innermost layer is the medulla, an unstructured central core that is not always present. When present, it can appear as a continuous, discontinuous, or fragmented channel running through the center of the hair.
Microscopic Markers for Identification
Distinguishing human from animal hair often relies on examining specific microscopic features within the cuticle, medulla, and cortex. The cuticle’s scale pattern offers clues; human hair typically exhibits an imbricate pattern, with flattened, smoothly overlapping scales. Animal hair can display varied patterns, such as coronal (crown-like, in rodents) or spinous (petal-like, in cats).
The medulla provides further differentiation. In human hair, the medulla is usually very thin, fragmented, or may be entirely absent. If present, it often appears amorphous, meaning it lacks a distinct structure. Animal hair commonly features a continuous or interrupted medulla with a more defined shape, including ladder-like, cellular, or patterned structures specific to species. The medullary index, the ratio of the medulla’s diameter to the total hair shaft’s diameter, is a key distinguishing metric. Human hair generally has an index of less than 1/3, while animal hair typically has an index of 1/2 or greater.
The cortex also presents differences in pigmentation. Human hair tends to have pigment granules evenly distributed, or sometimes slightly denser towards the cuticle. Animal hair often shows more centrally distributed pigment, sometimes clumped, or can exhibit radical color changes along the shaft, known as banding. Cortical fusi, air spaces within the cortex, can be observed; their size and distribution vary between human and animal hair.
Additional Distinguishing Characteristics
Beyond internal microscopic structures, other hair strand characteristics aid in identification. The morphology of the hair root, the part embedded in the skin, differs notably. Human hair roots are typically spade-shaped or club-shaped, often possessing a follicular tag, which is a small amount of tissue attached to the root. Animal hair roots exhibit a wide range of shapes depending on the species and frequently lack a follicular tag.
The hair tip, the distal end of the hair shaft, also offers insights. Human hair tips often show evidence of cutting, appearing blunt or frayed due to grooming. Animal hair, if undisturbed, usually tapers to a natural, fine point. The hair shaft’s overall shape and cross-section also vary; human hair is generally round to oval in cross-section. Animal hair can display a broader spectrum of cross-sectional shapes, including flattened, kidney-bean shaped, or irregular forms that change along the shaft’s length.
Macroscopic observations, such as overall length, flexibility, and diameter consistency, can provide initial clues. Human hair can grow to significant lengths with a relatively uniform diameter. Animal hair often varies more in length and diameter along a single strand. These combined characteristics aid in differentiation.