Sections Aid in Identifying Hair

1. What they did

Hairs found in the stomachs and droppings of carnivores can show what they have eaten, but only if the hairs can be identified. Ned Dearborn set out the structural features of guard hairs from mammals of one region, as an aid to identifying hairs of unknown origin.

He explains that many mammals in temperate and cold regions have two coats. The underfur is made of fine, short, closely packed hairs that shield against cold and damp. Scattered among them are longer, coarser guard hairs, which widen above the underfur to form the outer coat and carry the animal’s colour. The wide part of the guard hair, called the blade, is where the useful features are.

He made camera lucida drawings (a drawing aid that projects the microscope image onto paper) of two views of guard hair blades from 19 small mammals of the northeastern United States: shrews, moles, mice, voles, squirrels, a chipmunk, a muskrat, a rat, three bats, a hare and a cottontail:

• Cross sections: the shape and size of the hair when cut across.
• Optical longitudinal sections: a lengthwise view through the hair under the microscope, showing the pattern of the medulla, the central core of the hair.

2. What they found

• The broad part of a guard hair above the underfur can be shaped so distinctively that it identifies a family or even a genus. Size and the pattern of the medulla can also be distinctive.
• Underfur hairs, and the part of guard hairs hidden in the underfur, are more or less round in cross section and have no distinctive shape, so they are not useful for identification.
• Now and then a hair among the normal guard hairs is larger, longer and rounder. The author calls these bristles and says they are of no use in identification.
• Guard hairs of young animals grow larger as the animal matures, but keep the same shape.
• Very often the size and shape of the cross section alone is enough to identify a hair.
• When species have hairs of similar size and shape, the medulla pattern in the lengthwise view is needed. For example, the medulla patterns of the meadow jumping mouse (Zapus) and the muskrat (Ondatra) are far more distinctive than their cross sections.
• Comparing hairs from mammals around the world shows curious peculiarities in some species and interesting parallels in others.

3. What they concluded

The author recommends a practical method. In advance, make camera lucida drawings of cross and lengthwise sections of hair from the less familiar mammals of the area. Arrange the drawings in a vertical column and slide them beneath the mirror of the camera lucida, so the image of an unknown hair under the microscope can be laid over each drawing in turn. Because size matters as well as shape, the unknown hair must be viewed at the same magnification as the drawings; otherwise the identification is questionable.

4. How tracking was or was not used

No fieldwork or tracking is described. The paper is about one kind of sign: identifying hair, especially hair found in carnivore stomachs and droppings, by its microscopic structure.

6. Other technologies, skills or innovations

• Microscope views of hair in cross section and in optical lengthwise section.
• Camera lucida drawings as a reference collection, compared directly against an unknown hair by laying its image over each drawing.