One of the distinguishing characteristics of mammals is the protective growth known as hair. Two major kinds of hair provide either protection (guard hairs) or insulation (underfur).
This image of river otter fur was taken from a folded portion of a tanned hide. The spikes are guard hairs projecting above the soft but densely packed mat of underfur. It is the guard hairs that are most used in identification.
Guard hairs of mammals vary not only in color but also in microscopic structure. There are three major components to a guard hair: the outer layer, called the cuticle; the middle layer, called the cortex; and the inner layer, called the medulla. The cuticle has a variable scale pattern, the cortex contains the pigment of a colored hair, and the medulla ranges from absent to very complex.
Guard hairs of the opossum typically are white. The medulla is unbroken and fills most of the shaft. Intrusions from the cortex extend into the air-filled medulla (bottom photo). When air has not been removed from the medulla, air bubbles remain, resulting in a grayish appearance (top photo).
In most smaller rodents, the medulla fills most of the shaft, but pigmented cortical intrusions project into the medulla. Note that the cortex on the outside has no pigment; the pigment is placed in the cortical intrusions only. This condition has been called the "rodent base."
The top photo shows a variety of colors. Detail (bottom photo) reveals patterns of cuticular scales.
Note extreme development of cuticular scales on these three hairs.
Moles live underground and are subjected to dirt and water. Each hair is made with a broad spatulate shield region (tip). If wet, these shields stick together and help prevent dirt or water from penetrating the fur to the skin. In a burrow, a mole may have to back up if it comes in contact with a hard object. The resulting change in direction of the hair could allow dirt to fall among the hairs. As an adaptation against this, there are several thin areas (strictures), which allow the upper part of the hair to move without disturbing the basal region. Shields and strictures both can be seen in this photo.
The structure of mole hair makes it so soft that it is the namesake of a bandage called "moleskin." Historically, women with a need for a powder puff but without access to cotton supposedly used the skin of a mole as a substitute.
The medulla in river otters fills about one-third of the shaft width. Note the pigment in the cortex (brown flecks).
In the beaver, the cortex fills most of shaft. In fact, the medulla may be absent, very thin (and sometimes broken), but at most only about a third of the shaft width. All three conditions can be seen in hairs in the photo.
In felids, the medulla is wide and typically vacuolated. The vacuoles are various sizes and the shape of a spindle, or football.
The medulla in rabbits practically fills the shaft. The medulla has the form of a multiserial ladder, meaning that it looks like several wavy ladders side-by-side. The black dots in these two photos are air bubbles in compartments of each ladder that were not removed during preparation of this slide.
The hairs of deer are easily identified because the medulla about fills the shaft and is in the form of a unbroken lattice (honeycomb). A poacher who is accused of having killed a deer illegally may argue that the wisp of hairs taken as evidence from the back of his truck are actually those of a rabbit. Compare the deer hair photo (two hairs shown) with the previous rabbit hair photos to see why his argument can easily be proven false.
Hairs of human often are studied when they appear at scenes of crimes. There are several traits that can be compared to determine whether two hair samples match to evaluate whether they might have come from the same individual.
These hairs from three individuals show some variation in width and color.
Human hairs either have no medulla (upper photo) or a thin medulla which is amorphous--without a clear form (lower photo). Because a hair shaft is round and thick, you must focus on the center of the shaft to be able to see the medulla, if present.
When the microscope is focused on the surface of the hair shaft, the pattern of cuticular scales can be seen. The cuticle is the outer surface of the hair, and the appearance of the scales varies among individuals. The presence of cuticular scales allows some of the hair styles that change with fashion. You can feel these scales on your own hair. If you rub from the scalp outward, hair feels smooth. However, if you hold your hair out and rub your fingers toward the scalp, you will notice a rougher texture. Any hair style that requires "teasing" is making use of the cuticular scales.