A Microscopical Study of Exotic Animal Hair: Part 1

Editor’s note: This article is Part 1 in a two-part series; read Part 2.

A microscopist might not ever think to focus on the wild animal realm. Brookfield Zoo, in Brookfield, IL, is the home to numerous exotic animals from all over the world. A collection of animal hairs from an array of animals at the Zoo was examined, and photomicrographs were made using polarized-light microscopy (PLM) and scanning electron microscopy (SEM). After a long proposal, and approval from the Zoo, a project was initiated that would benefit not only McCrone Associates, but the Zoo as well. The plan was to set up a display at the Zoo featuring the images of the exotic hairs; McCrone Associates would be acknowledged as the provider of the images.

What is hair? Webster defines hair as a slender threadlike outgrowth of the epidermis of an animal. Hair comes in an array of colors, textures, sizes, and shapes. There are two distinct parts, the hair bulb, or root, and the hair shaft. Just beneath the surface of the epidermis lie tiny cup-shaped pits within the fat of the scalp; these cavities are known as hair follicles. The hair bulb lies within the hair follicle; it is the structure that contains actively growing and dividing cells. As new cells are formed, they steadily push the previously formed cells upwards. Also within the follicle, lies the mid-follicle region. Here the actively growing cells die and harden, and consist of the protein, keratin. Eventually these “scales” are pushed upwards, and out of the skin, creating the shaft.

There are three main regions that make up the shaft of the hair: the cuticle, cortex, and medulla. The cuticle is a thin translucent layer on the exterior of the shaft. Cuticle scales, formed as the hair grows outward and consisting of keratinized cells, are usually overlapping, non-nucleated, and pigment-free. The cortex is the main body of the shaft, and is composed of elongated, filamentous cells that grow parallel to the shaft. The cortex may also contain tiny air spaces called fusi, and even smaller pigment granules. The medulla is a hollow, cellular, central core that, when present, runs through the center of the cortex. Its structure may be fragmentary, continuous, or discontinuous. Medullas from lower animals tend to be very regular and well defined. These medulla types are known as uniserial or multiserial ladder, cellular or vacuolated, and lattice. The main function of the medulla is to provide protection and temperature stability by adding internal air spaces. The medullary index is also a valuable tool for species differentiation; it can be calculated by dividing the average width of the medulla by the average width of the cortex. However, the medulla may differ from one hair to another on the same species.

Why study hair? One reason is for animal identification; it is helpful to differentiate between animals of similar species. The scale pattern on a hair is one of the important identifying characteristics and can almost always be used as a “finger print”. Every species has its own scale pattern unique to them. Another reason why one may study hairs is for animal distribution; certain areas are surveyed to determine if a species is present in that area. A third reason why one may study hair is because it’s fun, it’s inspiring, and it’s educational too.

The animals reviewed here have been chosen because they are all either endangered or threatened in the wild. When this happens, a species survival plan (SSP) may be developed to help prevent the animal from becoming extinct. SSPs are cooperative breeding programs between zoos that aim to increase the captive population of endangered species. Currently there are 87 SSPs covering 116 species throughout 183 zoos within North America. Brookfield Zoo has 44 endangered species within their collection.

African Lion (Panthera leo)

Let us start with Panthera leo, commonly known as the African Lion. The African lion inhabits the open plains of Africa and Asia. Their coloring can range from a yellowish-red to a silver-gray. Their body color is usually uniform, with a buff to whitish underside. In Meltmount™ 1.662, you will notice the continuous medulla in both transmitted light and with crossed polarizers. Figures 1 and 2 are X300 photomicrographs of African Lion hair (both polarizer and analyzer have been rotated 45°).

Notice the moderately-high birefringence using crossed polarizers. You can just make out the scale pattern along the edge of the hair in the transmitted light photo. When several cross-sections were examined, the medullary index calculated out to be 0.44. The cross-sections are a circular shape with a medium size medulla (Figure 3). The image was taken at X150.

Casts were made of the hairs using clear nail polish, in order to provide optimum study preparation; the contrast and definition are both greatly improved (Figure 4).

Now the scale pattern is extremely visible, and you can see the irregular mosaic pattern of the scales. Images were made at X950 using the scanning electron microscope (SEM), and even more detail is visible at this higher magnification. This sample is ~83 µm wide and has ~5 to 10 scales per 100 µm. The margins appear to be relatively smooth and near to one another (Figure 5).

Orangutan (Pongo pygmaeus)

Next, we have Pongo pygmaeus, otherwise known as the Orangutan. From its name one may guess that orangutan comes from the word orange; this is incorrect. “Orangutan” is a Malay word for “forest people” or “man of the forest”. Inhabiting the islands of Sumatra and Borneo in Indonesia and Malaysia, the orangutan spends most of its time in the tree tops. Their coloring is anywhere between orange to reddish-orange to maroon. In Meltmount™ 1.662, notice the narrow, fragmented medulla in both transmitted light and crossed polarizers; both images were taken at X300 (Figure 6 and 7).

The orangutan hair with crossed polarizers shows relatively high birefringence (Figure 7). Examination of the cross-section reveals a circular shape with a medium-size medulla. Also notice the lighter brown area showing the pigmentation. The medullary index calculated out to 0.35. The image was taken at X150 (Figure 8).

From the scale cast of this sample you will notice the prominent, irregular wave pattern; the margins are near, with crenate edges (Figure 9). The SEM shows that the width of this sample is ~127 µm and has 10 to 15 scales per 100 µm (Figure 10).

Rodrigues Fruit Bat (Chiroptera pteropidae)

Next, Chiroptera pteropidae, or the Rodrigues Fruit Bat. The only location you will find this species is on the Island of Rodrigues in the Indian Ocean, off the coast of Madagascar. They possess black to silver, yellow, orange, or red hair color. In Meltmount™ 1.662 you will notice that the medulla is absent and that the birefringence is low. Both images were taken at X300 (Figure 11 and 12).

The cross-section reveals a unique dumb-bell shape with a narrow medulla, when present. The medullary index calculated out to be 0.33. The image was taken at X300 (Figure 13).

Another clear nail polish cast was made, and the scale pattern shows a coronal pattern (Figure 14). The SEM shows that this sample is ~21 µm in width, with 5 to 10 scales per 100 µm; the margins are distant and dentate; the coronal pattern is here even more obvious (Figure 15).

Northern Fur Seal (Callorhinus ursinus)

For a little different species, we have Callorhinus ursinus, the Northern Fur Seal; found in the temperate North Pacific and sub-arctic coastal waters. These creatures are mammals, therefore have hair. Along with their long obvious whiskers, they have tiny hairs covering their entire body to help keep them warm. Northern fur seals, along with sea lions and walruses, are classified as pinnipeds, which means “fin-footed” animal. They are usually brownish-gray to silver in color. In Meltmount™ 1.662, at X300 you can see the unusual lattice medulla, along with a suggestion of the scale pattern; in crossed polarizers there is low birefringence (Figure 16 and 17).

The cross-section shows the presence of a medulla and a few with an absent medulla. The cross-sections have a cigar-shape with a small, narrow medulla when present. The medullary index calculated out to be 0.21. The image was taken at X150 (Figure 18).

The clear nail polish cast of this sample shows a distinct diamond petal scale pattern with margins that are noticeably near and smooth (Figure 19). The SEM is a bit different from the ones already seen, shown in a pattern unique to this species; here the hair is ~152 µm in width, having 3 to 5 scales per 100 µm (Figure 20).

Aardvark (Orycteropus afer)

He’s so ugly he’s cute! Orycteropus afer, inhabiting most of Africa south of the Sahara Desert, the aardvark is an extraordinarily unique animal. Aardvarks are a mish-mash of several different species; they have the long nose of an anteater, the ears of a donkey, and the body of a pig. Not only are they unique, but their hairs are too. At X300 in Meltmount™ 1.662 you will notice the absence of a medulla, and at least four orders of interference colors between fully-crossed polarizers (Figure 21 and 22).

The cross-sections are considerably larger than the ones viewed previously. They are oval in shape with an absent medulla, therefore the medullary index is 0. The image was taken at X150 (Figure23). The darker areas you see near the center are pigment granules.

The clear nail polish cast was difficult to make due to the thickness of the hair. The SEM of this sample shows an irregular wave pattern with near, crenate margins; the width of the hair is ~272 µm and has 10 to 15 scales per 100 µm (Figure 19 and 20).

Even from the few exotic animal hairs described here, the great diversity of morphological features has been obvious; the continuous medulla represented in the lion hair; the fragmentary medulla within the orangutan hair; the absence of a medulla in the large aardvark hair; and the unique lattice-type medulla in the northern fur seal hair.


The cooperation of the Brookfield Zoo, Brookfield, IL, in the collection of the exotic animal hair samples used in this study is greatly acknowledged and appreciated. Thanks are also due to Brian Bierman, and Paul Toepfer, staff members of McCrone Associates, for assistance in capturing the SEM micrographs.


Bisbing, R. E. (1982). The Forensic Identification and Association of Human Hair. Forensic Science Handbook (Saferstein, R.; ed.). Prentice Hall, Englewood Cliffs.

Brunner, Hans, and Brian Coman (1974). The Identification of Mammalian Hair. Inkata Press, Melbourne.

Hicks, John W. (1977). Microscopy of Hair – A Practical Guide and Manual, FBI Laboratory Technical Supplement. Washington, D.C.

Moore, Tommy D., Liter E. Spencer, and Charles E. Dugnoll, (1974). Identification Of the Dorsal Guard Hairs of Some Mammals of Wyoming. Wyoming Game and Fish Department. Cheyenne, Wyoming.

Brookfield Zoo Web Site: http://www.brookfieldzoo.org.

P & G Beauty Science: http://www.pantene.com/haircare/hair_twh_12.htm.

Crime Library: http://www.crimelibrary.com

Hair Analysis: http://www.bergen.org


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