During the course of a criminal investigation, the identification
and comparison of human and animal hairs can be helpful in demonstrating
physical contact with a suspect, victim, and crime scene. Hairs
can provide investigators with valuable information for potential
leads. In Microscopy of Hair Part I: A Practical Guide and Manual
for Human Hairs, the characteristics of human hairs were described
in detail as well as their significance to an investigation (Deedrick
and Koch 2004). In Microscopy of Hair Part II: A Practical Guide
and Manual for Animal Hairs, the focus is on animal hairs.
This manual is intended to focus on animal hair evidence in the
forensic setting and to provide a foundation for its proper identification
and comparison. Although Part I and Part II are intended to be a
complete manual for the examination of hair, each part can stand
alone. Some repetition between the two documents will, therefore,
occur because the basics are the same for microscopy of hair, whether
human or animal.
Basic Structure of Hair
Hair can be defined as a slender, thread-like outgrowth from a follicle in the skin of mammals. Composed mainly of keratin, it has three morphological regionsthe cuticle, medulla, and cortex. These regions are illustrated in Figure 82 with some of the basic structures found in them. The illustration is a diagram used to emphasize structural features discussed in this guide. Certain structures may be omitted, and others enhanced for illustrative purposes.
Figure 82. Hair Diagram
A hair grows from the papilla and, with the exception of that point of generation, is made up of dead, cornified cells. It consists of a shaft that projects above the skin and a root that is imbedded in the skin. Figure 83 diagrams how the lower end of the root expands to form the root bulb. Its basic components are keratin (a protein), melanin (a pigment), and trace quantities of metallic elements. These elements are deposited in the hair during its growth and/or absorbed by the hair from an external environment. After a period of growth, the hair remains in the follicle in a resting stage to eventually be sloughed from the body.
Figure 83. Diagram of Hair in Skin
The cuticle is a translucent outer layer of the hair shaft consisting of scales that cover the shaft. Figure 84 illustrates how the cuticular scales always point from the proximal or root end of the hair to the distal or tip end of the hair.
Figure 84. Transmission Electron Photomicrograph of Cuticular
Scales Provided to the FBI Laboratory by Charles Lynch
There are three basic scale structures that make up the cuticlecoronal (crown-like), spinous (petal-like), and imbricate (flattened). Combinations and variations of these types are possible. Figures 85-90 illustrate scale structures.
- The coronal, or crown-like scale pattern, is found in hairs of very fine
diameter and resemble a stack of paper cups. Coronal scales are
commonly found in the hairs of small rodents and bats but rarely
in human hairs. Figure 85 is a diagram depicting a longitudinal
view of coronal scales, and Figure 86 is a photomicrograph of
a free-tailed bat hair.
Figure 85. Diagram of Coronal Scales
Figure 86. Photomicrograph of Free-Tailed Bat Hair
- Spinous or petal-like scales are triangular in shape and protrude from the hair shaft. They are found at the proximal region of mink hairs and on the fur hairs of seals, cats, and some other animals. They are never found in human hairs. Figure 87 is a diagram of spinous scales, and Figure 88 is a photomicrograph of the proximal-scale pattern in mink hairs.
Figure 87. Diagram of Spinous Scales
Figure 88. Photomicrograph of Proximal-Scale Pattern (Mink)
- The imbricate or flattened-scale type consists of overlapping scales with narrow margins. They are commonly found in human hairs and many animal hairs. Figure 89 is a diagram of imbricate scales, and Figure 90 is a photomicrograph of the distal-scale pattern in mink hairs.
Figure 89. Diagram of Imbricate Scales
Figure 90. Photomicrograph of Distal-Scale Pattern (Mink)
The medulla is a central core of cells that may be present in the hair. If it is filled with air, it appears as a black or opaque structure under transmitted light, or as a white structure under reflected light. If it is filled with mounting medium or some other clear substance, the structure appears clear or translucent in transmitted light, or nearly invisible in reflected light. In human hairs, the medulla is generally amorphous in appearance, whereas in animal hairs, its structure is frequently very regular and well defined. Figures 91 through 94 are photomicrographs of medullary types found in animal hairs. Figure 91 exhibits a uniserial ladder, and Figure 92 exhibits a multiserial ladder, both found in rabbit hairs. Figure 93 exhibits the cellular or vacuolated type common in many animal hairs. Figure 94 exhibits a lattice found in deer family hairs.
Figure 91. Photomicrograph of Uniserial-Ladder Medulla
Figure 92. Photomicrograph of Multiserial-Ladder Medulla
Figure 93. Photomicrograph of Cellular or Vacuolated-Type
Figure 94. Photomicrograph of Deer Medulla
When the medulla is present in human hairs, its structure can be described as fragmentary or trace, discontinuous or broken, or continuous. Figure 95 is a diagram depicting the three basic medullary types.
Figure 95. Diagram of the Three Basic Medullary Types
The cortex is the main body of the hair composed of elongated and fusiform (spindle-shaped) cells. It may contain cortical fusi, pigment granules, and/or large oval-to-round-shaped structures called ovoid bodies.
Cortical fusi in Figure 96 are irregular-shaped airspaces of varying sizes. They are commonly found near the root of a mature human hair, although they may be present throughout the length of the hair.
Figure 96. Photomicrograph of Cortical Fusi in Human Hair
Pigment granules are small, dark, and solid structures that are granular in appearance and considerably smaller than cortical fusi. They vary in color, size, and distribution in a single hair. In humans, pigment granules are commonly distributed toward the cuticle as shown in Figure 97, except in red-haired individuals as in Figure 98. Animal hairs have the pigment granules commonly distributed toward the medulla, as shown in Figure 99.
Figure 97. Photomicrograph of Pigment Distribution in
Figure 98. Photomicrograph of Pigment Distribution in
Red Human Hair
Figure 99. Photomicrograph of Pigment Distribution in
Ovoid bodies are large (larger than pigment granules), solid structures that are spherical to oval in shape, with very regular margins. They are abundant in some cattle and dog hairs as well as in other animal hairs. To varying degrees, they are also found in human hairs.
Figure 100. Photomicrograph of Ovoid Bodies in Cattle
Figure 101. Photomicrograph of Ovoid Bodies in Dog Hair
Figure 102. Photomicrograph of Ovoid Bodies in Human Hair
Animal Versus Human Hairs
Human hairs are distinguishable from hairs of other mammals. Animal hairs are classified into three basic types.
- Guard hairs that form the outer coat of an animal and provide protection
- Fur or wool hairs that form the inner coat of an animal and provide insulation
- Tactile hairs (whiskers) that are found on the head of animals and provide sensory functions
Other types of hairs found on animals include tail hair and mane hair (horse). Human hair is not so differentiated and might be described as a modified combination of the characteristics of guard hairs and fur hairs.
Human hairs are generally consistent in color and pigmentation throughout the length of the hair shaft, whereas animal hairs may exhibit radical color changes in a short distance called banding. The distribution and density of pigment in animal hairs can also be identifiable features. The pigmentation of human hairs is evenly distributed, or slightly more dense toward the cuticle, whereas the pigmentation of animal hairs is more centrally distributed, although more dense toward the medulla.
The medulla, when present in human hairs, is amorphous in appearance, and the width is generally less than one-third the overall diameter of the hair shaft. The medulla in animal hairs is normally continuous and structured and generally occupies an area of greater than one-third the overall diameter of the hair shaft.
The root of human hairs is commonly club-shaped, whereas the roots of animal hairs are highly variable between animals.
Figure 103. Photomicrograph of Human Hair Root
The scale pattern of the cuticle in human hairs is routinely imbricate. Animal hairs exhibit more variable scale patterns. The shape of the hair shaft is also more variable in animal hairs.
It may be necessary to make a scale cast of the hair specimen in order to see the scale pattern more clearly, particularly in the identification of some animal hairs. Ogle and Mitosinka (1973) devised a quick and easy method of making a scale cast using a Polaroid film-print coater. A thin layer is applied to a glass microscope slide with two or three passes of the Polaroid print coater. The hair specimen is lightly pressed onto the film and allowed to stand until the film is dry. The hair is then pulled from the film, and the cast remains.
A method developed by Crocker (1998) at the Centre of Forensic Sciences in Toronto, Canada, uses clear tape and coverslip together as a mounting medium, which allows for quick observation of such surface features as the scale pattern.
Scale casts may also be prepared using clear nail polish. A thin coat is painted on a glass microscope slide or, if the lacquer is thinned with acetone, a drop may be allowed to run down the surface of the slide. The hair to be cast is placed on the slide and allowed to dry. When the surface has dried, the hair is removed to reveal the scale pattern.
Animal Hairs Identification
The animal hairs discussed in this manual will be limited to those animals most likely to be encountered in casework. An adequate reference collection is essential for accurate identification of questioned specimens. In most cases, specific identification can only be accomplished with guard hair specimens. In some instances, however, as with some commercial garment furs, specimens can be identified on the basis of the microscopic appearance of the fur or down hairs alone.
The animal hairs presented here can be classified into three major groups on the basis of their microscopic appearance.
- Deer family and antelope
- Commercial fur animals
- Domestic animals
For individual identification in these groups, deer family and antelope hairs are distinguished on the basis of their scale patterns, whereas commercial fur animals are distinguished on the basis of their color, color bands, scale patterns, and medullary structure. Domestic animals are distinguished primarily through their root structure, medullary structure, and pigmentation.
Deer Family and Antelope
The deer family and antelope hairs are not readily distinguishable on the basis of their gross appearance or microscopic appearance in whole mount. However, when scale cast impressions are carefully studied, certain patterns become apparent that can be used to separate the different members of this group. It is emphasized that these patterns are impressions perceived through general observation of the entire hair specimen.
- Very coarse overall diameter (approximately 300m)
- Medulla composed of spherical cells that occupy entire hair (Figure 104)
- Diameter constant throughout most of hair
- Wineglass-shaped root (Figure 105)
- Regular wave or crimp
Figure 104. Photomicrograph of Lattice Medulla
Figure 105 Photomicrograph of Wineglass-Shaped Root
Deer (White-Tailed Deer and Mule Deer)
Scale margins are round and isodiametric and resemble fish scales.
Figure 106. Diagram of Scale Pattern of Deer Hair
Figure 107. Photomicrograph of Scale Pattern of Deer Hair
Scales are hexagonal and usually longer than wide.
Figure 108. Diagram of Scale Pattern of Caribou Hair
Figure 109. Photomicrograph of Scale Pattern of Caribou
Scales are elongated and five- or six-sided. Scale margins are narrow and ends are pointed.
Figure 110. Diagram of Scale Pattern of Elk Hair
Figure 111. Photomicrograph of Scale Pattern of Elk Hair
Scales are relatively large with irregular polygons. Overall hair diameter is considerably larger than other members of this group.
Figure 112. Diagram of Scale Pattern of Moose Hair
Figure 113. Photomicrograph of Scale Pattern of Moose
Scales are diamond-shaped and frequently give impression of being arranged in diagonal rows.
Figure 114. Diagram of Scale Pattern of Antelope Hair
Figure 115. Photomicrograph of Scale Pattern of Antelope
Deer family hairs exhibit similarities to goat hairs in general
form (medullary structure). However, goat hairs are generally finer
in overall diameter (approximately 220μ) and exhibit a narrow, pigmented
cortex. No cortex is apparent in deer family hairs (cortex is only
occasionally visible in elk hairs). The scale pattern of goat hairs
shows flattened scale margins and no regular pattern.
Commercial Fur Animals
The commercial fur group includes several specimens commonly encountered in fur garments that are distinctive in their microscopic appearance. These are rabbit, seal, mink, muskrat, and chinchilla. Of these, seal and chinchilla are identified on the basis of the appearance of their down or fur hairs, not by their guard hairs. This is because guard hairs are frequently plucked from seal pelts for a more pleasing appearance. Chinchilla pelts have few, if any, guard hairs. The remaining specimens in the commercial fur group are identifiable largely by their characteristic colors and color banding.
- Very fine to medium overall diameter (20 to 150μ)
- Characteristic medullary formations (serial or vacuolated)
- Wide diameter variations in single hair
- Hairs generally banded
Rabbit: Extensively Used in Felted Fabrics, Glove Linings,
Trim, Coats. Sheds Readily.
- Medulla uniserial in fur hairs to multiserial ladder in guard hairs
- Ribbon-like appearance
- Cross section flattened
- May be various colors and lengths
Figure 116. Photomicrograph of Rabbit Fur Hair Medulla
Figure 117. Photomicrograph of Rabbit Guard Hair Medulla
- Medulla absent
- Diameter very fine and uniform
- Scales spinous (elongated-petal shape) and easily visible even in whole mount
- Pelts usually dyed and sheared (guard hairs frequently plucked)
Figure 118. Photomicrograph of Seal Hair
Figure 119. Photomicrograph of Seal Hair
Mink: Coats, Stoles, Hats, Trim
- Characteristic shape: basal one-third (root portion) of hair very fine, rapidly widening to four or five times basal diameter and then reducing to a pointed tip (spear-shaped)
- Scales: spinous (elongated-petal shape) through fine basal portion of hair, changing to imbricate through blade or shield portion
- Color: characteristically dark brown (other color phases possible)
Figure 120. Photomicrograph of Shield Area of Mink Hair
Figure 121. Photomicrograph of Scale Cast of Mink Hair
Figure 122. Photomicrograph of Scale Cast of Mink Hair
Muskrat: Coats, Trim
- Similar to mink in general shape and appearance
- Scales imbricate throughout full length
- Maximum length: 43mm; maximum shield diameter: 110μ
Figure 123. Photomicrograph of Shield Area of Muskrat
Figure 124. Photomicrograph of Scale Cast of Muskrat Hair
Chinchilla: Coats, Hats, Trim
- Diameter: very fine and uniform
- Medulla: uniserial ladder (fur hairs) (Figure 126)
- Pigment usually clumped in segments between medullary cells
- Very dense, even coat; several hairs may emanate from a single follicle; few, if any, guard hairs
Figure 125. Photomicrograph of Chinchilla Hair
Figure 126. Photomicrograph of Chinchilla Hair Bundles
Raccoon (Procyon): Coats, Trim
- Medulla: unbroken; amorphous through basal portion changing to vacuolated; broad
- Banded: white base to dark brown midshaft to yellow (or white) to black tip
- Maximum length: 74mm; maximum diameter: 154μ
- Scales: diamond-petal shape (basal) to imbricate
Figure 127. Photomicrograph of Raccoon Hair
Figure 128. Photomicrograph of Scale Cast of Raccoon Hair
Figure 129. Photomicrograph of Scale Cast of Raccoon Hair
Red Fox: Stoles, Trim
- Medulla: unbroken; amorphous or vacuolated; broad
- Banded: brownish-gray base to yellow (or white) to reddish-brown tip
- Maximum length: 156mm; maximum diameter: 110μ
- Scales: vary throughout shaft; mosaic pattern (basal) to diamond-petal shape to elongated-petal to imbricate
Figure 130. Photomicrograph of Red Fox Hair
Figure 131. Photomicrograph of Scale Cast of Red Fox Hair
Figure 132. Photomicrograph of Scale Cast of Red Fox Hair
Beaver: Coats, Hats
- Medulla (fur hairs): continuous, cellular (beaded appearance); in guard hairs medulla is unbroken, cellular, and usually relatively narrow
- Diameter: fine, uniform
- Pelts always sheared
Figure 133. Photomicrograph of Beaver Guard Hair
Figure 134. Photomicrograph of Beaver Fur Hair
Bear (Black and Grizzly): Coats, Rugs
- Medulla: continuous, amorphous, usually less than half overall shaft diameter
- Unbanded: pigment coarse, granular, and fairly even distribution
- Scales: imbricate
- Maximum length: 108mm (Black), 70mm (Grizzly); maximum diameter: 153μ
Figure 135. Photomicrograph of Bear Hair
Figure 136. Photomicrograph of Scale Cast of Bear Hair
There are wide variations in color and length of the hair specimens in the domestic animal group. The identifying characteristics given are general and apply in most cases. In order to distinguish between dog and cat and between beef (cattle) and horse, it is usually necessary that the root be present.
- Medium overall diameter (75 to 150μ)
- Medulla generally amorphous
- Moderate diameter variation in single hair
- Hairs generally unbanded
- Characteristic root shapes
- Diameter: fine; little variation
- Medulla: uniserial ladder (fur hairs) continuous; occasionally vacuolated in coarser hairs
- Scales: spinous; very prominent
- May be banded
- Root: elongated, no distinct shape; fibrils frayed at base of root
Figure 137. Photomicrograph of Cat Root
Figure 138. Photomicrograph of Cat Hair
- Diameter: fine to coarse (usually coarser than cat hairs); diameter may vary to give short hairs a barrel-like appearance
- Medulla: continuous, vacuolated to amorphous, occasionally very broad
- Scales: generally not prominent
- Unbanded: pigment occasionally very coarse and extending into root
- Root: spade-shaped
Figure 139. Photomicrograph of Dog Root
Figure 140. Photomicrograph of Dog Hair
- Diameter: coarse
- Medulla: absent or continuous, amorphous or vacuolated; width may be narrow to very broad (without mosaic pattern)
- Scales: imbricate and with no protrusions from hair shaft
- Unbanded: ovoid structures abundant, pigment coarse
- Root: elongated; medullary structure continues into root area; traces of follicular tissue may be present
Figure 141. Photomicrograph of Cattle Root
Figure 142. Photomicrograph of Cattle Hair
- Diameter: very coarse
- Medulla: absent to unbroken; cellular or amorphous (mosaic pattern)
- Scales: imbricate; without protrusions from hair shaft
- Characteristic color; pigment fine, evenly distributed; few ovoid structures
- Root: area adjacent to root tapers to bulb-shaped root
Figure 143. Photomicrograph of Horse Hair
- Diameter: coarse, uniform
- Medulla: absent to continuous; amorphous; generally broad to very broad
- Scales: imbricate; without protrusions from main shaft
- Root: not remarkable
- Texture stiff, brittle; tips split
Figure 144. Photomicrograph of Hog Hair
- Diameter: coarse
- Medulla: unbroken lattice; occupies nearly entire width of hair shaft
- Scales: imbricate (absence of characteristic scale shapes or patterns distinguishes goat from deer family and antelope hairs)
Figure 145. Photomicrograph of Goat Hair
Figure 146. Photomicrograph of Scale Cast of Goat Hair
There are three basic conclusions that can be reached from a microscopic examination and comparison of hairs.
- The hairs from the questioned (Q) source exhibit the same microscopic characteristics as the hairs in a known (K) hair sample and can be associated to the source of the known hairs and others of similar breed.
- The hairs from the questioned source are microscopically dissimilar to the hairs in a known hair sample and cannot be associated with the source of the known hairs.
- The questioned hairs exhibit both similarities and slight differences to hairs found in a known hair sample, and no conclusion can be reached whether they could have originated from the known source. It may be that, in the opinion of the examiner, the differences are not sufficient to eliminate the source of the known hairs as being a possible source of the questioned hairs. At the same time, the presence of these differences precludes an association being made between the questioned and known hairs.
In the first conclusion, it is stated that the questioned hairs
can be associated with the source of the known hairs. Hairs are
biological specimens and subject to variation. During the analysis
of hair, the examiner must establish the range of variation in the
sample and then determine whether the questioned hair fits in that
range. The possibility cannot be dismissed that there may be two
hair samples whose ranges of variation overlap and distinguishing
between the samples is not possible.
The information contained in the report should be limited to a factual statement of findings concerning the examinations conducted. An interpretation of the evidence is saved for court testimony and explanation about the basis for the examinations. The report should be clear, concise, and easily understood. Technical terminology that is foreign to a layperson or contributor does not serve a useful purpose. The report should contain information pertinent to the requests made by the contributor of the evidence and to the examinations performed. A listing of the items of evidence and their origin, either a description or the contributor's reference number, should be included. Results of examinations should be written clearly, followed by a statement of the examiner's conclusions. A statement may follow to clearly state the limiting factors of hair examinations.
Item 1 = item from crime scene
Q1 n Knit cap
Item 2 = item from suspect=s dog
K1 n Dog hair sample from Rover
Results of Examinations: Brown dog hairs found on specimen Q1 exhibit the same microscopic characteristics as the hairs comprising specimen K1. Accordingly, these hairs can be associated to Rover, the identified source of the K1 dog hair sample. Dog hairs do not possess enough individual characteristics to associate them to a particular dog to the exclusion of other similar dogs.
Hair comparisons are not a means of absolute personal identification. The
statement of results sets forth, fairly completely, those determinations
that can be made (i.e., that the hairs came from a brown dog, that
they exhibit characteristics of a specific breed or animal type,
and that the Q hairs are consistent with the K hairs in microscopic
appearance). The resulting conclusion is that given these results,
the Q hairs can be associated with the suspect's dog Rover or another
of similar breed or type.
Mitochondrial DNA analysis can be conducted on animal hairs, but it is not currently done in any state or federal forensic laboratory.
Expert witness testimony should include an education component on hair evidence for the jury and judge and a statement of the results as reported. The witness should be prepared to discuss the procedures used in reaching the conclusion(s) and to defend opinions. An expert witness should endeavor to promote a better understanding of the methods of examination, the theory of the transfer of trace materials, and the strengths and limitations of the science.
Significance and Value
The forensic analysis of hair has been accepted in courts of law for many years, but this does not necessarily validate the science. The reliability of hair examinations must be weighed with the education and training of the examiner, as well as with the procedures used in the analysis. The examinations must be objective and impartial, and the weight placed on the results must be in accordance with the experience and training of the examiner.
Hair identifications are subjective interpretations of objective criteria. The variability and distribution of the microscopic characteristics are useful in determining whether or not a questioned hair could have originated from a particular individual.
Animal hairs are different in this respect from human hairs. Animal hairs do not posses enough individual characteristics to differentiate individual animals from other similar animals. However, differentiation between breeds and types of animals is possible and may be useful in an investigation.
Studies (Bisbing and Wolner 1984; Gaudette and Keeping 1973) have been conducted to determine the significance of hair associations. Some of these studies attempted to establish a mathematical probability of a match. The FBI Laboratory does not use the mathematical calculations of other researchers nor does it support the feasibility of establishing a numerical probability of a hair match.
The ability to analyze and interpret hair characteristics is a skill gained by training and testing. New examiners must study hair samples from different animal species and breeds and take hair-matching tests to demonstrate the ability to correctly associate hairs with a particular source.
Hair associations in casework should be subject to a confirmation process that has another qualified examiner review the match before a report is issued. The significance of a hair match is dependent on the distinct qualities of the hairs and the experience of the examiner. The greater the number of associations found in casework and the greater the number of cross matches, the greater the chance of association.
Anagen: Actively growing root stage.
Catagen: A transitional root stage between the actively
growing anagen stage and the resting telogen stage.
Comparison microscope: Two microscopes joined by an optical
bridge with a split screen to see both fields of views at the same
Cortex: Middle portion of hair extending from the cuticle
to the medulla and containing the pigment granules, cortical fusi,
and ovoid bodies.
Cortical fusi: Air spaces located in the cortex of hairs.
Cuticle: Translucent outer layer of the hair shaft consisting
of overlapping scales.
Fur hairs: Fine hairs that make up the undercoat of mammals
and provide warmth.
Guard hairs: Coarse hairs that provide protection and
are usually longer than fur hairs.
Keratin: Fibrous protein forming the chemical basis for
hair, nails, and feathers.
Medulla: Central portion of hair, the core area.
Melanin: A pigment that gives hair its color.
Miscible: Able to be mixed together.
Ovoid bodies: Dark bodies of unknown origin that are
a useful discriminatory characteristic in their pattern of appearance.
Papilla: Connective tissue from which hair is generated
from the follicle.
Pigment granules: Melanin granules whose size, shape,
density, and distribution vary.
Scales: Outermost portion of the cuticle, flattened and
imbricate in humans, pointing toward the distal end of the hair
Tactile hair: (Vibrissae): Whiskers only found on animals.
Telogen: Resting stage when the root takes on club shape
and is ready to be naturally shed.
Vellus: Fine body hair.
Bisbing, R. E. and Wolner, M. F. Microscopical discrimination
of twins' head hair, Journal of Forensic Sciences (1984)
Crocker, E. J. A new technique for the rapid simultaneous examination
of medullae and cuticular patterns of hairs, Microscope
Deedrick, D. W. and Koch, S. L. Microscopy of hair Part I: A practical
guide and manual for human hairs, Forensic Science Communications
[Online]. (January 2004). Available: www.fbi.gov/hq/lab/fsc/backissu/jan2004/research/
Gaudette B. D. and Keeping, E. S. An attempt at determining probabilities
in human scalp hair comparison, Journal of Forensic Sciences
Ogle, R. R. and Mitosinka, G. T. A rapid technique for preparing
hair cuticular scale casts, Journal of Forensic Sciences