This article incorporates, in modified form, material from the not-yet-published Illustrated Guide to Forensics Investigations: Uncover Evidence in Your Home, Lab, or Basement.
As you might expect, hair from non-human mammals can be difficult or impossible to discriminate from human hair under macroscopic examination. Fortunately, under microscopic examination, it’s relatively easy to discriminate most non-human animal hair from human hair. (Hair from some non-human primates is the major exception.)
Animal hair, particularly pet hair, is frequently examined in forensics labs. Although in the past animal hair was of limited evidentiary value, the advent of DNA testing–which can as easily be applied to animal hair as to human hair–has made animal hair testing an important part of the work of modern forensics labs. It is, for example, now possible to convict criminals based on DNA matches between animal hairs from the criminal’s pet that were found on the victim, or vice versa.
As is true of human hair, DNA testing of animal hair is relatively expensive and time-consuming, so macroscopic and microscopic examination is used for preliminary screening. In fact, DNA testing of animal hairs is still relatively rare. As late as 2004, no state, federal, or private forensics labs were doing DNA testing of animal hair, and as of early 2009, only a few private forensics labs routinely do DNA testing of animal hair.
Animal hairs are more differentiated by somatic region and purpose than human hair. Animal hairs are classified as members of four broad types:
- Guard hairs form the outer coat of the animal, shed water, and protect the inner hair and skin
- Fur or wool hairs form the inner coat and provide insulation
- Tactile hairs, also called whiskers, are found on the head (the snout or ears), where they provide sensory functions
- Special-purpose hairs, such as tail hairs and mane hairs, whose morphology may differ substantially from the main body hairs of the animal.
Human hair differs noticeably from animal hair in the following respects:
- Most human hairs are consistent in color for the entire length of the hair shaft, while many animal hairs vary significantly in color over relatively short sections of the shaft, a phenomenon called banding
- The pigment granules in human hair are generally relatively evenly distributed throughout the cortex, with perhaps some concentration toward the cuticle, while the pigment granules in most animal hair are generally strongly concentrated centrally in close proximity to the medulla
- The medulla in human hair, if present, is generally amorphous, often broken or fragmented, and occupies a third or less the width of the hair shaft, while the medulla in animal hair is generally present, continuous, sharply defined and structured, and occupies one third or more (sometimes nearly the entire) width of the shaft
- Cuticle scaling in human hair, if present, is often subtle and usually of the imbricate pattern (see the preceding lab session) or, much more rarely, the coronal pattern, while cuticle scaling in animal hair is often more readily visible and may be imbricate, coronal, or spinous pattern, which is never found in human hair.
In this session, we’ll examine various animal hair specimens macroscopically and microscopically and produce scale casts to learn how to differentiate animal hair from human hair.
Required Equipment and Supplies
- goggles, gloves, and protective clothing
- stereo microscope, loupe, or magnifier
- compound microscope (100X and 400X, with ocular micrometer)
- microscope slides and coverslips (as required)
- disposable pipettes (as required)
- ruler (graduated in mm)
- forceps or tweezers
- mounting fluids (see Substitutions and Modifications)
- animal hair specimens (see Substitutions and Modifications)
All of the specialty lab equipment and chemicals needed for this and other lab sessions are available individually from the Maker Shed or other laboratory supplies vendors. Maker Shed also offers customized laboratory kits at special prices, and a wide selection of microscopes and microscope accessories.
Although none of the activities in this lab session present any significant risks, as a matter of good practice you should always wear splash goggles, gloves, and protective clothing when working in the lab, if only to avoid contaminating specimens. Obviously, you may need to work without goggles when using a microscope or magnifier to examine specimens.
Substitutions and Modifications
- For temporary wet mounts, you can use distilled water (RI ~ 1.33), although glycerol (RI ~ 1.47), castor oil (RI ~ 1.48), or clove oil (RI ~ 1.54) is a closer match for the refractive index of hair. Permount, Canada balsam, or a similar mounting fluid can be used to make permanent mounts. Unless you are short of slides and coverslips, you may want to label and mount your animal hair specimens permanently for later use as reference standards.
- Obtain at least one shed or (ideally) plucked hair specimen from as many animals as possible. Dog and cat hair are readily available, from your own pets or from those of family members, friends, and neighbors. You may also be able to obtain hair specimens from someone who keeps a rabbit, rat, ferret, gerbil, hamster, or other small mammal as a pet. Specimens from wild animals such as squirrels, raccoons, opossums, and others can sometimes be obtained from road kill. Store each specimen you obtain in a labeled container.
The procedures for this lab session are identical to the procedures for the preceding two lab sessions.
- Examine an animal hair specimen under low magnification and record your observations in your lab notebook or Table 6-5.
- Wet-mount the animal hair specimen.
- Examine the animal hair specimen under medium and high magnification (100X and 400X) and record your observations in your lab notebook or Table 6-6. Compare and contrast these microscopic observations with those you made of your scalp hair.
- Make a scale cast of the animal hair specimen, and examine the scale cast under high magnification, recording your observations in your lab notebook or in Table 6-7. If you have the necessary equipment, shoot an image of the scale cast for your lab notebook.
- Repeat steps 1 through 4 for each of your animal hair specimens.
Figure 6-10. A canine hair at 100X
Table 6-5. Morphology of Animal Hair – observed macroscopic data
Table 6-6. Morphology of Animal Hair – observed microscopic data
Table 6-7. Morphology of Animal Hair – observed scale pattern data
Working Without a Comparison Microscope
Professional forensics labs use comparison microscopes to do side-by-side comparisons of hair specimens. We can’t afford the $5,000 or more cost of a decent comparison microscope, but there is a workaround that we’ve found to be just as useful for learning purposes. Simply wet-mount two or more hair specimens side-by-side and as close together as possible on one slide. That way, you can do a comparison of both or all specimens in one field of view. You can use this method to compare human hairs against each other, human hairs against animal hairs, or different types of animal hairs.
Q1: What microscopic characteristics can be used to readily differentiate an animal hair specimen from a human hair specimen?