Animal and Human Hair Evidence

Forensic Science Guide: Characteristics of Animal and Human Hair

One of the first determinations that a forensic investigator must make when analyzing a hair sample is whether its origin is human or animal.  Fortunately, human hairs have several unique characteristics that can be used to distinguish them from animal hairs, with perhaps the exception of hairs taken from other primates.  These distinct features are summarized below.  Human hairs vary depending on the body part from which they come and on the subject’s ethnic background.  This variation sometimes makes human hair exhibit certain animal characteristics.  To avoid confusion, descriptions of these differences between humans are also provided.

Typical Hairs in Mammals

Hair is one of the defining features of mammals, including humans.  Seen lengthwise, almost all hair can be divided into three main sections: the bulb (or root), the shaft (the middle section), and the tip (where the hair thins).  The innermost part of a single hair is called the medulla, which is surrounded by a mid-layer known as the cortex.  Finally, the outermost layer of a hair is the cuticle.  In general, hair is made up of proteins called keratins, although other substances that are secreted by the body can be found on hairs too.  Coming in contact with secretions is a natural part of each hair’s being rooted in the skin in a compartment called the follicle.

Hair grows in three stages.  The first, and most active, growth spurt is the anagen stage, which lasts for several years.  For forensics purposes, this is the best stage in which to extract a hair for DNA testing because the root is firmly anchored in the follicle and samples of tissue will cling to the root when the hair is being dislodged.  Next comes the multi-week catagen stage in which the hair stops growing and the bulb takes on a stretched-out look.  Last is the talagen stage in which the bulb begins to resemble a spoon shape.   This final phase usually lasts for several months until the hair falls out.

Human Hair Structure and Classifications

Although there are separate categories of hair found on human fetuses, adult hair can be classified into two main categories:

  1. Vellus hair is extremely fine and somewhat hard-to-see.  These hairs can be found almost all over the body and do not have a medulla.
  2. Terminal hair is the kind we typically see: eyebrows, beards, head hair, pubic hair, arm and leg hair, etc.  These hairs are longer and coarser, and may have a medulla.

Terminal hairs can be further classified by body part and ethnic background, for which three broad categories are used in forensics:

  1. Mongoloid hair is unique in that the medulla is usually unbroken, running the length of each hair.  In most other human hair types, the medulla is either missing or fragmented.  Other distinctions of Mongoloid hair include the thickest cuticle and thickest overall diameter (at up to 120 micrometers), and a round cross-section.  People of Asian and Native American descent typically have this kind of hair.
  2. Caucasoid hair, found in people of European, Middle Eastern, and Latin American descent, is oval in shape and each hair is fairly even in color (pigmentation) from one to the next.
  3. The finest hairs are Negroid with a diameter as small as 60 micrometers, found in people of African descent, although there is much overlap in diameter between these and Caucasoid hairs.  The cross-section of a Negroid hair is the flattest of the three.  Pigmentation tends to concentrate in specific areas, so the color of each hair is less uniform than in the other two.  Finally, Negroid hairs tend to twist and curve far more than Mongoloid and Caucasoid hairs.

These categories become somewhat less useful for identifying people of mixed ancestry, and there is a great deal of overlap in average diameter and overall shape between the three.

Besides ethnicity, the part of the body from which a specimen of human hair comes is marked by unique features:

  1. Head hair is the longest, typically growing up to 1,000 millimeters.  Each hair is fairly uniform in diameter.  Of special note to investigators is that head hair is the most likely to be dyed or otherwise treated.
  2. Facial hair is usually the coarsest and likely to have a blunted tip from shaving and trimming.
  3. Pubic hairs are noted for their coarseness and lack of uniformity from one to the next, and for a distinctive buckling in the middle section.
  4. Eyebrows, or superciliary hairs, stand out for their gradual curve and thicker than normal medulla.
  5. Arm and leg hairs can be identified by the lack of uniformity in their medullas, milder pigmentation, and narrow tip.

Animal Hair Structure and Classifications

Because of the wide range of animals, non-human hair can have many different characteristics.  However, during forensic investigations, these four classifications generally apply:

  1. The fur or wool serves to insulate the animal from the elements.  Fur is an inner layer of hair on most animal coats.
  2. The vibrissa, or tactile, hairs are used to guide animals by sense of touch.  Whiskers are the prime example.
  3. The guard hairs, or bristles, protect animals’ skin from pests and water, and comprise the outermost layer of a coat.
  4. Other hairs, such as the manes of large cats and tails of horses, have unique properties that set them apart from the other categories.

In general, animal hairs can be distinguished from human hairs in the following ways:

  1. If, in the cross-section of a hair, the medulla runs most of the hair’s length, the hair is likely animal in origin, especially if the medulla is unbroken or only occasionally interrupted.  Human hairs tend to have fragmented, inconsistent medullas or no medulla at all.
  2. Animal medullas tend to be thicker and more pronounced as well, sometimes composing more than 2/3 the diameter of a hair’s shaft.  The ratio of medulla to hair diameter is called the medullary index.
  3. Pigments in animal hairs tend to clump around the medulla, while they are more uniformly distributed in human specimens.
  4. Animal hairs often show distinct banding: different sections of a hair will differ dramatically in color.  Human hairs are colored more evenly in most cases.
  5. The cuticles of animal hairs display a wide variation in pattern.  Many are serrated, ragged, scaly, and irregular while human cuticles tend to show no obvious patterning and are typically flat.

The way in which animal hairs contrast to human samples can also help forensics experts determine the animal’s species.  For example, the serrated pattern of cats and rabbits’ cuticles cause their hairs to interconnect.  This is not true of most other animals.

Investigating Hair

Samples are collected a number of ways, depending on whether or not the hairs are immediately visible.  Tweezers, for example, can be used to collect specimens that the investigator can easily see, but run the risk of causing damage to the hair.  Otherwise, clear tape, special illumination (infrared, for example), vacuuming, and brushing or shaking of clothes in a container, such as a plastic bag, work best for revealing hair samples that aren’t instantly visible.  Clear tape is one of the better ways to avoid tampering.

Once extracted, hair is best examined under a microscope with a magnification of 40X to 800X, depending on which of the characteristics the investigator wants to identify.  Solvent, such as clear nail polish or glycerin, can be placed on the microscope slide to help stabilize the hair, although nail polish should be allowed to dry first and the investigator should be wary of its flammability.  The clear tape used to extract the sample can also work as a medium in the microscope slide for a quick analysis.

For more details on the forensic examination of human and animal hair, please see the resources below.

Works Consulted

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