Variability among hairs of humans is dependent on body source and ethnicity. Variability in the hairs of non-human mammals is also dependent on somatic source, but may exhibit a wide range of characteristics among hairs from any given site. Humans do not have secondary guard hairs as defined below, but forensic scientists involved in trace analysis must recognize a different suite of variability that exists when examining animal hairs. Hairs other than primary guard hairs from the dorsum, which are often the only representative hairs illustrated in animal hair atlases, may be present as trace evidence in a forensic case in which the origin of the hair is unknown. An overview of the different grades of animal hair that could be present in a forensic case or natural history inquiry places the secondary, tertiary, etc. guard hairs in a continuum that is based on relative diameters.