|The relationship between facial hair whorl patterns and objective measures of stress and reactivity will be examined among 15 mustangs of the same herd, obtained from the Bureau of Land Management. Data collection will include assessments of salivary cortisol levels and heart rates at rest, during, and after exposure to a potentially frightening stimulus or situation. These tests will include trailer loading and isolation from other horses, interactions with trainers, and exposure to novel visual and auditory stimuli such as an opening umbrella or a moving tarp. Data from the heart rate monitors and salivary cortisol test kits will be analyzed and compared to the number, direction, and vertical and lateral position of each horse’s facial hair whorl(s). Correlations between certain whorl patterns and higher heart rates or cortisol levels may support long-standing claims that a horse’s hair whorls can be indicative of his temperament or reactivity, which could provide a simple method of predicting animal behavior.