Seasonal variations in hair pigmentation of white-tailed deer and their relationship to sexual activity and plasma testosterone

Intensity of hair pigmentation of dorsal scrotum, nose, cheek and forehead areas of seven mature, male white-tailed deer were determined from close-up colour slides taken once a month during a 2-year period. Blood samples and skin biopsies from forehead areas were taken at the same time as the photographs. Plasma testosterone (T) levels were measured by radioimmunoassay and T in the skin was investigated by immunohistology. Seasonal variations of hair pigmentation are most pronounced in the forehead region followed by the cheek, scrotum, and nose area. Peak blood levels of T (15.4 ng/ml) were detected in November. The highest correlation between T levels and pigmentation of the forehead area (R = 93%; R² = 0.87), was established when pigmentation values were shifted two months ahead. Immunohistologically detectable T was localized in hair follicles, hair sheets and apocrine glands but not in the sebaceous glands. It is hypothesized that pigmentation of head regions might serve as a visual cue indicating the sexual status of an individual.