Seasonal pattern of cortisol fluctuation in horsehair samples from three different body areas: A year long study

This study aimed to assess the seasonal changes of the cortisol concentrations in horsehair (HCC), as an indicator of long-term or chronic stress. Hair samples were taken four times throughout one year from 12 horses (aged from 1.5 to 30 years), and analysed by the ELISA method. The sampling interval (interval between two collections of the samples representing two different seasons) ranged between 102 and 115 days. Seasonal dynamics have been confirmed with a pattern: August ˃ November ˃ January ˃ April (F (3.93) = 50.28, P < 0.001). The lowest cortisol concentrations were detected in April samples (mean value 10.05 ng/ml, SD 1.80 (n = 36)), whereas the highest levels were detected in August samples (mean 18.63 ng/ml, SD 4.61 (n = 36)). The correlations across samples taken from various body areas (from the mane, tail, and carpal area of the right forelimb) were investigated, but the body area has no effect on the cortisol concentrations (F (2.31) = 1.22, P = 0.308). In relation to the variable "sampling area" difference was found only for the winter - between the mane/leg, and tail/leg (F (2.32 = 3.79, P < 0.05), but not for mane/tail (P > 0.87). No difference in HCC associated with the variable "age" was revealed. Due to the small sample size and because of several various factors that may contribute to seasonal patterns of HCC, these results must be interpreted with caution.