Cumulative testosterone levels in horses detected in hair: relationship with season, physiological features and stress hormones

Hair is a matrix able to cumulate steroid hormones, mainly demonstrated with stress hormones in several species, including the horse. High hair cortisol concentrations have been related to chronic stress environments or situations. There are not studies assessing testosterone concentrations in hair in relation to cortisol levels, season and age. This study was conducted to characterize the seasonality of concentrations of hair testosterone (HTC) in relation to hair cortisol concentrations (HCC) and age. Nine stallions with average age 8.2±2.6 years old (range: 5 - 13) were submitted to 12 consecutive monthly hair sample extraction through by shaving an area of 10×17 cm of hair coat from the left side of abdomen to the level of the skin at each sampling time. Biochemical validation tests for cortisol and testosterone determination in horse hair were performed by enzyme immunoassay with reliable results following precision, specificity, accuracy and sensitivity criteria. Pearson’s correlation between HTC, HCC and age did not showed any associations (P>0.05). Average HTC was 0.99±0.39 pg/mg. There were no significant differences in HTC among seasons (P>0.05; autumn: 1.08±0.45 pg/mg, winter: 1.04±0.37 pg/mg, spring: 0.92±0.40 pg/mg, summer: 0.89±0.28 pg/mg) and age (P>0.05). The levels of HCC were higher in summer (4.45±1.92 pg/mg) than in spring, autumn and winter (3.36±1.46 pg/mg, 3.18±1.48 pg/mg, 2.58±1.16 pg/mg, respectively; P<0.05) but not in age (P>0.05). Our study showed that cortisol and testosterone concentrations can be measured in hair of adult horses which may be useful as a non-invasive indicator of horse well-being and long-term retrospective endocrinology.