Diurnal Variation of Plasma Testosterone in Wild Stallions

Abstract

Three wild horse stallions from the Pryor Mountain National Wild Horse Range in Montana were captured and blood samples collected over a 24 h period. Androgens were extracted from the plasma, testosterone was isolated by Sephadex LH-20 column chromatography and quantitated by a competitive-protein-binding assay. All three horses exhibited similar qualitative changes in testosterone concentrations during the 24 h, with the nadir occurring at 2300 (1.37 ± 0.75 ng/ml) and a peak at 0800 (3.48 ± 0.12 ng/ml). One of the horses, an eight year old bachelor stallion which has never made an effort to gather a harem since sexual maturity, had a significantly lower (P<0.01) 24 h mean testosterone concentration (1.52 ± 0.89 ng/ml) than either of the two stud stallions (2.68 ± 0.59 ng/ml and 3.20 ± 0.68 ng/ml), both of which have possessed harems and sired colts. It is concluded that all three stallions possessed a diurnal testosterone rhythm. Whether the lower testosterone concentrations in the bachelor stallion are a cause or an effect of sexual disinterest remains unanswered.