Biology of Reproduction, Vol 15, 98-101, Copyright © 1976 by Society for the Study of Reproduction

Diurnal Variation of Plasma Testosterone in Wild Stallions

¹ Section of Animal Physiology, Endocrinology and Reproduction, Department of Biological Sciences, Eastern Montana College, Billings, Montana 59101

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.

Note:
ACKNOWLEDGMENTS This study was supported in part by the Eastern Montana College Foundation. We gratefully acknowledge and thank Mr. Lowell Brown of the Billings Office of the Bureau of Land Management, Mr. Gene Nunn, and Mr. Lynn Taylor for the expert capture and handling of the horses, Marlane Angle and Debbie McGowen for their able technical assistance. We are particularly grateful to Dr. V. K. Ganjam of the University of Pennsylvania School of Veterinary Medicine for help in developing the assay procedure.