CIRCULATING ESTROGEN LEVELS IN THE RAM: INFLUENCE OF SEASON AND MATING, AND THEIR RELATIONSHIP TO TESTOSTERONE LEVELS AND MATING FREQUENCY

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Abstract
Two experiments were conducted to investigate the relationship between circulating estrogen and testosterone levels in the adult ram. Experiment 1 involved assessing the mean concentration of these hormones in blood collected from eight rams during 8-h periods of both sexual inactivity and mating with estrus-induced ewes prior to and during the fall breeding season (early August, mid-September, early November and mid-December). Rams were bled at 20-min intervals by venipuncture of the jugular vein during all periods. Serum estrogen concentration increased approximately 40% to an average (± SE) of 11.2 ± 0.8 pg/mL between August and November, and in direct relation to testosterone concentration (r = 0.88); estrogen values were generally less than 0.2% of testosterone values. Mating activity was associated with increments in circulating estrogen in September only, while testosterone levels increased in both August and September. Frequency of mating (ejaculation) was highly and positively correlated with serum estrogen concentration among rams in August (r = 0.86) and within rams across months (r = 0.78). In exp. 2, the relationship between hour-to-hour fluctuations in estrogen and testosterone levels was examined for six rams in early winter. Rams were bled as in exp. 1 for 7 h, and two of the rams were given frequent intravenous injections of NIH-LH-S14 following the first 2 h of bleeding. Short-term variations in serum estrogen concentration were not consistently related to those in testosterone. Key words: Estrogen, season, mating, ram, testosterone, circulation
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