Testosterone levels in plasma obtained from the jugular vein of rams were measured using a radioimmunoassay. The assay antiserum had a specificity for testosterone such that chromatography was not used during determinations. Blood was taken every ½-1 h under conditions approximating to the natural environment at the particular time of year. Measurements during a 48 h period in January demonstrated that marked episodic bursts of testosterone occurred. Two rams showed eight peaks of testosterone in 24 h with levels reaching 12–14 ng/ml. Two other animals showed four peaks in 24 h, the levels reaching 8 ng/ml. The pattern for all animals was similar in the second 24 h period. In one animal, a testosterone profile obtained over 24 h in the previous November corresponded to that obtained in January although the animal had been serving ewes in the interim period and suggests tentatively that a particular pattern may be inherent in an individual animal.
The rams were maintained in the absence of females until early March and sampled again for 7 h during which time they were allowed contact with an ovariectomized ewe induced into oestrus. The libido of the rams appeared normal. Testosterone levels were low throughout, exceeding 5 ng/ml on one occasion only, and were not affected by copulation. A testosterone profile obtained for two of the rams over a 24 h period in late March demonstrated that the levels were still low. A further testosterone profile was obtained for one of these rams in June. Levels were high and the pattern had reverted to that found the previous November and January.
To obtain evidence that the low levels of testosterone found in March were a result of season rather than lack of regular copulation at this time, blood was obtained from four rams maintained at the Meat and Livestock Commission A.I. Centre, Selby. Two of these animals had not been used for semen collection since January and two were in regular use with a teaser female and semen was collected twice weekly. Maximum testosterone levels of 5 ng/ml were obtained over 12 h in both groups.
During this work, in periods when the testosterone levels were high, there were disruptions in apparently periodic fluctuations in testosterone levels which could be correlated with the proximity of other animals.
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