SEASONAL-VARIATIONS IN TESTIS SIZE, LIBIDO AND PLASMA TESTOSTERONE CONCENTRATIONS IN BRITISH GOATS

The effects of months and seasons of the year on the testis size, libido and plasma testosterone (T) concentrations were studied in 10 young sexually mature male goats over a 12-month period. Testis size was expressed in terms of scrotal circumference, testis diameter and paired testes width while the libido was measured as stimulation and reaction times, number of mounts and services, and libido index. These measurements were recorded fortnightly. The body weight increased from 33.5 (s.e. 0.95) kg in October, to a maximum of 62.8 (s.e. 1.55) kg in July. All parameters of testis size decreased from October through January with a sharp increase noted in July and August. These parameters were significantly higher in the summer and autumn than in the winter and spring (P < 0.01). The libido in the goats increased from October to a maximum in December, declined gradually to reach the minimum in July, and then improved again in August. Thus, libido was lowest during the summer and highest in winter, the difference being significant (P < 0.02). Plasma T concentrations were high during October, with the lowest values noted in June. An increase in the T concentrations was observed in August, reaching the maximum value in September, the difference being significant (P < 0.01). The T concentrations were significantly higher during autumn than in all other seasons (P < 0.01).