CIRCANNUAL CHANGES IN SERUM LEVELS OF PITUITARY HORMONES AND TESTOSTERONE AND IN TESTIS SIZE OF SEXUALLY ACTIVE AND INACTIVE ADULT RAMS

Abstract

The influence of the social environment of adult rams on testicular function and the secretion of pituitary hormones was investigated between May 1978 and October 1979. Twelve rams, 3–6 yr of age, were penned in three groups of four, either as a sexually inactive all-male group or as sexually active groups adjacent to or together with ovariectomized ewes that were hormonally induced into estrus every 10 days. At approximately monthly intervals measurements of testis size were recorded and rams were bled by venepuncture from the jugular vein at 20-min intervals for 8 h (0900–1700 h). Sera were assayed for luteinizing hormone (LH), follicle stimulating hormone (FSH), prolactin (PRL) and testosterone. Since seasonal changes in testicular and endocrine characteristics for the sexually active groups were similar, data for these groups were combined. Differences in the circannual testicular rhythm were not observed between ram groups until the onset of the second breeding season; at this time (August 1979), sexually active rams had significantly (P < 0.05) higher scrotal circumference measurements (36.0 ± 0.9 vs. 32.2 ± 1.0 cm) and circulating testosterone levels (10.7 ± 1.6 vs. 6.7 ± 1.5 ng/mL), and significantly (P < 0.05) lower LH levels (0.75 ± 0.10 vs. 2.04 ± 0.40 ng/mL) than sexually inactive rams. The increase in rate of testicular redevelopment for sexually active rams during late summer was not preceded by significantly (P > 0.05) higher seasonal elevations in circulating LH and FSH levels, although PRL levels in May through July were only 50% of those observed for the sexually inactive rams. Sexually active rams also exhibited significant (P < 0.01) increases in LH and testosterone secretion in the nonbreeding season when ewes were sexually receptive. Ejaculation frequency during a 4-h test period at the conclusion of the study was similar for both groups of rams. Data indicate that long-term exposure of adult rams to estrual ewes results in an increase in rate of testicular redevelopment during the summer in preparation for the fall breeding season. Key words: Circannual changes, pituitary hormones, testosterone, testis, mating, ram