Seasonal cycles in gonadal activity and plasma gonadotropin in the musk turtle, Sternotherus odoratus

Abstract

Plasma gonadotropin cycles were examined in relation to the seasonal gonadal cycle in freshly captured musk turtles, Sternotherus odoratus, from South Carolina. Acute and chronic stress effects of captivity on testicular growth and circulating plasma testosterone (T) and follicle-stimulating hormone (FSH) also were examined. Monthly mean FSH levels in freshly captured males were correlated significantly with plasma T. Both T and FSH were minimal in spring when testes were small, and highest in the fall in males that had begun spermiation and whose testes had begun to regress. Individual plasma T and FSH values were significantly correlated during the fall when both hormones were at elevated levels. In contrast, plasma luteinizing hormone (LH) remained undetectable throughout the year in males. Females had nondetectable levels of both plasma LH and FSH throughout the year. There was a seasonal pattern in plasma 17β-estradiol which was correlated with follicular growth. Holding males in captivity (outdoors) reduced plasma T by 35–60% within 24 hr of capture (depending on season) and plasma T remained at these lower levels after 2 weeks. However, FSH was not significantly affected by such short-term “stress.” Testicular weight and FSH cycles of captive males were similar to those of males in the field. Monthly plasma T of long-term captives also exhibited a seasonal cycle but at a level significantly lower than in the field and with a delayed peak. Thus, while the data suggest that FSH may be important to both seasonal testicular growth and androgen secretion, fluctuations in FSH alone cannot account for all observed patterns or changes in spermatogenesis or plasma androgen levels.