Effect of Nutrition on Testicular Growth and Plasma Concentrations of Gonadotrophins, Testosterone and Insulin-like Growth Factor I (IGF-I) in Pubertal Male Soay Sheep

Nutritional effects on puberty were studied in Soay rams. Testicular growth is initiated at birth in April and testes reach maximum size in October. Groups of eight lambs were fed for 18 weeks, starting in August, a ration that restricted growth (Group R), the same diet ad libitum (Group F), or a restricted diet for 8 weeks followed by ad libitum feeding (Group R/F). Seasonal increases of plasma FSH, testis size, sexual skin flush and plasma testosterone occurred with similar timing but reduced magnitude in Group R compared with Group F lambs. Testis size and sexual skin flush peaked in all groups at 11 weeks (30 October); the testes of Group F animals were larger before the peak, but similar in size thereafter, compared with testes from Group R/F, and larger throughout the experimental period than testes from Group R. Plasma testosterone was higher in Group F than in Group R lambs from 7 to 17 weeks, but in Group R/F was similar to Group R before 10 weeks (23 October) and similar to Group F thereafter. Testis size, plasma testosterone, plasma insulin-like growth factor I (IGF-I) and liveweight were positively correlated. Ad libitum feeding in August-September (Group F) stimulated increased plasma FSH and LH above values for Group R, but ad libitum feeding initiated in October did not affect gonadotrophin concentrations (Group R/F). Therefore, the effects of improved nutrition on the hypothalamo-pituitary axis, which may have been mediated by circulating IGF-I, were season- or age-dependent, and those on the testes included direct stimulation, independent of changes in gonadotrophin concentrations. Nutrition modified the intensity, but not the timing, of peak pubertal reproductive activation.