Summary. Pituitary–gonadal function was examined in male lions free-ranging in the Serengeti Plains or geographically isolated in the Ngorongoro Crater of Tanzania. Lions were classified by age as adult (6·1–9·8 years), young adult (3·3–4·5 years) or prepubertal (1·4–1·6 years, Serengeti Plains only). Each animal was anaesthetized and then bled at 5-min intervals for 100 min before and 140 min after i.v. administration of saline or GnRH (1 μg/kg body weight). Basal serum LH and FSH concentrations were similar (P > 0·05) among age classes and between locations. In Serengeti Plains lions, net LH peak concentrations after GnRH were ∼ 25% greater (P < 0·05) in prepubertal than in either adult or young adult animals. GnRH-stimulated LH release was similar (P > 0·05) between adult and young adult lions, and these responses were similar (P > 0·05) to those measured in Ngorongoro Crater lions. Basal and GnRH-stimulated testosterone secretion was higher (P < 0·05) in adult than in young adult lions and lowest (P < 0·05) in prepubertal lions. Age-class differences in testosterone production were related directly to the concentrations of LH receptors in the testis (P < 0·05). Basal and GnRH-stimulated testosterone secretion and gonadotrophin receptor concentrations within age classes were similar (P > 0·05) between lions of the Serengeti Plains and Ngorongoro Crater. Lower motility and higher percentages of structurally abnormal spermatozoa were observed in electroejaculates of young adult compared to adult Serengeti Plains males (P < 0·05) and were associated with decreased steroidogenic activity. In contrast, there were no age-related differences in ejaculate characteristics of Ngorongoro Crater lions. Seminal quality in the Crater population was poor in adult and young adult animals and was unrelated to alterations in pituitary or testicular function. In summary, only seminal quality in adult male lions was affected by location, whereas age significantly affected both basal and GnRH-stimulated testosterone secretion and seminal quality (Serengeti Plains only) in sexually mature males. The striking seminal/endocrine differences among pride (breeding) males of different ages raises questions about the impact of age on individual reproductive performance in this species.
Keywords: lion; GnRH; LH; testosterone; receptors; testis; spermatozoa