Pituitary responses to gonadotrophin-releasing hormone (GnRH) and prolactin and steroid secretory profiles were examined in two populations of adult, female lions in the Serengeti (one outbred in the Serengeti Plains and one inbred in the Ngorongoro Crater) to determine whether reductions in genetic variability adversely affected endocrine function. GnRH-induced gonadotrophin secretion was also examined after adrenocorticotrophic hormone (ACTH) treatment to determine whether acute increases in serum cortisol altered pituitary function. Anaesthetized lions were administered (i) saline i.v. after 10 and 100 min of blood sampling; (ii) saline at 10 min and GnRH (1 μg kg−1 body weight) after 100 min; or (iii) ACTH (3 μg kg−1) at 10 min and GnRH after 100 min of sampling. Basal serum cortisol and basal and GnRH-induced gonadotrophin secretion were similar (P > 0.05) between females of the Ngorongoro Crater and Serengeti Plains. After ACTH, serum cortisol increased two-to threefold over baseline values and the response was unaffected (P > 0.05) by location. ACTH-induced increases in serum cortisol had no effect on subsequent basal or GnRH-stimulated luteinizing hormone (LH) or follicle-stimulating hormone (FSH) secretion. Overall mean serum progesterone concentrations ranged from 0.2 to 5.4 ng ml−1 with the exception of four females (two in the Serengeti and two in the Crater; progesterone range, 18.4–46.5 ng ml−1) that were presumed pregnant (three of these females were observed nursing cubs several weeks later). There were no differences (P > 0.05) between Serengeti and Crater lions in mean serum progesterone, oestradiol or prolactin concentrations, and hormone secretion was not influenced (P > 0.05) by GnRH or ACTH treatment. Although Ngorongoro Crater lions have decreased genetic variability, the reproductive–endocrine system of females appears functionally normal compared with outbred counterparts living in the Serengeti Plains. Furthermore, the acute rise in serum cortisol after ACTH administration in lions fails to alter subsequent GnRH-induced gonadotrophin release, suggesting that short-term changes in adrenal activity do not markedly affect pituitary responsiveness in this species.
J. L. Brown, M. Bush, C. Packer, A. E. Pusey, S. L. Monfort, S. J. O'Brien, D. L. Janssen and D. E Wildt
J. L. Brown, M. Bush, C. Packer, A. E. Pusey, S. L. Monfort, S. J. O'Brien, D. L. Janssen and D. E. Wildt
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