Impact of season on seminal characteristics and endocrine status of adult free-ranging African buffalo (Syncerus caffer)

Pituitary, gonadal and adrenal activity were compared in free-living, adult African buffalo bulls during the breeding and nonbreeding seasons. Frequent blood samples were collected for 2 h from anaesthetized bulls treated intravenously with saline, gonadotrophin-releasing hormone (GnRH, 200 μg), human chorionic gonadotropin (hCG, 10,000 i.u.) or adrenocorticotrophic hormone (ACTH, 1.5 mg). Electroejaculates also were collected from anaesthetized bulls during the breeding and nonbreeding seasons. Pretreatment testosterone concentrations among bulls varied more during the breeding (0.17-23.0 ng/ml) than the nonbreeding (0.15-2.21 ng/ml) season. The variation within the breeding season was attributed to 8 of 25 bulls producing higher (P < 0.05) serum testosterone (High-T; 16.28 ± 2.03 ng/ml) and testicular LH receptor (1.53 ± 0.22 fmol/mg testis) concentrations compared with their seasonal counterparts (Low-T; 0.95 ± 0.26 ng/ml; 0.38 ± 0.04 fmol/mg) or with all bulls during the nonbreeding season (0.90 ± 0.27 ng/ml; 0.31 ± 0.04 fmol/mg). The magnitude of GnRH- and hCG-induced increases in serum testosterone was similar (P > 0.05) between Low-T bulls and bulls during the nonbreeding season. In the High-T animals treated with GnRH or hCG, serum testosterone did not increase, suggesting that secretion was already maximal. Peak serum LH concentrations after GnRH were greater (P < 0.05) in bulls during the nonbreeding than the breeding season; FSH responses were similar (P > 0.05). ACTH treatment did not increase serum cortisol concentrations above the 2-fold increase measured in bulls treated with saline, hCG and GnRH (P > 0.05). Ejaculate volume, sperm motility and the proportion of morphologically normal spermatozoa were greater (P < 0.05) during the breeding than in the nonbreeding season, but the total number of spermatozoa/ejaculate was similar (P > 0.05). We suggest that season effects endocrine function and seminal quality in free-living, male African buffalo. During the breeding season some, but not all, adult bulls produce high circulating concentrations of testosterone associated with increased testicular LH receptor binding. These findings suggest that this species may experience selective gonadal suppression, perhaps related to differences in social status.