Circannual inter-relationships among reproductive hormones, gross morphometry, behaviour, ejaculate characteristics and testicular histology in Eld's deer stags (Cervus eldi thamin)

Blood samples, morphometric measurements and behavioural data were collected weekly for 52 weeks from six adult Eld's deer stags exposed to natural fluctuations in photoperiod (38°N latitude). Mean (±SEM) serum LH concentrations reached peak values in the autumn (October), three months before FSH and testosterone concentrations reached peak values in early winter (January). Prolactin concentrations were inversely related (r = 0.733, P < 0.001) to LH and directly related to daylength, and maximal concentrations were observed during mid-summer (July) and minimal concentrations during early winter January). The temporal pattern of circulating inhibin was positively correlated with FSH (r = 0.88, P < 0.001), but lagged behind the seasonal FSH increase by 1-3 weeks. Antler length, body weight and chest girth were maximal during pre-rut (December/January). Maximal scrotal circumference and combined testes volume were observed in mid-winter (February), whereas peak neck girth and behavioural aggression occurred 1-3 months later (March-May). On the basis of quarterly electroejaculation results, motile spermatozoa were produced in all seasons. However, the greatest number of motile spermatozoa per ejaculate was observed during the winter and spring, whereas the highest incidence of sperm pleiomorphisms (> 80%) was detected in the autumn. Histological assessments of the regressed testis (July) revealed fewer germ cells undergoing spermatogenesis and an increased incidence of degenerating and abnormal cell types. In summary, Eld's deer exhibit a circannual hypothalamic-pituitary-gonadal cycle with onset of pituitary activation occurring during the autumn and winter, whereas gonadal activity peaks during the winter and spring as daylengths are increasing. Marked circannual variations in circulating prolactin suggest that Eld's deer may use photoperiodic cues to modulate seasonal fertility; however, the existence of an endogenous seasonal rhythm operating independently of photoperiod cannot be excluded.