Seasonal changes in the plasma testosterone profile in Merino rams

Abstract

The pattern of testosterone secretion (i.e. profile of plasma testosterone sampled every hour for 24 h) was characterized for Merino rams at various times during the year at latitude 35¦S to assess seasonal trends. The profiles of plasma testosterone were always characterized by random peaks regardless of season. However, at any particular time of the year, there were large differences between individual rams in the number of peaks124 h (range 1-7). Mean plasma testosterone levels were higher and a greater number of peaks were recorded per 24 h in summer (December-January) than in winter (July) and spring (September-October). Seasonal changes observed in the plasma testosterone profile of Merino rams were consistent with the concept that activity of the hypothalamic-pituitary-gonadal axis in sheep is entrained to changes in the photoperiod. Merino rams at latitude 35¦S are therefore markedly seasonal from an endocrine point of view, although they are not generally regarded as seasonal breeders based on mating behaviour.