Role of gonadal hormones in the regulation of the seasonal antler cycle in female reindeer, Rangifer tarandus.

The hormonal control of the seasonal antler cycle was investigated over 4 years in four adult female Norwegian reindeer, Rangifer tarandus, maintained in captivity at Tromsø, Norway (69 degrees N). The antlers were fully redeveloped in August, cleaned of velvet in September, and were retained in hard antler throughout the winter before being cast in March or April. The cleaning of the antlers occurred at the seasonal onset of ovarian activity and was associated with an increase in the blood plasma concentrations of testosterone, androstenedione, oestradiol and progesterone. Ovariectomy in October resulted in premature casting of the hard antlers in that year. This was paralleled with a decrease in the plasma concentrations of oestradiol and progesterone, but not of testosterone and androstenedione. Antlers were redeveloped, cleaned of velvet and cast each year in the ovariectomized females, but the antler cycle was abnormal since the cleaning of the antlers began 1-3 months later than in intact females and was often incomplete, and the casting surface of the old antler was deformed. The hardening of the antlers in the ovariectomized animals in autumn was correlated with seasonal increases in the plasma concentrations of androstenedione, but not in the concentrations of other steroid hormones; provocation tests using GnRH and arginine vasopressin (AVP) illustrated that the adrenal gland was a likely source of androgens. The initiation of antler growth in spring always followed the seasonal increase in the secretion of prolactin. The overall conclusion is that a gonadal steroid hormone, possibly oestradiol, normally acts to induce and maintain the hard antler state in female reindeer. This hormonal mechanism acts to synchronize the antler cycle to the seasonal reproductive cycle with the hard antlers functioning as weapons in intrasexual competition in winter.