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Reindeer (or caribou), Rangifer tarandus, is the only extant species of deer in which females as well as males normally develop antlers that are cast and regrown each year. This study investigated the role of ovarian oestradiol in the regulation of the seasonal antler cycle in female reindeer. Ovariectomized Norwegian reindeer living outdoors in northern Norway (69 degrees N) were treated with continuous-release subcutaneous Silastic implants containing oestradiol, which maintained the blood concentrations of oestradiol within the physiological range for the mating season from June to October-November. The treatment with oestradiol induced the synchronized maturation of the antlers and rapid cleaning of the velvet-like skin in August-September in the ovariectomized reindeer, a pattern very similar to that observed in ovary-intact controls living under the same conditions. The removal of the steroid implant in October-November caused the premature casting of the antlers in early winter in two of five animals, while the remainder cast at the normal time in spring; this response was seen whether the animals had received one or two oestradiol implants in autumn. The antlers developed by the ovariectomized, oestradiol-treated females were significantly heavier and carried more branches than the ovariectomized animals without oestradiol replacement, and were marginally heavier than the antlers of intact controls. These results support the view that oestradiol is the biologically active steroid secreted by the ovary in intact female reindeers that induces the normal development of the antlers. Oestradiol stimulates the growth and mineralization of the antler bone, the cleaning of the velvet, and suppresses the casting of the hard antlers. This endocrine control ensures that the hard antlers, which function as weapons, are retained throughout the autumn and winter when the females are normally pregnant and when competition between females over food in the snow is most intense; hence there is a reproductive advantage to explain the evolution of antlers in females.

Role of oestradiol in the regulation of the seasonal antler cycle in female reindeer, Rangifer tarandus