Antlers are developed in 36 of the 40 species of deer. The shape of the antlers is characteristic of the species and the size varies allometrically with body size. In most species, the antlers are produced only by males and grow at puberty under the stimulus of testosterone from the developing testes. The antlers then undergo an annual cycle of calcification, cleaning, casting and regeneration which is also regulated by the changes in the secretion of testosterone from the testes. Regeneration of the new antlers occurs during the period of minimal testicular activity, while maturation of the antlers occurs in response to increasing circulating concentrations of testosterone as the testes redevelop before the mating season. At this stage, the antlers become fully calcified and cleaned of the velvet-like skin, and the underlying bone dies forming the insensitive hard antlers which are used as weapons and display symbols when males compete for dominance and access to females in the rut. The hard antlers remain attached to the living pedicles on the skull as long as the testosterone levels remain increased. This varies from 3–9 months between species and usually reflects the period of male fertility. Once testicular activity declines, the hard antlers are cast and a new set rapidly regenerate through a process resembling wound healing. The successive sets of antlers developed by an individual increase in size and complexity in parallel with the increase in body size. Males continue to grow for at least half their life-span and are usually most successful in gaining access to females for mating when physically mature with large antlers. In reindeer/caribou, both males and females grow antlers, and the growth and seasonal replacement of the antlers is not totally dependent on the activity of the gonads, unlike other species. However, gonadal hormones still act to synchronize the antler cycle with the seasonal reproductive cycle. Female reindeer use their antlers as weapons in intra-sexual competition and the main conflict occurs over access to food in winter.