The cycle of antler growth, hardening, casting (dropping off) and regrowth is a hormonal response to seasonal changes in the amount of daylight. This cycle follows the rhythmic rise and fall of testosterone in blood levels. Testosterone is produced principally by the testes, and if certain levels of testosterone are not achieved, as in the case of castrates, the antlers will remain in velvet. Insufficient production of testosterone causes small, deformed antlers among old bucks. Conversely, too much testosterone can cause some does to sprout antlers.