According to the Alaska Department of Fish and Game, while in the summer both male and female reindeer grow antlers each year, male reindeer drop their antlers at the beginning of winter, usually late November to mid-December.

Female reindeer retain their antlers till after they give birth in the spring. Therefore, according to EVERY historical rendition depicting Santa's reindeer, EVERY single one of them, from Rudolph to Blitzen, had to be a girl.