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

Therefore, according to every historical rendition depicting Santa's reindeer, every single one of them, from Rudolf to Blitzen........had to be a girl.

We should've known when they were able to find their way.