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 later November to Mid
December. Female reindeer however, retain their antlers till after
giving birth in the spring.
Therefore, according to every historical rendition depicting Santa's
reindeer, every single one of them, from Rudoloph to Blitzen had to be
a girl.
We should have known. Only a woman, while pregnant, would be able to
drag a fat man in a red velvet suit around the world in one night and not get
lost.