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's in mid-December. Female reindeer retain their antlers till after they give birth in the spring. Therefore, according to every historical rendition depicting them - From Rudolph to Blitzen - Santa's reindeer had to be girls.We should've known. Only women would be able to drag a fat-ass man in a red velvet suit all around the world in one night and not get lost!
(I know; I know - you're supposed to wait until after Thanksgiving to start on Christmas, but this was just too funny.)