Duh!

"According to the Alaska Dept. of Fish and Game, while both male and female reindeer grow antlers in the summer each year, mail reindeer drop their antlers at the beginning of winter. Female reindeer retain their antlers until spring, after they give birth. According to every historical and artistic rendition depicting Santa's reindeer, they all have antlers. Therefore, every single one of them, from Rudolph to Blitzen, are female. We should have known. Only women would be able to drag a fat man in a red velvet suit all around the world in one night and not get lost."