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 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.

We should've known. Only women, while pregnant, would
be able to drag a fat man in a red velvet suit all
around the world in one night and not get lost.