According to the Alaska Department of Fish and Game,
while both male and female reindeer grow antlers in the summer
each year (reindeer are the only  members of the deer family, Cervidae,
to have females grow antlers). 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 Rudolf to Blitzen........had to be a girl.

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