Both male and female reindeer grow antlers in the summer each year (the only members of the deer family, Cervidae, to have females do so).
Male reindeer drop their antlers at the beginning of winter, usually late November to mid-December. Female reindeer retain their antlers until 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!
This makes absolute sense of course as only the female of the species would be:
1.organized enough to drag around a fat old bearded guy seated in a totally loaded sleigh to points all over the world;
2. know exactly what each child wants to have;
3. make all the deliveries in one night; and
4. NEVER ONCE HAVE TO STOP AND ASK FOR DIRECTIONS!