And she had found an old article (from 1976): Bergerud, A.T. 1976. The annual antler cycle in Newfoundland caribou. – Canadian Field-Naturalist 90: 449-463, in which it said that male reindeer (or caribou, since this was in North America) can have antlers in January, but she thought it to be very rare and could not find anything newer or more reliable on the subject, so I think you can quite fairly say that even though there is a small chance that some of Santa's reindeer are indeed, males, as indicated by the name of Rudolph, it is highly unlikely, especially when you consider the fact that they keep the antlers every year, according to legend.