According to the Alaska Department of Fish and Game, 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. We should've known that when they were able to find their way.

The eight (or nine, including the red-nosed one) flying reindeer that pull Santa's sleigh as he delivers toys to all the good girls and boys around the world on Christmas Eve have traditionally been assigned masculine names (such as "Rudolph" and "Blitzen") and depicted with masculine personalities. However, the perennial bit of Christmas humor quoted above offers physiological evidence -- specifically, the presence of antlers -- demonstrating that Santa's reindeer are actually all female, a suggestion borne out by the University of Alaska Fairbanks' Reindeer FAQ, which notes that only female reindeer retain their antlers much past the beginning of winter:

Reindeer and caribou are the only deer (Cervidae) where males, females, and calves produce antlers. Substantial growth of calf and female antler can be obtained with good nutrition. Some females can grow small shovels (eye-guards), normally grown only by bulls. Like other deer, reindeer shed their antlers annually. Bulls drop their antlers by January, and pregnant cows and calves retain theirs until spring. Normally, pregnant cows retain their antlers a week or so after calving for advantage over food resources. New antler growth in the spring and summer is nourished by a highly vascular covering called velvet which is shed in August. Bull antler hardens (ossifies) in June and cow antler in July. The primary function of antler is for gaining social dominance.

Does the presence of antlers indicate that Santa's reindeer are really all female? Well, it's not impossible that a male reindeer could retain his antlers as late as December 24; it's just not typical. And there are some other possible explanations for Santa's reindeer having antlers, such as the following:

Santa's sleigh helpers might also be castrated males, known as steers, said Greg Finstad, who manages the Reindeer Research Program at the University of Alaska Fairbanks. Sledders most often use steers because they maintain their body condition throughout the winter, he said. Bulls are tuckered out from rutting season when they mate with as many as a dozen females in the months leading up to December. That leaves them depleted and too lean to pull a sleigh or sled through heavy snows.

Besides, as the web site of Alaska's Department of Fish and Game notes, Santa's reindeer are an uncommon subspecies (saintnicolas magicalus) that exhibit characteristics atypical of other subspecies of reindeer:

Among the common subspecies of reindeer, the largest bulls shed their antlers in late October and the small bulls and non-pregnant cows shed their antlers in April. It is not known when or if Santa's reindeer shed their antlers. We do know, from a few furtive sightings, that their antlers appear to be extremely velvety and robust in late December. Santa's reindeer also have the unique and remarkable ability to fly. The Claus' have helped to further develop and condition this ability enabling them to fly great distances in a very short time period, provided they receive frequent carrot snacks.

The way we see it, male reindeer retaining their antlers into late December is not nearly as unusual as a fellow who lives at the North Pole, spends the year making toys for millions of children all over the world, and manages to deliver all of them in a single evening. If Santa could develop a way to make reindeer fly, his coming up with a way to ensure that his bulls kept their antlers until late December must have been a piece of cake.