You probably will never spot Rudolph, but there are plenty of kinds of deer that exist around the world.

About 41 species of deer occur worldwide, and two of them, the elk and the white-tailed deer (Pennsylvania's state mammal), are found in Pennsylvania.

Incidentally, the familiar reindeer is actually a domesticated caribou. Reindeer have a slightly larger body size but shorter legs than wild caribou, because reindeer are bred for meat and milk production.

By the way, what did the reindeer say before telling Santa a joke? "This joke will sleigh you."

A distinguishing characteristic of deer is the presence of antlers. The only deer that lacks antlers is the Chinese water deer. Antlers grow from two permanent, skin-covered projections on the skull, known as pedicels.

Antlers are shed annually, in contrast to horns, which are retained throughout the life of such animals as mountain sheep, bison and the familiar domestic cow. The Père David's deer, native to China, is the only deer that grows two sets of antlers per year.

Antlers are typical of male deer, with the exception of caribou -- in this deer, antlers are found in both sexes.

However, in other deer species, like the white-tailed deer, antlers may occur in about one out of every 1,000 females. Antlers in females are retained for life (rather than being shed annually), remain covered in velvet and are not branched. Usually, females with antlers are fertile and can produce young.

Why do deer have antlers? At least five reasons have been proposed for the evolution and role of antlers.

The first -- and least likely -- reason for the evolution of antlers is that they help dissipate body heat because they are highly vascularized, which means they contain lots of blood vessels. The large ears of elephants, for instance, help them to lower body temperature. But if antlers in deer evolved for heat dissipation, we should expect the largest ratio of antler-to-body size to exist in tropical deer species. However, tropical deer, like muntjac of China, have very tiny antlers, whereas the antler-to-body size ratio of elk and moose in cooler, temperate latitudes is quite large.

A second reason for antlers is that larger antler size in males acts as a female attractant. There is no evidence for this, but females may select older, larger males over young, smaller males during the breeding season, or rut.

A third reason for the evolution of antlers is that they act as a signal to assess dominance in males. If this were true, a male during rut could assess a rival's strength without actually fighting and risking injury or possible death. However, the behavior of deer during male-to-male encounters -- for instance, the use of vocalizations or ritualized parallel walks -- is probably more indicative of a male's fighting ability than size of antlers.

A fourth reason for the evolution of antlers is that they are used as a defense against predators. If antlers were used as weapons, then antlers would be expected to occur in adult female deer because it is the female that cares for the young. Instead, deer use their hooves when attacked by predators. In a 12-year study of moose and gray wolves in Isle Royale National Park in Michigan, nine dead wolves were found. Two of these wolves died from malnutrition, perhaps related to broken ribs and associated infected wounds inflicted by hooves of moose, which are a large species of deer.

A fifth -- and most plausible -- reason for the evolution of antlers is their use in intraspecific (male-to-male) combat. In small deer, like muntjac, antlers are used to knock other males off balance, which then is followed by attacks with the canines. In large deer, antlers are used to damage rival males, protect from blows by a rival and engage in pushing contests to determine dominance.

In conclusion, the primary reason for the evolution of antlers in male deer is in their use in male-to-male combat during rut. The abnormal presence of antlers in female deer, which is caused by excessive secretions of the hormone testosterone, probably serves no purpose. Relatively small antlers in female caribou (and reindeer) may provide an important source of calcium during winter; in this species, antlers in females are retained throughout winter.

So, to impress your roommate with your wildlife knowledge -- what deer has antlers and loves cheese? Mickey Moose.