Antlers are found in males of all deer species, with the exception of caribou (or reindeer), in which both male and females have antlers.

Reindeer, by the way, are domesticated caribou and have large bodies and short legs compared to wild caribou.

So, which of the reindeer have the shortest legs? - The shortest reindeer.

In all deer species, male antlers are deciduous, meaning they are shed each year.

Occasionally, antlers may occur in one out of every 1,000 white-tailed does (female deer) and typically are retained for life, remain in velvet, and are unbranched.

At least five hypotheses have been proposed for antlers in male deer. First, antlers have evolved to help dissipate body heat, particularly when the antlers are growing, because they are highly vascularized.

If this were true, we would expect antlers to grow during the hottest time of the year in all deer species.

Yet antlers in the roe deer of Europe are still growing in winter. Moreover, moose antlers are already "clean" in late summer, having gone through the mineralization and hardening phases of the antler cycle well in advance of the hottest times of the year.

However, tropical deer, like the Reeve's (Chinese) muntjac, has very tiny antlers, whereas the antler to body size ratio of elk and moose in cooler temperate latitudes is quite large. Furthermore, female deer rarely have antlers, but females presumably generate a similar amount of body heat as males.

Second, larger antler size acts as a female attractant.

There is no evidence for this hypothesis, but perhaps females may select older, larger body-sized males over young, smaller males.

Third, antlers act as a signal to assess dominance. If this were true, a male during the rut (breeding season) could assess age, social status, and strength without actually fighting or risking injury and perhaps death (about 5 percent of the mortality in male elk is attributed to injuries in male-male fights).

However, the behavior of deer during male-male encounters, e.g., use of vocalizations, ritualized parallel walks, are probably more indicative of a male's fighting ability than the size of antlers.

Fourth, antlers are used as a defense against predators. If antlers were used as a major weapon against predators, then antlers would be expected in adult female deer.

Furthermore, antlers would be most valuable in winter and early spring when predation pressures often are highest during the year. Small deer, such as the Reeve's muntjac, rely on flight or, when cornered, used razor-sharp canines against enemies. Large deer, like white-tailed deer and moose, typically use hooves against attacks by predators. For example, in a 12-year study of moose and gray wolves on Isle Royale National Park, nine dead wolves were found. Two of these wolves died from malnutrition, which is perhaps related to broken ribs and associated infected wounds inflicted by hooves of moose.

The fifth and most plausible reason for the evolution of antlers, is intraspecific (male-male) combat. In small deer, like muntjac, antlers are used to knock another male off balance, which then is followed by attacks with the canines. In larger deer, antlers are used to damage rival males, protect from blows by a rival, and engage in pushing contests to determine dominance. Hence, antlers are best developed in polygamous species where one male can mate with a large number of females.

Thus, in conclusion, the primary reasons for the evolution of antlers in male deer in decreasing order of importance are intra-specific combat, defense against predators, and assessment of dominance or fighting ability.

The abnormal antlers in females of some species, such as white-tailed deer, probably serves no purpose but is simply caused by excessive secretions of the hormone testosterone. The relatively small antlers in female caribou may serve as an important source of calcium during winter; in this species, antlers in females are retained throughout winter.

By the way, the white-tailed deer is Pennsylvania's state mammal. So, for all you hunters out there -- an ecology fact: Why do some deer where sunglasses during the hunting season in Pennsylvania? - So you can't recognize them.