Antlers are unique to cervids and found mostly on males: only caribou and reindeer have antlers on the females, and these are normally smaller than those of the males. Nevertheless, fertile does from other species of deer have the capacity to produce antlers on occasion, usually due to increased testosterone levels.
Each antler grows from an attachment point on the skull called a pedicle. While an antler is growing, it is covered with highly vascular skin called velvet, which supplies oxygen and nutrients to the growing bone. Antlers are considered one of the most exaggerated cases of male secondary sexual traits in the animal kingdom, and grow faster than any other mammal bones. Growth occurs at the tip, and is initially cartilage, which is mineralized to become bone. Once the antler has achieved its full size, the velvet is lost and the antler's bone dies. This dead bone structure is the mature antler. In most cases, the bone at the base is destroyed by osteoclasts and the antlers fall off at some point. As a result of their fast growth rate, antlers are considered a handicap since there is an incredible nutritional demand on deer to re-grow antlers annually, and thus can be honest signals of metabolic efficiency and food gathering capability.
In most arctic and temperate-zone species, antler growth and shedding is seasonal and controlled by the length of daylight. In tropical species, antlers may be shed at any time of year, and in some species such as the sambar, antlers last several years. Some equatorial deer never shed their antlers.
The ancestors of deer had tusks (long upper canine teeth). Antlers appear to replace tusks; two modern species, the musk deer and the water deer, have tusks and no antlers, the muntjac has small tusks and small antlers, and other deer have full-sized antlers and no tusks. Literature concerning the evolution of antlers concludes that the diversification of antlers, body size and tusk-like upper canines have been strongly influenced by changes in habitat and behavior (fighting and mating).
Caribou and reindeer use their antlers to clear away snow so they can eat the vegetation underneath. This is probably why females of this species evolved antlers. Females may have evolved antlers due to intraspecific female competition during winter foraging.
In moose, antlers appear to act as large hearing aids. Moose with antlers have far more sensitive hearing than moose without, and a study of trophy antlers with an artificial ear confirmed that the antler behaves like a parabolic reflector.