Antlers and Horns

views updated

Antlers and Horns

Antlers and horns are two kinds of ornamentation present on the front of the heads of mammals. Antlers consist of pure bone tissue and are shed and regrown annually, whereas horns consist of a bony knob and an exterior horn sheath and typically grow throughout life. Both structures were first seen in fossils from about 25 million years ago. Most even-toed ungulates (Artiodactyla) have head ornaments, such as deer, reindeer (antlers), antelopes, oxen, cows, and giraffes (horns). Some odd-toed ungulates (Perissodactyla), such as rhinoceroses, have horns. In most species, only males have antlers, but both females and males can have horns.

Antlers stem from the upper part of the frontal bone processes, called burrs. They grow by the accumulation of a cartilage -like bone matrix . The development of antlers begins in the spring with small nubs covered with "velvet," a layer of skin with oil and scent glands and nerves, as well as the sparse coating of hair that gives the velvet its name. Nutrients are supplied from the diet to the underlying bone through the velvet, and the antlers grow quickly throughout the summer. Not uncommonly in urban areas where the animal is malnourished or overpopulated the antlers are deformed, twisted and assymetrical because of inadequate nutrients. Toward the end of the velvet stage, growth slows and mineralization begins to increase. The velvet begins in spring and ends in the fall. During this period the interior of the bone becomes more dense, increasing the final weight from the beginning of mineralization process by about 70 percent. The velvet dies in the fall, and the animal rubs its antlers against tree trunks and branches to get the dead velvet off. Mating occurs later in the fall and in early winter when the antlers are at their peak size and weight, afterward the antlers are shed. Over a lifetime, the antlers usually grow more branches, or "points," so that an individual's approximate age can be estimated based on the number of points its antlers have.

Horns arise from an independent horn bone (the os cornu) that forms in the mesoderm (deep skin layers) of the forehead during fetal development, and the frontal bone fuses with the horn bones after birth. As the animal matures, the horn bones grow longer and wider, and the skin around the bone (the ectoderm) forms a horny outer covering, with the growth originating from the base. As it grows, the horn accumulates layers, and the horns of many species have grooves around them, the rings of which can be counted to determine the age of the animal. Although the structure of a horn is a single shaft, there are variations such as spirals and hooks or horn parts that are greatly broadened like those of the African buffalo, whose horn bases meet at the center of the head, and are broad and flat to form a giant shield over the top of the skull before curving up and outward.

Antlers and horns can be used as a defense against predators, but scientists believe that their primary function is to establish an individual's ranking within its species. Many animals point the ends of their horns or antlers away from their target during an advance, and adaptations such as branches, rings, and grooves facilitate direct, nonlethal combat with similarly adorned individuals. Antlers and horns are also used in ritual fights among individuals of the same species. Their size and scent indicate to other members of the same species an individual's rank and maturity, and this information often avoids the need to fight to establish the same ranking.

Jean K. Krejca