Deer of the Southwest: A Complete Guide to the Natural History, Biology, and Management of Southwestern Mule Deer and White

Chapter 4. Antlers

Chapter 4 Antlers H umans have been intrigued by antlers since the beginning of time. Although valuable to early man as tools, they undoubtedly elicited the same admiration and curiosity they do today. What factors affect antler growth? How did antlers evolve? Do genetic factors or injuries play a more important role in the occurrence of nontypical antlers? The questions are infinite, but not all can be definitively answered. Many people erroneously use the word horns when describing antlers. The term horns is actually reserved for cranial appendages that consist of a bony core sheathed in keratin, a material much like fingernails. True horns on animals such as bighorn sheep, cattle, and bison are never shed and continue to grow throughout the life of the animal. Antlers are different from horns in origin, form, and function. The branching appendages that define members of the deer family are called antlers and are regenerated each year. Except for caribou, females do not normally grow antlers. The term antler is derived from the old French antoiller, a derivative of the late Latin antoculorum, meaning “before the eyes,” because this latter term was originally used to describe the brow tines on European deer (Lydekker 1898:5). 78  chapter 4 Antlerogenesis Antlerogenesis is a complicated term for an even more complicated process—the annual physiological production of antlers. Male fawns are born with almost imperceptible bumps on the frontal bone of their skull. At about the age of four months, male fawns experience a rise in testosterone that causes the growth of small platforms, called pedicles, on the skull. These pedicles usually appear as small, fur-covered knobs during the fawn’s first autumn. At this point, the ends of the pedicles are not polished and therefore are not shed; they remain dormant until the buck’s second fall. These buck fawns have small fuzzy knobs and are sometimes referred to as “nub bucks.” In areas with exceptional nutrition, some of these buck fawns may produce small spikes that are then polished and shed. The pedicles provide the structural base for the future development of antlers. If a buck fawn is castrated before his fourth month, he will never grow pedicles, and thus remain antlerless the rest of his life because he lacks sufficient levels of testosterone. Pedicle and subsequent antler growth can be induced, however, with an injection of testosterone. When a buck fawn reaches the age of nine to ten months, antler growth is initiated from the pedicles, and the youngster thus starts his first real set of antlers. Although testosterone is necessary for the development of pedicles, antler growth does not require large amounts of this hormone. Even moderate levels of this hormone actually inhibit the growth of antlers. Testosterone levels fluctuate throughout the year and are at a very low level in the spring when antler growth starts (G. Bubenik 1990a:296). Testosterone levels appear to increase slightly at the very beginning of antler growth, only to subside again during the growth period. A large number of other hormones, including IGF-1 and derivatives of testosterone , increase and decrease during various phases of the antler-growing period. This complicated orchestration of hormones is not yet completely understood, but serves to promote and sustain antlerogenesis. In adult deer, antler growth begins within a few weeks after the loss of the last set of antlers. In the Southwest, antler growth is under way by May in Coues whitetails and mule deer (Villa 1954, Hanson 1955, Truett 1971, Hoffmeister 1986, Ockenfels et al. 1991, Weber et al. 1995). The scab that forms over the wound left by the shed antler heals and becomes covered with fine hairs. This finely haired skin forms the beginnings of the velvet that will nourish and protect the growing antlers. As antler growth begins, the underlying pedicle gives rise to new antler material , which at this point is firm tissue composed primarily (about 80 percent) of protein. This protein matrix of growing antler tissue is cartilage-like and inundated with blood vessels. Most of these nutrient-transporting blood vessels rise up through the velvet lining, but inside the antler are blood vessels that rise through the pedicles. The visible grooves on the base and beam of hardened antlers are impressions left by the blood vessels in the velvet. antlers  79 The antlers are also richly endowed with a dense network of nerves. These...