Horns and antlers

Horns, antlers, or hornlike structures are found in many ruminant herbivores. They are hard, pointed outgrowths that arise at the front of the heads of many herbivores. Animals exhibiting such growths include cattle, deer, giraffes, antelope, and rhinoceroses, although the actual composition of the structures varies between species. In the case of true horns, as on cattle, they are permanent appendages that enlarge the bony front of the skull and are covered in hard proteins including keratin. In the case of antlers, found on deer, the structures are temporary bones arising on—but separate from—the skull, and they fall off and regrow every year. In other cases, what is commonly called a "horn" is in fact another type of growth. For example, rhinoceroses have horns made purely of keratin, with no living bone core, and giraffes have skin-covered bone growths called ossicones that project from further back in the skull rather than from the frontal bone.

Tusks often serve similar purposes to horns or antlers, but are structurally very different, consisting of overgrown teeth. Some non-mammals also have hornlike structures, such as the horned lizard, various species of chameleons, and many insects, but these are not horns in the true sense.

Regardless of species, horns and antlers serve very important protective functions, both for the individuals having them and for the family groups or herds to which the individuals belong. Often, males use their horns or antlers to fight other males in the group for ascendency. In many horned species, an ascendant male does not fight much but faces down rivals with gestures, including displaying and threatening to use his horns.

The protective function of horns and antlers may explain why, most often, it is the males within a species who have horns, while the females are hornless. The males protect the herds or other family groups from predators. Although horns and antlers are most often found in males, females may have the structures as well, for example, as with giraffes, oxen, and reindeer or caribou. When females of a species have horns or antlers, they are almost always much smaller than those found in males of the same species.

The True Horns

True horns are pointed, permanent, bony structures most often seen on the heads of male ruminant mammals. The horns of females, where present, are smaller. Most horned animals—such as cattle, sheep, goats, and antelope—have a single pair of horns, although some species have other variations. True horns have cores of bone which are extensions of the skull’s frontal bone. Atop the bone core is a layer of skin, rich in a very tough, fibrous protein: keratin, which also makes up human hair and nails. The high keratin content in this skin makes it an extraordinarily tough and durable covering for the underlying bone.

Males and females of many animal species grow horns. The horns range from straight spikes to elaborately curved varieties. However, except in pronghorn antelope, the horns do not form branches, as in deer and other animals which shed their antlers every year. Animals having horns keep them for life. In pronghorn antelope the horn coverings are shed and grow again every year. This allows horn enlargement that would otherwise not occur.

Antlers

Antlers are horns which are shed each year and then grow again; they are found only on various species of deer in the family Cervidae. In most antlered species only males have antlers. Like true horns, antlers grow out of the bones of the skull. They arise from permanent frontal bone structures called pedicles. At first the antlers have soft, velvetlike coverings of skin over their bone. Instead of hardening, as in true horns, this covering dies off and is rubbed away by the animal. The rubbing is believed to be due to itching caused by the dead velvet. Many zoologists propose that an advantage associated with rubbing off dead velvet is the development of good spatial perception of a stag’s antler size and shape that helps to keep him from entanglement in underbrush and forests. Antlers have the same functions as true horns: protection and use or display to develop ascendancy in a herd.

Antlers begin their growth in early summer, on the skull’s frontal bone. They begin as small nubs that increase in length over time. The growing antler, sensitive to the touch, is covered with velvet, soft, smooth, and full of blood vessels. This gives it an abundant blood supply, carrying to growing antlers all the nutrients required for the two to three months of the growing period. At first, antlers are made of connective tissue. As time goes on, this tissue calcifies and becomes solid bone.

Antler growth is greatest in the late summer. At that time, hard tissue grows around the base of each antler, cutting off its blood supply. This kills the velvet, which loosens and is rubbed off by the stag. Antlers reach their full glory in the mating season, when they can be used to impress potential mates and to vanquish other males. Zoologists observe that the blood running through antler velvet cools down and this cool blood makes stags and bulls more comfortable on hot days. Antlers are shed between January and February, just following the mating season, when they are no longer useful.

The first growth of antlers occurs when a stag or bull elk is one to two years old. At first, the antlers are small, straight, and spikelike. At that time young males are often called spikehorns by hunters. As an antlered male grows older its yearly antler crop branches out, having more prongs (points). In old, antlered males these horns may spread to widths of six feet and be several feet tall. Hunters often value large antlers with many "points," or branches, as trophies.

Antler growth is regulated by hormone secretions from the pituitary gland and the testes. The growth begins outside of the breeding season, when the testes are inactive. When the testes begin to make androgens, in preparation for breeding, both antler calcification and velvet tissue death occur. Decreased androgen production in winter leads to shedding of antlers.

Antlers, Horns, and Species Life Cycles

In the fall (September to October), American elk—called wapiti by some Native Americans—mate, after crashing battles between bull elk, who joust with many-pointed antlers. This identifies winners, who thoroughly impress cow elk. Then each bull mates with his harem of cows and they wander off to form part of a huge winter herd of elk of both genders and all ages.

By early winter, bull elk shed their heavy, four- to six-foot-wide antlers, unneeded burdens that would diminish their chances of surviving the cold winter. Winter passes and by the late spring (May and June) the cows give birth to young, usually one per mother. At this time, new antlers begin to sprout from the pedicles on foreheads of bulls over a year old.

The antlers of elk follow the same growth pattern as in deer, leaving the bull elk with a strong bony rack of antlers in the fall, just right for jousting at the mating season. With some variation, in true-horned species—such as pronghorns, horned sheep, goats, and bovines—the overall story is about the same. Offspring are conceived in the fall after jousts, and life goes on. In some species females have horns or antlers, and in others small family groups are formed. Sometimes males hardly interact with females after copulation. However, a common thread to all their lives is the possession and use of horns or antlers.

Principal Terms

androgen: a male hormone (especially testosterone) made by the testes

frontal bone: the bone which, vertically, makes up the forehead and is important, horizontally, to formation of the top (roof) of the orbital and nasal cavities

keratin: a tough, fibrous protein which is a major component of hair, nails, hooves, and the outer covering of true horns

pedicle: a small bone spur from which an antler grows (for example, in deer)

ruminant: a hoofed animal, with a stomach divided into four parts, that chews a cud of regurgitated, partly digested food

true horn: the permanent horns found in animals such as cattle, sheep, and goats

Bibliography

Bies, LeAnn, and Phil Myers. "Horns and Antlers." Animal Diversity Web. Regents of the University of Michigan, 2014. Web. 3 Oct. 2016.

Bubenik, George A., and Anthony B. Bubenik, eds. Horns, Pronghorns, and Antlers: Evolution, Morphology, Physiology, and Social Significance. New York: Springer-Verlag, 1990. Print.

"Difference Between Antlers and Horns." Yellowstone National Park. National Park Service, 2016. Web. 3 Oct. 2016.

Goss, Richard J. Deer Antlers: Regeneration, Function, and Evolution. New York: Academic Press, 1983. Print.

Petersen, David. Racks: The Natural History of Antlers and the Animals That Wear Them. Santa Barbara: Capra, 1990. Print.

Russell, H. John. The World of the Caribou. San Francisco: Sierra Club Books, 1998. Print.

Van Wormer, Joe. The World of the American Elk. Philadelphia: J. B. Lippincott, 1969. Print.