December 7, 2007
By Susan Benson
Director of Education
Cable Natural History Museum
The nine-day gun-deer season has passed, but hunting continues to dominate conversations around town. Going into supermarkets, hardware stores, or gas stations, you can’t help but hear the stories, and those stories often include talk about antlers and horns. These two terms are often used interchangeably, but they actually are different physical structures. Antlers and horns, like hooves and claws, are both modified parts of the epidermal (skin) layer of mammals, but that is where the physical similarities end.
Horns are grown by both males and females in the family Bovidae (cows, sheep, and goats). They are modified bone covered by a cone-like layer of keratin, or modified hair tissue – the same material that forms our own hair and fingernails. Horns grow above the skull but below the skin, entirely separate structures that do eventually fuse to the skull during development. Horns are not shed each year, but are retained and, for many animals, continue to grow throughout life. Also, horns form a single tine and do not branch out. There is one exception to this rule. The pronghorn antelope (which is in a family separate from the cows, sheep, and goats) has horns that are branched, and it is the only horned mammal that annually sheds the keratin sheath that covers the horns.
Antlers are modified bones that do not have the keratin covering. Male deer, elk, and moose, which comprise the family Cervidae, grow antlers. The one exception to this is the caribou; both males and females of this species grow antlers. They are attached to the skull by way of a short base known as a pedicel. Antlers, unlike horns, are grown and then shed each year as the pedicel loses calcium, weakening its connection with the antler. Growth and development of antlers are controlled by hormones and, in the northern hemisphere, the increasing duration of daylight. The timing of the “drop” in the winter coincides with declining testosterone levels and decreasing daylight.
Antlers also have a covering over them, but it is not a permanent cover like the keratin sheath that grows over horns. The velvet that temporarily covers antlers as they grow is soft hair and skin that contains blood vessels and nerves. Growth of the antler stops and the velvet dies when testosterone levels reach their peak each fall. The velvet is then shed, in large part by the animal rubbing its antlers on trees and woody shrubs. This rubbing also polishes the antlers, which become stained by the blood that once pulsed through the vessels in the velvet.
Both horns and antlers are used by males to establish dominance and thereby gain access to females during the breeding season. Among the horned animals, males and females can be distinguished from one another by differences in the size of the horns. Male horns tend to be thicker at the base, while those on the females tend to be straighter and thinner, which may allow them to be used as a stabbing weapon.
Of course, there is variation among the horned animals, too. The horn of a rhinoceros is not considered a “true horn” because it is composed of epidermal cells and fibers rather than a bony core surrounded by a keratin sheath. The rhinoceros horn also does not grow from the skull, but from the tip of the snout. Giraffes also have horns. Their horns are composed of bone, and they arise from the skull like other “true horns,” but they do so from the rear of the skull rather than the front. Both male and female giraffes have horns, but unlike other horned animals, newborn young have them as well.
So if you are hunting and lucky enough to get a buck this season, or if you happen to find a skull or antlers during a walk in the woods, take a look at these fascinating structures. Compare them with horns (we have a few in the museum you can come and see) and you may be able to see some of the differences between them.
Nature Watch is brought to you by the Cable Natural History Museum. For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 43570 Kavanaugh Street or on the web at www.cablemuseum.org to learn more about exhibits and programs.