The earliest known horns appear as bony projections on the heads of certain ceratopsid dinosaurs whose skeletons have been recovered from the Cretaceous beds of North America. For instance, Triceratops, as its name indicates, sported a horn over each eye and a third one on its nose. Even among modern reptiles there are a few rare bizarre lizards with nose horns, for example Chamaeleon oweni and Ceratophora stoddaerti (Fig. 174). The “horned toad,” Phrynosoma, of our southwestern desert region, also a lizard and not an amphibian as its name would imply, is another reptilian specialist in horns. Aside from these few cases of horned lizards, horns are peculiar to mammals and are coupled with hoofs that characterize the ungulates.
In early Triassic times certain ungulates that lived in North America outdistanced dinosaurs in the number of their horns. Dinoceras, for example, had six large horns on its head. It is probable that these conspicuous bony projections were capped over by homy sheaths, although positive evidence is not furnished by the fossils of these animals. There are four general kinds of ungulate horns known today, namely, keratin-fiber horns, antlers, pronghorns, and hollow horns.
Keratin-fiber horns are made up of hairlike keratin fibers produced from the corneal layer of the epidermis and cemented together in a hard compact mass. They are entirely epidermal and have no bony core. The Indian rhinoceros carries one of these homs on its “nose,” as its name indicates (rhino, nose; ceros, horn), while the African rhinoceros has two, arranged tandem-fashion instead of side by side in the conventional way of paired structures. It is reported that Bos triceros, one of the kinds of native African cattle, also has a median horn of this curious type, as its species name indicates.
Antlers are commonly borne by the various representatives of the prolific and diversified deer family (Cervidae), ordinarily only by the males, but in the case of the reindeer and the caribou by both sexes. They consist of bony outgrowths from the skull, which at first are entirely covered over by hairy skin. While in that condition a stag is said to be “in velvet” (Fig. 175). Later the skin dries and becomes rubbed off, leaving the antlers as unadorned bone, at which time it is incorrect to include them among integumental structures. At the end of the second year, before the mating season, the antlers weaken at the base next to the skull by the breaking down and absorption of some of the bony tissue, and are broken off. The surrounding skin grows over the wound thus made and a new pair of antlers “in velvet” grows out (Fig. 176); this time with an additional prong. Thereafter each successive breeding season is celebrated by new antlers, usually with a regular increase in the number of prongs (Fig. 177). This physiologically expensive process of shedding and renewing the antlers does not occur in castrated bucks, which is evidence that it is determined by the secretion of sex hormones. The antlers of the fossil Irish elk that once roamed the boglands of Ireland grew to an enormous expanse, as if they started and could not stop. Doubtless their excessive size was a factor in the extinction of this ancient animal.
That “fantastic deer,” the giraffe, has a stubby pair of single antlers that are permanent, and remain in velvet throughout life.
The lateral prongs of the reindeer’s antlers are greatly flattened, serving as snow-shovels to aid these arctic dwellers in getting at the snow-covered “reindeer mosses” on which they feed in winter.
There are two species of ungulates with pronghorns. They are the pronghorn antelope, Antilocapra americana (Figs. 178), and the “saiga antelope” of the Russian steppes, Colas tatarica, both of which possess permanent bony horns covered with a thimble-like sheath of horny integument that is periodically shed and renewed without the loss of the bony core.
Finally, the most familiar kind of horns are the hollow horns of domestic and wild cattle, sheep, goats, and antelopes, which are usually present in both sexes.
Unlike pronghorns, hollow horns are not periodically shed. As the horn wears away it is renewed from the Malpighian layer of the epidermis, just as any other dead corneal structure is restored from within, the hollow corneal layer fitting over a core of living bone attached to the frontal region of the skull.
Hollow horns do not branch like antlers and pronghorns, but they assume a considerable variety of forms all the way from the majestic graceful spread characteristic of the Texas steer of the range, to the “cow with the crumpled horn.” Polled, or hornless cattle were known long before cattle were domesticated by mankind, as shown by rude drawings of Palaeolithic polled cattle, depicted on the walls of prehistoric caverns of the cave dwellers in France and Spain.