Growth and Casting of Horns by Pronghorns and Exfoliation of Horns by Bovids

Abstract

Growth of by mature male pronghorns (Antilocapra ameri- cana) in Montana ceased during August when the stratum germinativum became inactive under mature sheaths. During October after the rut, growth of new resumed under the old and the stratum germinativum proliferated soft, hairy integument. Annular swellings of this integument at the tops of the bony cores produced hard-keratin tips, which aided in loosening old sheaths that were cast in November. The stratum comeum of the new horn formed a tissue layer that permitted abscission to occur. Histologically, horn formation resembled that of skin. New consisted of bony cores approximately 134 millimeters (mm) long covered by gray hairy skin and topped by cones of hard horn 20-25 mm long. During December, rapid differential growth from the thickened integument at the tops of the cores extruded curved horn tips, and frontal prongs began forming approximately 50 mm above horn bases. By February, prongs and hooks were nearly developed. From March through July, hard keratin formed around the cores while skin around the bases of the cores thickened and extruded horny material distally, lengthening the horns. During keratinization, some was incorporated into the horn sheaths but the results are no more hair horns than those of bovids. Observations of four species of bovids, plus a search of the literature, revealed that at least one species from each of the five subfamilies of Bovidae exfoliate old horns, or outer layers of horns, once or twice during their lives. In those species, new horny material must de- velop under old in a manner similar to early horn growth by pronghorns. Further research is needed on exfoliation by Bovidae. At least six species of bovids possess lacrimal vacuities similar to those of antilocaprids. The diagnostic familial characteristics of Antilocapridae and Bovidae are similar, with the ex- ception of horn formation and casting which are shown here to be more alike than generally thought. Skeletal and dental characters of the earliest known fossils from each family are similar. We feel that reclassification of Antilocapridae to Antilocaprinae, the sixth subfamily of Bovidae, and the present subfamilies Merycodontinae and Antilocaprinae to tribes Merycodontini and Antilocaprini would more accurately reflect the true relationship of antilocaprids to other artiodactyls than does the present system.