Abstract

The horns of rhinoceros are unique in that they lack a bony core. Contrary to popular belief, it is not made of compacted hair rather keratinized tubules of squamous cells within an amorphous, keratinized, epithelial, fusiform, interstitial cell matrix. Each tubule arises from a dermal papilla at the base of the horn. Upon keratinization, the epithelial cells die, forming the rigid horn. All growth of the horn takes place from the base. The horn is analogous to the hooves of horses (Equus sp.), beaks of birds and turtles, and the baleen of whales.^2,3

Two individuals (an adult male and a subadult female) partially avulsed the superior horn on separate occasions from horizontally placed caging materials. In both cases, >80% of the horn was avulsed and attached only at the rostral margin. The horns did not progress to falling off after several weeks to months and were mechanically removed under stationary operant conditioning. An initial myiasis and resultant infection of the germinal tissues in the male was treated by physical removal and application of fly repellent, and systemic antimicrobials and topical disinfection. The infection resolved over the course of 6 months. The female did not incur infection or myiasis and minimal treatment was necessary upon removal of the horn.

These cases demonstrate that partial avulsions may not progress to falling off in a timely manner and intervention may be indicated. Captive environments for rhinoceros should be constructed with vertical posts/beams to minimize the possibility of horn trauma associated with horizontal structures.^1,4,5

Acknowledgments

We thank the staff of the Kansas City Zoo for their care and assistance in resolving these two cases.

Literature Cited

1.  AZA Rhino Husbandry Committee. 1996. Design. In: Fouraka, M., and T. Wagener (eds.). Rhinoceros Husbandry Resource Manual. Fort Worth Zoological Park, Fort Worth, Texas. 33.

2.  Hieronymus, T.L., L.M. Witmer, and R.C. Ridgely. 2006. Structure of white rhinoceros (Ceratotherium simum) horn investigated by X-ray computed tomography and histology with implications for growth and external form. J Morphol. 267:1172–1176.

3.  Homberger, D.G. 2001. The case of the cockatoo bill, horse hoof, rhinoceros horn, whale baleen, and turkey beard: the integument as a model system to explore the concepts of homology and non-homology. In: Dutta, H.M., and J.S. Datta Munshi (eds.). Vertebrate Functional Morphology: Horizon of Research in the 21st Century. Science Publishers, Enfield, New Hampshire. 317–343.

4.  Miller, R.E. 2003. Rhinoceridae. In: Fowler, M.E., and R.E. Miller (eds.). Zoo and Wild Animal Medicine. 5th ed. WB Saunders, Philadelphia, Pennsylvania. 559.

5.  Nelson, L., and M.E. Fowler. 1986. Rhinocerotidae. In: Fowler, M.E. (ed.). Zoo and Wild Animal Medicine. 2nd ed. WB Saunders, Philadelphia, Pennsylvania. 935.