Lawrence M. Witmer, PhD
Professor of Anatomy
Chang Professor of Paleontology

Dept. of Biomedical Sciences
Heritage College of Osteopathic Medicine
Life Science Building, Rm 123
Ohio University
Athens, Ohio 45701 USA

Phone: 740 593 9489
Fax: 740 593 2400



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Rhinoceros Horn Growth & Form
2006. Hieronymus, Witmer, and Ridgely. Journal of Morphology

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Image of sectioned rhino horn photographed under ultraviolet light.

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Drawing of rhino skull with CT-based images of horns in place. Redder colors represent denser portions.

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Image of CT slice through rhino horn. Whiter areas represent denser portions.

  Common Language Summary
  CT scanning sheds new light on the unique horns of rhinos. Many kinds of projecting structures on animals are lumped together under the term “horn.” There are horns on the heads of cattle, on the beaks of hornbills, and on the faces of chameleons, to name a few. In most cases, the external shape is defined by a bony horn core, and the surface of the horn is only a thin sheath of hardened keratins (the same materials in fingernails and hair). Rhinoceros are unique in having large horns that are composed entirely of tiny keratin tubes embedded in a keratinous matrix, without any bony core. CT scans and cross-sections of rhino horns show a dense central region that is reinforced by a combination of mineral (calcium) and melanin. These two components make the center of the horn more resistant to physical wear and breakdown due to UV light exposure whereas the softer outer part is more easily worn away during use. Much like a pencil, this difference in resistance leads to the characteristic elongate and sharply pointed shape. Dark patches of high mineral and melanin concentration within the horn match up very well with measurements of annual horn growth in wild rhinos, indicating that there are yearly “pulses” in the deposition of these compounds. The tissue of rhinoceros horn itself, with its tubules-within-matrix architecture, is very similar to a number of other hardened skin appendages, including the hooves of horses and cattle, whale baleen, and the tips of bird beaks, all of which have evolved this skin architecture independently.

News Release from Ohio University

Complete citation and full-text download (PDF):
Hieronymus, Tobin L., Lawrence M. Witmer, and Ryan C. Ridgely. 2006. Structure of white rhinoceros (Ceratotherium simum) horn investigated by x-ray computed tomography and histology with implication for growth and external form. Journal of Morphology 267(10)1172–1176.

Larger versions of Figures 1-3 from the original article

The Wilds: Cumberland, Ohio

The Phoenix Zoo

Research Partners: Sources of rhinoceros specimens

O'Bleness Memorial Hospital, Athens, Ohio
Research Partner: CT scanning site

Funding for this research comes in part from the following National Scientific Foundation (NSF) grants: NSF IBN-9601174 and NSF IBN-0343744 to Witmer, and NSF IOB-0517257 to Witmer & Ridgely. Other funding provided by Ohio University.
  note: Research in the Witmer lab does not involve experimentation on live animals.  Specimens of modern animals used in research are salvage specimens, obtained legally from commercial or governmental sources.
  Ohio University
Heritage College of Osteopathic Medicine
Irvine Hall, Athens, Ohio 45701
740-593-2530 740-597-2778 fax

Last updated: 11/23/2015