Structure of white rhinoceros (Ceratotherium simum) horn investigated by X-ray computed tomography and histology with implications for growth and external form.

The nasal and frontal horns of two individuals of Ceratotherium simum were examined by x-ray computed tomography (CT scanning), gross observation of sectioned horn, and light microscopy of histological sections of the horn tissue. CT scans of both sets of horns reveal a periodic banding pattern that is evident upon gross observation of sections as darker bands of tissue. The overlap of these bands in both histological and CT slices suggests the presence of both a photoabsorbent component (melanin) and a radiodense component (calcium phosphate salts, most likely hydroxyapatite or octocalcium phosphate). The distribution of these two components in the horns is hypothesized to contribute to the differential wear patterns that produce the characteristic sweeping conical shape of rhinoceros horn from what otherwise (in the absence of wear and UV exposure) would be cylindrical blocks of constantly growing cornified papillary epidermis. Although extant rhinocerotids are unique in possessing a massive entirely keratinous horn that approximates the functions of keratin-and-bone horns such as those of bovid artiodactyls, the tissue structures that make up the horn are strikingly convergent with other examples of papillary cornified epidermis found in horses, artiodactyls, cetaceans, and birds.