Diceros is a polytypic genus, represented by a single extant species, the Black Rhinoceros – Diceros bicornis. This species once was widespread in sub-Saharan Africa, from Niger R. to the west in Somalia to the northeast and to the south in South Africa. It was never found in the Congolese rainforests.
The animal exists in a wide variety of habitats, including savanna, bushveld, and dry thornbush, from sea level to high mountain forests. The members of Diceros, like the other extant African rhinoceros genus Ceratotherium, have two nasal horns.
Diceros is distinguished from Ceratotherium (White Rhino) by the shorter skull, pointed upper lip, swayed back, and generally smaller size.
Fossil remains of species referable to Diceros have been found in many parts of Africa. Diceros douariensis was found in the Miocene and Pliocene of Tunisia and Diceros australis in deposits from the middle Miocene of Namibia. In East Africa, the genus was represented by Diceros praecox in the early to late Pliocene, although previously some of the remains had been classified under Ceratotherium.
Diceros bicornis first appears at the 4 mya level in Pliocene deposits in Kenya and Ethiopia, and at 2.5 mya the molar crowns became as high as in the recent specimens.
Diceros bicornis – Black Rhinoceros
The second smallest of the rhinoceros species, the Sumatran rhinoceros Dicerorhinus sumatrensis being smaller. It is located around 1.6 m at the shoulder and has a mass of around 800–1350 kg. Sometimes known as the Hook-lipped Rhino on account of its hooked prehensile upper lip, which it can use like an elephant’s trunk to grab the branches and pull them into the mouth (unlike the grazing White Rhino, the Black Rhino is a browser eating primarily trees, bushes, herbs, and succulents).
In silhouette, the neck hump is not as pronounced as that of the White Rhino. The much smaller head is generally held higher than the White Rhino unless feeding at ground level. The back has a much more concave saddle-like appearance in profile compared with the White Rhino whose prominent vertebral crest two-thirds of the way along the back is much more pronounced.
Rounded, trumpet-shaped ears pointed tips upwards from the middle of the ear. Ears are fringed with hair (as in the White Rhino), and the amount of hair on the ears varies between individuals. Body hairs are otherwise sparse. Body barrel-shaped due to an extended ribcage.
Overall body color is a marginally darker battleship grey than that of the White Rhino. However, both species of rhino can take on the color of the soil and mud in which they have been wallowing. The eastern subspecies have rib-like skinfolds on the flanks. There are also prominent folds of skin near the top of the front legs and where the upper part of each hind limb joins the body.
Limbs thickset with three toes on each foot, each with large nails that mark clearly in the spoor. Toes and feet are smaller than those of the White Rhino and so the spoor of the Black Rhino is smaller, more rounded, and with smaller indentations than the White Rhino, whose spoor has a more elongated appearance with a more pronounced groove in the center of the reverse side of the print. Cushioned pads on the soles of the feet have a hard surface with a mosaic of uneven cracks; patterns can be used to identify individual animals for short periods of time, as they can change.
The tail is short with a sparse fringe of bristly hairs. Some animals may miss part of their tails, and it is thought this is caused by unsuccessful hunting Spotted hyena – Crocuta crocuta attempts. Two horns are present on the snout; these are made up of compressed keratinous fibers and do not have a bony core. In general, the anterior horn is invariably longer than the posterior horn.
Horn length and basal circumference are greater in adult males than in adult females. In a sample of horns from KwaZulu–Natal and Kruger N.P., the mass of anterior and posterior horns amounted to some 2.65 kg of horn for animals. The base of the anterior horn is circular in contrast to the White Rhino, whose anterior horn has a squarish base. In general, the horns of Black Rhino are thinner and less thick than those of White Rhino.
The intrinsic growth of the anterior horn is similar to that of the White Rhinoceros, determining regrowth rates for dehorned Black Rhino at 60 mm and 27 mm per year for the anterior and posterior horns respectively; regrowth was faster in the young, 89 mm and 44 mm per year respectively.
Horns from both African species are much larger than those of the three Asian species. Isotopic ratios of carbon and nitrogen of rhinoceros horn differ between the two species reflecting their different diets. Females bear two inguinal nipples. Males lack a scrotum, and the muscular penis points backward when enclosed in its sheath. Apart from genitalia, there is little sexual dimorphism, except that males attain a bigger body mass than females, and have noticeably more heavily set chests and necks.
In the south-central black rhino, females invariably have thinner and longer horns than the adult males, which tend to have shorter chunkier horns. However, the horns of the eastern black rhinoceros do not clearly exhibit this trend. Skull is shorter than that of the White Rhino, the supraoccipital crest does not extend upwards and backward to the extent seen in the White Rhinoceros.
The occipital crest does not have a large wrinkled area at the top characteristic of the White Rhino, while zygomatic arches are heavily built to give a firm attachment for the masseter muscles that activate the massive lower jaw.
One to two lower incisors in the deciduous dentition. In the adult dentition, sometimes only three premolars are present in the upper and lower jaws. The upper and lower second molars are the largest of the cheek teeth.
D. b. longipes (Western Black Rhino). It formerly occurred in the savannah of western central Africa and was more recently confined to N Cameroon; recently declared extinct by the IUCN following the failure of surveys to find any surviving individuals. Legs are slightly longer than in relation to body length (hence the subspecies name D. b. longipes).
The high degree of homozygosity among the nine loci studied in the single Western Black Rhino sample showed the least genetic diversity of all the individuals examined and was consistent with a major loss of genetic diversity in this population.
D. b. michaeli (Eastern Black Rhino): historically, in S Sudan, Ethiopia, from Somalia to Kenya, NC Tanzania, and Rwanda; currently the only confirmed survivor in Kenya, northern Tanzania, and Rwanda, with extralimital population established in South Africa in Limpopo Province. Longer, finer, and more curved horns than two southern subspecies; visible skin folds, reminiscent of ribson on its flanks (these folds are not obvious in the other subspecies).
D. b. minor (Southern-central Black Rhino): KwaZulu–Natal in South Africa through Swaziland, Mozambique (where now possibly extinct), Zimbabwe, N Botswana (reintroduced), and Malawi into W and S Tanzania. Formerly in N Angola and S DR Congo. The smallest of the subspecies and in some parts of its range is vulnerable to developing large red sores on its chest.
D. b. bicornis (South-western Black Rhino): formerly in Namibia, S Angola, W Botswana and Western Cape and Eastern Cape, south of Kei R. (South Africa), though now extinct in Botswana with perhaps a few individuals surviving in S Angola. Largest of the subspecies; in some parts of the range, and most noticeably in the very arid west Kunene region in NE Namibia, often possesses very thick straight horns (the anterior horns of other Black Rhino subspecies are invariably slim and curved when the animal is an adult).