Rhinoceroses (Rhinocerotidae)

(Rhinocerotidae)

Class: Mammalia

Order: Perissodactyla

Family: Rhinocerotidae

Thumbnail description
Large, heavily built ungulates with three toes on each limb, one or two horns on the snout, and skin mostly devoid of hairs

Size
Shoulder height: 54–73 in (135–185 cm); head and body length: 100–150 in (250–380 cm); body mass: 1,750–5,000 lb (800–2,300 kg)

Number of genera, species
4 genera; 5 species

Habitat
From rainforest through savanna to semidesert

Conservation status
Critically Endangered: 3 species; Endangered: 1 species; Lower Risk/Near Threatened: 1 species

Distribution
Africa and tropical Asia; formerly also Eurasia

Evolution and systematics

The rhinoceros lineage split from the tapirs and equids in the late Eocene. The family was far more abundant and species-rich during the later Tertiary period than today. Among the Oligocene rhinoceroses, Indricotherium asiaticum, standing 16.4 ft (5 m) tall at the shoulder, was the largest land mammal ever. Teleoceros was a squat North American form with a single small horn on the end of the nose, while the Diceratheres had two horns side by side on the snout. Elasmotherium sibiricum was a Pleistocene giant with a huge single horn in the frontal region. The five extant species of rhinoceros fall into three distinct subfamilies. The Asian twohorned rhinos, or Dicerorhinae, may be traced back 40 million years to Dicerorhinus tagicus, an animal the size of a small tapir. One of its descendants was the woolly rhinoceros, Coelodonta antiquitatis, which was widespread through northern Eurasia during the Pleistocene ice ages. This species was primarily a grazer, as attested by its lengthened head, lack of incisors and canine teeth, and high-crowned cheek teeth. Two other rhinos from this subfamily occurred in Europe during the Pleistocene: the steppe rhinoceros, Dicerorhinus hemitoechus, and Merck's rhinoceros, Dicerorhinus kirchbergensis, which was more a forest inhabitant. The sole surviving species, the Sumatran rhinoceros, Dicerorhinus sumatrensis, has changed little from the Oligocene form.

The Asian one-horned rhinoceroses in the subfamily Rhinocerotinae can be traced back to Gaindatherium browni from mid-Miocene deposits in India. Of the two surviving species, the Javan rhinoceros, Rhinoceros sondaicus, is the more ancient, having changed little during the late Pleistocene in the last two million years.

The two African rhinoceroses represent the subfamily Dicerotinae. The earliest form was Paradiceros mukiri, which was found in Miocene deposits at Fort Ternan in Kenya and Beni Mellal in Algeria, dated to 12 million years ago. Rhinoceroses from this subfamily were found from Spain to Turkey and Iran during the late Miocene. The genus Ceratotherium first appears in late Pliocene deposits at Lange-baanweg in the Cape and elsewhere. The modern species

Ceratotherium simum is especially numerous in Pleistocene deposits at Olduvai Gorge in Tanzania. Some workers regard the distinctions between Ceratotherium and Diceros as insufficient to warrant the generic distinction.

Physical characteristics

Together with the elephants and the hippopotamus, rhinoceroses constitute the "megaherbivores," those species weighing over 2,200 lb (1,000 kg) as adults. Rhinos are large, graviportal animals with relatively short limbs and barrelshaped bodies. The three toes on each foot leave a track resembling the ace-of-clubs. The skull includes enlarged nasal bones and an extended occipital crest, with the eyes perched strangely on the sides of the head. The cavity occupied by the nasal sinuses exceeds that of the brain. The chewing teeth comprise three molar-like premolars and three true molars in each half-jaw. In grazers like the white rhino and Indian rhino (Rhinoceros unicornis), these are high crowned with complex grinding surfaces, while in browsers the teeth are lower crowned with prominent cusps. The African species completely lack incisor and canine teeth, while the Indian and Javan rhinos retain a pair of tusk-like incisors in the lower jaw, and the Sumatran rhino (Dicerorhinus sumatrensis) has tusk-like lower canines as well as upper incisors. The skeleton is massively constructed to support the heavy body weight, with the vertebral spines greatly extended in the shoulder region and in the posterior thoracic region. The neck is short, as is the tail.

Rhino horns lack the bony core that is typical of the horns of cattle, goats, and antelope, but consist of the same proteinaceous substance, keratin, that forms the outer material of such horns, as well as the material of hooves, fingernails, and hairs. They are made up of tubular filamentous rods, resembling a mass of adherent hairs. Rather than being part of the skull, the horns adhere to roughened areas of bone. If knocked off during fighting, or through some other accident, the horns re-grow. Indeed, they continue growing throughout the life of a rhino, with increase in length counteracted by wear from the tip.

The skin thickness varies between 0.5 and 1.8 in (13–45 mm) over different regions of the body, and among species.

However, the outer epidermis is quite thin (about 0.04 in, or 1 mm), and well supplied with blood vessels, so that biting flies have only to penetrate this distance to draw blood. Instead of the usual type of sweat gland, rhinos have exceptionally large apocrine glands scattered over the skin, well designed for rapid and copious discharge of fluid.

The penis is muscular as in equids, and backwards pointing when retracted. There are laterally projecting lobes associated with extensions of the corpus cavernosum, and the tip terminates in an enlarged flattened flange. The testes are located close to the skin between the prepuce and the attenuated nipples, and there is no scrotum. Females possess two teats located between the hindlegs. Pedal glands are present in the genus Rhinoceros, but lacking in the African species. Preputial glands are present in the white rhinoceros only.

Distribution

Two species occur in Africa, and three species in Southeast Asia. All five species are much more restricted in their distribution today than they were in the past because of human impacts.

Habitat

Modern rhinoceroses occupy a diversity of habitats: dense rainforests for the Sumatran and Javan rhinos, swamplands and adjoining meadows for the Indian rhino, grassy savannas for the white rhino, and dry bushland or semi-desert for the black rhino (Diceros bicornis).

Behavior

Rhinos are largely solitary animals, apart from the mother-offspring association, but the white rhino is more social and forms small groups. They have poor vision, and seem unable to recognize a stationary human observer at distances exceeding about 100 ft (30 m). Their hearing is good, with the ear pinnae moved independently to scan for sounds from different directions. Their sense of smell is acute, and rhinos can detect traces of human scent, and also follow the tracks of other rhinos, after many hours have passed. They can be frightening animals to encounter, because they often charge human intruders, or their vehicles, but when not threatened can be quite docile, and become very tame in captivity.

Feeding ecology and diet

The stomach is simple, and the capacious cecum and colon (or large intestine) in the hind-gut serve as the main sites of fermentation of plant food, with the help of bacteria. Large body size prolongs the period of retention, facilitating efficient digestion. Grass-feeders such as the white rhino and Indian rhino have a relatively longer colon than the other three rhino species, which are primarily browsers on the leaves and stems of woody plants.

Reproductive biology

The gestation period is 15–16 months for all rhinoceros species, even the small Sumatran rhino, and the inter-birth interval is correspondingly between two and four years. Nursing generally continues for over a year, and the older calf is driven away by the mother around the time of birth of the next offspring. Estrous cycling begins while the mother is still nursing, and there is no narrow birth season. Rhinos are renowned for the extended duration of copulations, which last between 20 minutes and an hour or longer, with multiple ejaculations. They have proved surprisingly difficult to breed in zoos, with many strange features of their reproductive biology being revealed. For example, white rhinos show no reproductive activity if housed in pairs, and the presence of more than one male, or at least an exchange of males, seems necessary for females to show overt estrous behavior. For the Sumatran rhino, mating induces ovulation. Courtship behavior can be surprisingly aggressive and frequently results in injuries. Rhinos are polygynous, and generally males and females do not associate with each other outside of mating.

Conservation status

Both the Javan rhino and Sumatran rhino are Critically Endangered, with low numbers of animals persisting in just a few sanctuaries in Southeast Asia. Although more numerous, the black rhino is also classed as Critically Endangered, because no area harbors a large population and because it remains extremely vulnerable to poaching. The Indian rhino is marginally less insecure in numbers, and thus classed as merely Endangered. Only the white rhino is no longer seriously in danger, having recovered amazingly from its critically low numbers at the beginning of the twentieth century in southern Africa. Nevertheless, the distinct northern sub-species persists as just a relict of 30 animals in one park in northeast Congo. The threat to all of these species comes from illegal hunting driven by the high market value and legendary powers of their horns.

Significance to humans

Rhinoceroses have fascinated humans from early times, as is shown in cave art from the Early Stone Age in Europe, depicting the extinct woolly rhino, and in rock paintings and engravings spread across Africa, from the Cape to Algeria. The engraving of an Indian rhino by Durer in 1515, received as a gift to the Portuguese king from an Indian sultan (actually intended for the Pope), brought these animals to the attention of Europeans. However, Marco Polo described the Sumatran rhinos that he had seen during his travels through the Far East early in the fourteenth century, and the early Romans had imported some rhinos of unknown affinities. Rhinoceroses have become especially valued for their horns, used for the making of prestigious dagger handles in Yemen and adjoining parts of the Middle East, and in powdered form as a fever-reducing drug in China and aphrodisiac potion in India. The horns were also carved into cups, used by Indian and Far Eastern potentates to test whether beverages contained poison. The claimed medicinal power is without pharmaceutical foundation, since the substance of the horn is no different from that of hooves or fingernails. The legends about aphrodisiac properties seem to derive from the prolonged copulations typical of the family, perhaps supported by the phallic appearance of the anterior horn.

Species accounts

Resources

Anonymous. Die Nashorner. Begegnung mit urzeitlichen Kolossen. Furth: Filander Verlag, 1997.

Owen-Smith, Norman. Megaherbivores. The Influence of Very Large Body Size on Ecology. Cambridge: Cambridge University Press, 1988.

Van Strien, N. J. The Sumatran Rhinoceros in the Gunung Leuser National Park, Sumatra, Indonesia: Its Distribution, Ecology and Conservation. Berlin: Mammalia Depicta, Verlag, Paul, and Parey, 1986.

Dinerstein, E. "Effects of Rhinoceros unicornis on Riverine Forest Structure in Lowland Nepal." Ecology 73 (1992): 701–704.

Dinerstein, E., and L. Price. "Demography and Habitat Use by Greater One-horned Rhinoceros in Nepal." Journal of Wildlife Management 55 (1991): 401–411.

Dinerstein, E., and C. M. Wemmer. "Fruits Rhinoceros Eat: Dispersal of Trewia nudiflora (Euphorbiaceae) in Lowland Nepal." Ecology 69 (1988): 1768–1774.

Laurie, Andrew. "Behavioural Ecology of the Greater Onehorned Rhinoceros." Journal of Zoology, London 196 (1982): 307–341.

Owen-Smith, Norman. "The Social Ethology of the White Rhinoceros." Zeitschrift für Tierpsychologie 38 (1975): 337–384.

Schenkel, R., and L. Schenkel-Hulliger. "The Javan Rhinoceros in Ujung Kulon Nature Reserve, Its Ecology and Behaviour." Acta Tropica 26 (1969): 98–135.

Van Gyseghem, R. "Observations on the Ecology and Behaviour of the Northern White Rhinoceros." Zeitschrift fur Tierpsychologie 49 (1984): 348–358.

[Article by: Norman Owen-Smith, PhD]