A rhinoceros is any of five surviving species of odd-toed ungulate in the family Rhinocerotidae. All five are native to Africa or Asia. Several other species became extinct within geologically recent times, notably the Wooly Rhinoceros: the extent to which climate change or human predation was responsible is debated.
Rhinoceros-like animals first appeared in the Eocene as rather slender animals, and by the late Miocene there were many different species. Most were large. One, Indricotherium weighed about 30 tons and (so far as is known) was the largest terrestrial mammal that ever lived. Rhinos became extinct during the Pliocene in North America, and during the Pleistocene in northern Asia and Europe.
The five living species fall into three tribes. The critically endangered Sumatran Rhinoceros is the only surviving representative of the most primitive group, the Dicerorhinini, which emerged in the Miocene (abut 20 million years ago). There are two living Rhinocerotini species, the endangered Indian Rhinoceros and the critically endangered Javan Rhinoceros, which diverged from one another about 10 million years go. The extinct Wooly Rhinoceros of northern Europe and Asia was also a member of this tribe. The two African species, the White Rhinoceros and the Black Rhinoceros, diverged during the early Pliocene (about 5 million years ago) but the Dicerotini group to which they belong originated in the middle Miocene, about 14 million years ago.
Rhinoceros horns are used in traditional Asian medicine, and for dagger handles in Yemen and Oman. None of the five rhinoceros species have secure futures: the White Rhino is perhaps the least endangered, the Javanese Rhino survives in only tiny numbers (estimated at 60 animals in 2002) and is one of the two or three most endangered large mammals anywhere in the world.
Rhino protection campaigns began in the 1970s, but rhino populations have continued to decline dramatically. Trade in rhinoceros parts is forbidden under the CITES agreements, but poaching is a severe threat to all rhinoceros species.
The most obvious distinguishing characteric of the rhinos is a large horn on the nose. The word rhinoceros comes from the Greek words rhino (nose) and keros (horn). Rhinoceros horns, unlike those of other horned mammals, consist of densely compacted hair.