Rhinoceroses are characterized by the possession of one or two horns on the upper surface of the snout; these horns are not true horns but are composed of keratin, a fibrous protein found in hair.
Modern rhinoceroses are large animals, ranging from 2.5 metres (8 feet) long and 1.5 metres (5 feet) high at the shoulder in the Sumatran rhinoceros to about 4 metres (13 feet) long and nearly 2 metres (7 feet) high in the white rhinoceros. Adults of larger species weigh 3–5 tons. Rhinoceroses are noted for their thick skin, which forms platelike folds, especially at the shoulders and thighs. All rhinos are gray or brown in colour, including the white rhinoceros, which tends to be paler than the others. Aside from the Sumatran rhinoceros, they are nearly or completely hairless, except for the tail tip and ear fringes, but some fossil species were covered with dense fur. The feet of the modern species have three short toes, tipped with broad, blunt nails.
Most rhinoceroses are solitary. Individuals usually avoid each other, but the white rhinoceros lives in groups of up to 10 animals. In solitary species the home territory is crisscrossed with well-worn trails and often marked at the borders with urine and piles of dung.
Rhinoceroses have poor eyesight but acute senses of hearing and smell. Most prefer to avoid humans, but males, and females with calves, may charge with little provocation. The black rhinoceros (Diceros bicornis) is normally ill-tempered and unpredictable and may charge any unfamiliar sound or smell. Despite their bulk, rhinoceroses are remarkably agile; the black rhinoceros can attain a speed of about 45 km (30 miles) per hour, even in thick brush, and can turn around rapidly after missing a charge. Like elephants, rhinoceroses communicate using infrasonic frequencies that are below the threshold of human hearing. The use of infrasonic frequencies is likely an adaptation for rhinoceroses to keep in touch with each other where they inhabit dense vegetation and probably for females to advertise to males when females are receptive to breeding.
Rhinoceroses are by far the largest of the perissodactyls, an order of hoofed mammals that also includes the horses and zebras. One of the features of very large body size in mammals is a low reproductive rate. In rhinoceroses, females do not conceive until about six years of age; gestation is long (16 months in most species), and they give birth to only one calf at a time. The period of birth between calves can range from 2 to 4.5 years. Thus, the loss of a number of breeding-age females to poachers can greatly slow the recovery of rhinoceros populations. However, an Indian rhinoceros female will conceive again quickly if she loses her calf. In this species tigers kill about 10–20 percent of calves. Tigers rarely kill calves older than 1 year, so those Indian rhinoceroses that survive past that point are invulnerable to nonhuman predators.
The rhinoceros’s horn is also the cause of its demise. Powdered rhinoceros horn has been a highly sought commodity in traditional Chinese medicine—not as an aphrodisiac, as is often widely reported, but as an antifever agent. Substitute agents have been found, particularly pig bone and water buffalo horn, but rhinoceros horn commands tens of thousands of dollars per kilogram in Asian markets. Today poaching remains a serious problem throughout the range of all species of rhinoceros.