Rhinoceros - biology. I INTRODUCTION Rhinoceros, common name for five species of enormous mammals with one or two horns projecting from their snouts. Rhinos are among the biggest and heaviest land animals alive today, exceeded in size only by the elephant and the hippopotamus. They have thick, pillar-like legs, with three toes covered with broad, hooflike nails. Rhinos are legendary for their poor eyesight, but their senses of smell and hearing are acute. Although they look clumsy, rhinos can swivel around rapidly to face danger, and if threatened, can charge at speeds of up to 50 km/h (about 30 mph). Rhinos live in warm climates, and are found only in Africa and Asia. The two African species are the white rhinoceros, a relatively placid animal that is the largest of all rhino species, reaching a weight of over 3 metric tons, and the black rhinoceros, which is about half as heavy as the white rhinoceros but much more aggressive. The Asian species include the Indian rhinoceros, which has skin that looks like a suit of armor, and two much rarer species, the Sumatran rhinoceros and the Javan rhinoceros. African rhinos have two horns, but depending on their species and sex, Asian rhinos have two horns, a single horn, or none at all. As a result of illegal trade in rhinoceros horn and habitat destruction, the populations of the black, Sumatran, and Javan rhinos have suffered a sharp decline in recent years, and are now close to extinction in the wild. Rhinos first evolved over 40 million years ago from animals that looked similar to tapirs, and today's species represent the surviving remnants of a once thriving group of animals. Prehistoric ancestors of the rhinoceros include Indricotherium and Paraceratherium, hornless giants that may have weighed up to 20 tons, making them among the largest mammals that ever lived. During the last ice age, woolly rhinos roamed the tundra of northern Europe and Siberia. These two-horned animals with long, shaggy coats died out 15,000 years ago, but their frozen remains are occasionally discovered today. II RANGE AND HABITAT Although they all eat plants, rhinos live in different habitats and feed in different ways. A In Asia In Asia the Sumatran and Javan rhinos inhabit dense tropical forests, where they feed on leaves, twigs, and fruit. The Indian rhino also browses on trees and shrubs, but instead of living in forests, it feeds in flat, open areas. In the rainy season, its main food is swampland grass, which can grow over 5 m (16 ft) high. The Indian rhino once ranged across northern India, Pakistan, and Nepal, but by the beginning of the 20th century less than 100 animals survived. By 2001, thanks to years of rigorous protection, there were about 2,400 of these rhinos, concentrated in Indian and Nepali national parks. At one time, the Sumatran and Javan rhinos were also widespread throughout Asia, but their numbers have plunged and show no signs of recovering. Sumatran rhinos, numbering about 300 animals, are scattered in small, fragmented groups across Southeast Asia, primarily in Malaysia and Indonesia. Found only in Indonesia and Vietnam, the Javan rhino now numbers only about 60 animals, making this species the most endangered rhinoceros in the world. B In Africa In Africa both black and white rhinoceroses live in open woodland and tree-studded plains, but their feeding habits are quite unalike. The black rhinoceros feeds on leaves and twigs, while the white rhinoceros is the world's biggest grazing animal, living almost entirely on low-growing grass. Black rhinos were once found in many parts of Africa south of the Sahara Desert, but unrestrained hunting in the last half of the 20th century caused the population to collapse by more than 90 percent. Anti-poaching efforts have helped the black rhino population to rebound somewhat and by 2001, about 2,700 animals were scattered across central, southern, and southeastern Africa. The white rhinoceros has fared better, with over 10,400 surviving, mainly in southern Africa. Some white rhinos also live in East Africa, and a small number are found in Central Africa west of the White Nile in Sudan, but even so, the species occupies only a fraction of its former range. III PHYSICAL DESCRIPTION The word rhinoceros is derived from the Greek words rhino, meaning "nose," and keras, meaning "horn," a reference to the most eye-catching feature of these huge and often awkward-looking animals. Rhino horns are made of a mass of thousands of fibers of keratin, the same substance found in hair. Unlike the true horns found in, for instance, cows and deer, rhinoceros horns do not have a bony core, and they are embedded in skin rather than in the animal's skeleton. However, as with true horns, rhino horns are permanent and grow steadily throughout their owner's lifetime. Some rhinoceros horns are little more than bumps, but in the white rhinoceros particularly, they can reach an impressive size. The largest one on record, from a female white rhino in South Africa, measured nearly 1.6 m (5 ft) from tip to base. Rhinos sometimes use horns to get at food, but a horn's main function is for defense: With so much muscle-power behind them, horns make formidable weapons. Unfortunately, horns are also the main reason for the rhino family's endangered plight. On the black market, rhinoceros horns command astronomical prices for use as dagger and sword handles in the Middle East and as an ingredient for traditional Chinese medicine. Rhinoceros skin is remarkable, both for its thickness and its texture. African rhinos have fairly smooth skin, but in the Asian species skin hangs down in heavy folds. The skin of the Indian rhinoceros forms platelike folds that give the appearance of armor plating. It is covered with tubercles, hard lumps that look like rivets holding the animal's armor together. Four out of the five rhino species are almost hairless, but the Sumatran rhino is born with a thick coat of hair, which becomes more straggly and bristly as it ages. Rhinos have elongated heads, with small, puffy-looking eyes. Because their eyes face sideways, they have to turn their heads one way and then the other to look at anything in front. This impediment coupled with their weak eyesight means that rhinos often fail to spot danger--a motionless person 30 m (100 ft) ahead is more or less invisible to a rhinoceros. However, rhinos quickly scent anything that is upwind, and their ears flick in different directions to pick up the faintest sounds. Rhinos have few or no front teeth, but their cheek teeth, or molars, are well developed for grinding up food. A rhino's lips also play an important part in feeding, and their shape helps to show what it eats. In most species the upper lip is pointed and prehensile (able to grasp), and is used for collecting twigs and leaves. But in the white rhino, the upper lip is wide, enabling this species to pull up mouthfuls of low-growing grass. Because rhinos are such heavy animals, their legs are shaped primarily for bearing weight. As in other heavyweight mammals, such as hippos and elephants, rhinoceros legs are almost straight--a kind of anatomy that prevents buckling when the animal stands. Rhino feet are large, which helps to spread the load they carry, and they leave characteristic three-lobed tracks that look like an ace of clubs. Rhinos normally move with a ponderous walk, but when they are alarmed they break into a rapid canter. As they pick up speed, they develop a huge amount of momentum, making them potentially lethal to anything standing in their way. Male and female rhinos have a similar physical appearance, although male rhinos are usually larger than females, with the size difference varying between species. In the wild, some rhinos probably live into their late 40s, and they have survived into their 30s in captivity. IV TYPES OF RHINOCEROSES Until recently, mammalogists divided the rhinoceros family into three subfamilies, each with different characteristics. Scientists now believe that living rhinos belong to a single subfamily, although some of the species are clearly more closely related than others. A Asian Rhinos With its unique gray or reddish-brown coat, the Sumatran rhinoceros is the smallest rhino species, and also the most distinctive. It is believed to be the only survivor of the lineage that included the woolly rhinoceros. Sumatran rhinos stand up to 1.4 m (4.6 ft) at the shoulder and weigh up to 1,000 kg (2,200 lb). They have folded skin, like the other Asian rhinos, except that it is covered with coarse, bristly hair. The Sumatran is the only rhino in Asia with two horns. The front horn is usually the longest, reaching a length of 90 cm (36 in), and the rear one is sometimes so small that the animal looks as if it is single-horned. Sumatran rhinos live mainly in forest-covered hills near water and are known to be good at climbing slopes and swimming. They move mainly at night, and spend most of the day wallowing in mud-holes and pools. The Indian and Javan rhinos are similar to each other in many ways, the most conspicuous being that they are both single-horned. The Indian rhino's horn reaches a length of up to 50 cm (20 in), but the Javan's horn is only half this size, and in females is often absent altogether. Both species have gray-brown skin thrown into thick folds--a feature particularly prominent in male Indian rhinos, which develop extra folds around their necks as they age. Unlike the Sumatran rhino, both these species are practically hairless, except for a fringe around their ears and a tassel of hair on their tails. Indian rhinos are the biggest Asian species, standing up to 1.8 m (6 ft) at the shoulder. Mature males weigh up to 2,200 kg (4,900 lb), which makes them Asia's second heaviest land animal (after the Asian elephant). The Javan rhinoceros has a maximum weight of about 2,000 kg (4,400 lb), and unusually, the females are bigger than the males. Both these animals need surroundings with plenty of water, but the Indian rhino is the only one that spends much of its time in open grassland, instead of remaining concealed in thick forest. B African Rhinos In Africa the white and the black rhinos evolved from a shared ancestor about 3 million years ago. They have misleading names, because the white rhinoceros is usually light gray, while the black rhinoceros is often dark brown. The white rhino got its name from the Afrikaans word weit, meaning "wide"--a reference to the shape of its mouth--while the black rhino may have been named because the animal wallowed in dark soil. African rhinos have two large horns and some black rhinos even have the beginnings of a third horn. The white rhino is the biggest member of the rhinoceros family, with a combined head and body length of nearly 4 m (13 ft). Adult males have an average weight of about 2,500 kg (5,500 lb), but exceptionally heavy specimens can reach 3,500 kg (7,700 lb). Their barrel-shaped bodies are almost hairless, and from a distance their skin looks smooth. The front horn is often twice as big as the rear one, with a leading edge that comes almost as far forward as the broad top lip. The black rhino is slightly smaller, but considerably lighter, with a maximum weight of up to 1,400 kg (3,100 lb). Its forehead is steeper, and its front horn is set slightly further back, leaving room for its prehensile top lip to reach out and collect food. In the wild, white and black rhinos can be distinguished not only by their color, size, and the shape of the heads, but also by their stance. Like other grazers, white rhinos spend much of their time with their heads down, using their tough lips to tear up mouthfuls of grass. Despite their huge proportions, they are relatively placid animals and will often allow humans to approach them. Black rhinos are more often seen with their heads up, reaching into shrubs and small trees. They are much more wary of human intrusion and will charge if disturbed, making it hazardous for anyone to get too close. African rhinos often wallow in mud, a habit that probably helps to protect their bare skin from biting flies. These rhinos also play host to oxpeckers, sharp-clawed birds that scamper over rhino bodies in search of ticks. Oxpeckers rid the rhinos of these troublesome parasites while gaining themselves a meal. V BEHAVIOR Rhinos are often portrayed as aggressive and unintelligent, but neither of these labels is completely true. The Indian rhino and black rhino have been known to attack people without warning, but the white rhinoceros is a more docile animal, despite its immense size. The Javan and Sumatran rhinos are now so rare that, as far as humans are concerned, they present very little danger at all. In intelligence, probably rhinos rank as high as other hoofed mammals, but their poor eyesight can sometimes make their behavior seem hesitant and slow-witted. Rhinos are predominantly solitary animals. The only stable rhino social unit consists of a nursing mother and her calf. Female white rhinos sometimes gather in small herds, and males will also socialize when they are young. But in all five species, adult males are strongly territorial and do not tolerate the presence of any rivals. Males mark their territories by leaving piles of dung, and by spraying urine on the ground in a powerful backward jet. Rhinos also communicate with their neighbors by sound, and their calls range from deep grunts to a loud puffing that indicates alarm. When neighboring males meet, one animal may show signs of subordination, for example by turning back its ears. If subordination does not occur, the stage is set for a contest that can quickly turn violent. The African rhino attacks rivals with its horns, first using its front horn like a club, and then, if this does not deter the intruder, jabbing with the tip. Their Asian counterparts, which have smaller horns, tend to use their mouths and their teeth when fighting. In both African and Asian rhinos, fights can result in serious injury. With the exception of these territorial clashes, a rhino's life is normally one of quiet routine. Most rhinos feed by day or by night, but they are least active during the midday heat. They rarely stray far from water, and the Indian rhinoceros spends much of its time lying in pools. Rhinos can travel up to 25 km (16 mi) every day in search of water and food. VI REPRODUCTION Female rhinos reach sexual maturity at about the age of five or six, although the exact age varies from one species to another. The males mature slightly later, typically between the ages of seven and eight. They do not father calves until they have claimed a territory, which may take them three or four years. When a female is receptive, she may mate with several males, but males do not take any part in bringing up or protecting the young. Rhino courtship can be a protracted business, because two essentially solitary animals have to overcome their wariness of each other. In white rhinos, the male serenades his prospective partner with a characteristic courtship call, but despite this, she often responds by charging. It takes repeated approaches before the male can overcome her hostility, and even then he may remain with her for nearly three weeks until she is finally prepared to mate. A few days after mating has taken place, the male departs, leaving the female on her own. Rhinos always have a single calf, born after a gestation period of 15 to 18 months. The calf may feed on its mother's milk for up to two years, and it usually remains with its mother until she is about to give birth once more. While the female is nursing, the bond formed with her calf is a strong one, and the two animals are rarely far apart. A rhino's early years are normally the only time when it is at risk to natural predators. Nursing mothers are quick to respond to any threat when they have a calf at their side. In Africa, black rhinos have been known to kill lions when defending their young, and in Asia, female Indian rhinos guarding their young are responsible for the deaths of several people each year. VII CONSERVATION STATUS Until the late 1800s, rhinos were common animals throughout much of Africa and southern Asia. Since then, the twin pressures of habitat change and hunting have brought the rhino population close to collapse. By 2001, after three decades of particularly intense hunting for rhinoceros horn, less than 16,000 of these animals existed in the wild. With such small populations, rhinos run the risk of inbreeding (mating between close relatives). Inbreeding may cause future generations of rhinos to lack genetic variation, which makes them less able to adapt to changes in their environment, such as infectious disease or a climate change. The Sumatran and Javan rhinos are listed as critically endangered on the Red List of Threatened Species prepared by the World Conservation Union (IUCN), a nongovernmental organization that compiles global information on endangered species. Both have been hunted for many years, but a rapid increase in logging that has destroyed their natural habitat has put further pressure on their survival. The Sumatran rhino population has declined by 50 percent in just 10 years, from about 600 animals in 1990 to about 300 animals today. A captive breeding program is underway to help the Sumatran rhino population rebound. No such programs are available for Javan rhinos and scientists hold little hope for the survival of the 60 or so remaining animals in the wild. The situation of the black rhino is also very disturbing in terms of the sheer speed of its decline. In 1970 about 65,000 black rhinos were thought to live in the wild--about 25 times as many as today. Illegal poaching for rhinoceros horn has caused this headlong drop in numbers. In response, the IUCN has listed the black rhino as critically endangered and has launched a program to save the remaining wild populations, while working to stem the rhinoceros horn trade. National parks use armed guards to protect the animals in the wild. Some conservationists have advocated the drastic step of sawing off the horns of adults, so that the dehorned animals are no longer a target for hunters. Dehorning has been carried out on an experimental basis, but there are fears that it may reduce the rhinos' ability to defend their young. The outlook for the Indian and white rhinos is somewhat less dire. Both these animals have responded well to conservation management, and their numbers have increased gradually over recent years. However, both are now dependent on human intervention: Without round-the-clock protection from poaching, their future too would be in doubt. Scientific classification: Rhinoceroses make up the family Rhinocerotidae. The Indian rhinoceros is classified as Rhinoceros unicornis, the Javan rhinoceros as Rhinoceros sondaicus, and the Sumatran rhinoceros as Dicerorhinus sumatrensis. The black rhinoceros is classified as Diceros bicornis, and the white rhinoceros as Ceratotherium simum. Contributed By: David Burnie Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
