Rhinoceros
A rhinoceros, commonly abbreviated to rhino, is a member of any of the five extant species of odd-toed ungulates in the family Rhinocerotidae. It is the only living family in the superfamily Rhinocerotoidea. Two of the extant species are native to Africa, and three to South and Southeast Asia.
Rhinoceroses are some of the largest remaining megafauna: all weigh over half a tonne in adulthood. They have a herbivorous diet, small brains for mammals of their size, one or two horns, and a thick, protective skin formed from layers of collagen positioned in a lattice structure. They generally eat leafy material, although their ability to ferment food in their hindgut allows them to subsist on more fibrous plant matter when necessary. Unlike other perissodactyls, the two African species of rhinoceros lack teeth at the front of their mouths; they rely instead on their lips to pluck food.
Rhinoceroses are killed by poachers for their horns, which are bought and sold on the black market for high prices, leading to most living rhinoceros species being considered endangered. The contemporary market for rhino horn is overwhelmingly driven by China and Vietnam, where it is bought by wealthy consumers to use in traditional Chinese medicine, among other uses. Rhino horns are made of keratin, the same material as hair and fingernails, and there is no good evidence of any health benefits. A market also exists for rhino horn dagger handles in Yemen, which was the major source of demand for rhino horn in the 1970s and 1980s.
Taxonomy and naming
The word rhinoceros is derived through Latin from the, which is composed of wikt:ῥίς#Ancient Greek and wikt:κέρας#Ancient Greek with a horn on the nose. The name has been in use since the 14th century. Rhinoceros can either be referred to in plural as rhinoceros or as rhinoceroses. The first species of rhinoceros to be scientifically named were the Indian rhinoceros and black rhinoceros by Carl Linnaeus in his 10th edition of Systema Naturae in 1758, which also coined the genus Rhinoceros, which was originally used for all rhinoceroses. In his original classification scheme, Linnaeus placed the genus Rhinoceros within the group Glires, erroneously considering them closely related to rodents, because Indian rhinoceroses had incisor teeth. Initially in the 18th and the early 19th century there was confusion about the true number of living rhinoceros species. The family Rhinocerotidae was first named in 1821 as Rhynocerotidæ by John Edward Gray in his paper "On the natural arrangement of vertebrose animals". Some authors alternatively credit Richard Owen's 1845 work "Odontography" as the authority of Rhinocerotidae, which uses the modern spelling of the family.Description
Rhinoceroses are among the largest living land animals, with living species ranging in average weight from in the Sumatran rhinoceros, to in the white rhinoceros. Some extinct rhinocerotids were considerably smaller and larger than living rhinoceroses, with the genus Menoceras from the Early Miocene of North America having an estimated body mass of, comparable to sheep, or a pig, while Elasmotherium sibiricum from the Pleistocene of Eurasia has an estimated body mass of approximately. The skulls of rhinoceroses are generally saddle-shaped and low, with rhinoceroses being primitively characterised by the presence of a chisel-shaped upper first incisor and a tusk-like lower second incisor, with all other incisors and the canines typically being lost. Black and white rhinoceroses completely lack incisors. Living rhinoceroses have either one or two horns, which are formed from columns of densely packed corneocytes originating from dermal papillae. The development and growth of rhinoceros horns is similar to that of human nails, with both being largely made of keratin. The horns are attached to a rugose area on the surface of the skull. Horns are not a universal feature of rhinocerotids, with horns thought to be absent in many extinct rhinocerotids.The brains of rhinoceroses are relatively small compared to body size, around in an adult black rhinoceros. The limb bones tend to be robust. All living and the vast majority of extinct rhinoceroses have three toes on each foot. The body is covered in an armour of thick skin made of a dense crosslinked network of collagen fibres that is stronger and stiffer than those of other mammals. The skin exhibits prominent folding. The skin in living species is grey to brown in colour, and typically sparsely covered in hair or hairless as adults, with the exception of the eyelashes, ears, and the tail-brush. The exception is the Sumatran rhinoceros, which is often covered with a considerable amount of hair.
While the black rhinoceros has 84 chromosomes, all other rhinoceros species have 82 chromosomes. Chromosomal polymorphism might lead to varying chromosome counts. For instance, in a study there were three northern white rhinoceroses with 81 chromosomes.
Behaviour and ecology
Living rhinoceroses' gregariousness varies between species. Adult males tend to be solitary, and this is also true of female Asian rhinoceroses, though the females of African species sometimes form groups, with these groups being more common in white than black rhinoceroses. Rhinoceroses have widely varying diets ranging from strict grazing to largely browsing to a mixture between both. As bulk feeders of low quality vegetation, rhinoceroses spend a majority of their time foraging. Rhinoceroses are hindgut fermenters.All living rhinoceroses have a polyandrous and polygynous mating system where both males and females seek to mate with multiple individuals of the opposite sex. Male rhinoceroses guard reproductive age females until they are in full estrous though the females sometimes may drive away males until they are receptive. Male rhinoceroses taste the urine of female rhinoceroses and perform a flehmen response with the upper lip to determine their reproductive status. Adult males in the vicinity of oestrous females may become aggressive towards other males. These confrontations can range from ritualized behaviour to serious fighting that can result in significant injuries. In some species, male rhinoceroses are territorial, while in other species they are not or are only territorial depending on local environmental conditions. Females will sometimes reject males they consider undesirable, which results in them fleeing or fighting the male if cornered. During copulation, the male slides his neck up the back of the female, before using his neck as a lever to get his forelegs off the ground, before moving the front legs behind the shoulders of the female. Copulation can last several hours. Pregnancy lasts for over a year, around 460 days in the black rhinoceros and 504 days in the white rhinoceros.
The female generally gives birth in a secluded area and becomes aggressive towards other rhinoceroses for a while after giving birth. Calves typically stand up within 30 minutes of birth and begin to suck on their mother's teats within two hours of birth. The mother generally has a strong bond with her most recently born calf. The calf generally remains close to its mother the majority of the time, although at least in some species they are sometimes left considerable distances away. Up until they are around three years old, juvenile rhinoceroses are vulnerable to predation. Mothers are vigorously protective of their calves against potential predators. Juvenile one-horned rhinoceroses are rejected by their mothers around the time of the birth of her next calf. There is generally a gap of several years between females giving birth again after having her previous calf, though the gap can be as short as a year and a half. Rhinoceroses become sexually mature at around five to eight years of age, generally around a year later in males than in females in black and Sumatran rhinoceroses, though male white rhinoceroses become socio-sexually mature at around 12 years of age, four years after females start giving birth.
Extant species
White
There are two subspecies of white rhinoceros: the southern white rhinoceros and the northern white rhinoceros. As of 2013, the southern subspecies has a wild population of 20,405—making them the most abundant rhino subspecies in the world. The northern subspecies is critically endangered, with all that is known to remain being two captive females. There is no conclusive explanation of the name "white rhinoceros". A popular idea that "white" is a distortion of either the Afrikaans word wyd or the Dutch word wijd, meaning "wide" and referring to the rhino's square lips, is not supported by linguistic studies.The white rhino has an immense body and large head, a short neck and broad chest. Females weigh and males on average, though exceptional specimens can reportedly weigh up to. The head-and-body length is and the shoulder height is. On its snout it has two horns. The front horn is larger than the other horn and averages in length and can reach. The white rhinoceros also has a prominent muscular hump that supports its relatively large head. The colour of this animal can range from yellowish brown to slate grey. Most of its body hair is found on the ear fringes and tail bristles, with the rest distributed rather sparsely over the rest of the body. White rhinos have the distinctive flat broad mouth that is used for grazing.
A subspecific hybrid white rhino was bred at the Dvůr Králové Zoo in the Czech Republic in 1977. Interspecific hybridisation of black and white rhinoceroses has also been confirmed.
Black
The name "black rhinoceros" was chosen to distinguish this species from the white rhinoceros. This can be confusing, as the two species are not truly distinguishable by color. There are four subspecies of black rhino: South-central, the most numerous, which once ranged from central Tanzania south through Zambia, Zimbabwe and Mozambique to northern and eastern South Africa; South-western which are better adapted to the arid and semi-arid savannas of Namibia, southern Angola, western Botswana and western South Africa; East African, primarily in Tanzania; and West African which was declared extinct in November 2011. The native Tswanan name keitloa describes a South African variation of the black rhino in which the posterior horn is equal to or longer than the anterior horn.An adult black rhinoceros stands high at the shoulder and is in length. An adult weighs from, exceptionally to, with the females being smaller than the males. Two horns on the skull are made of keratin with the larger front horn typically long, exceptionally up to. Sometimes, a third smaller horn may develop. The black rhino is much smaller than the white rhino, and has a pointed mouth, which it uses to grasp leaves and twigs when feeding.
During the latter half of the 20th century, their numbers were severely reduced from an estimated 70,000 in the late 1960s to a record low of 2,410 in 1995. Since then, numbers have been steadily increasing at a continental level with numbers doubling to 4,880 by the end of 2010. As of 2008, the numbers are still 90% lower than three generations ago.