Rhinoceros Iguana, Rhino Iguana

A rhinoceros ( ; ; ; : rhinoceros or rhinoceroses), commonly abbreviated to rhino, is a member of any of the five extant species (or numerous extinct species) of odd-toed ungulates (perissodactyls) in the family Rhinocerotidae; it can also refer to a member of any of the extinct species of 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 (''rhino-'', "of the nose") and (, "horn") with a horn on the nose. The name has been in use since the 14th century.

The family Rhinocerotidae consists of only four extant genera: ''Ceratotherium'' (white rhinoceros), ''Diceros'' (black rhinoceros), ''Dicerorhinus'' (Sumatran rhinoceros), and ''Rhinoceros'' (Indian and Javan rhinoceros). The living species fall into three categories. The two African species, the white rhinoceros and the black rhinoceros, belong to the tribe Dicerotini, which originated in the middle Miocene, about 14.2'' ''million years ago. The species diverged during the early Pliocene and the Javan rhinoceros, which diverged from one another about 10 million years ago. The Sumatran rhinoceros is the only surviving representative of the Dicerorhinini.

A subspecific hybrid white rhino (''Ceratotherium s. simum'' × ''C. s. cottoni'') was bred at the Dvůr Králové Zoo (Zoological Garden Dvur Kralove nad Labem) in the Czech Republic in 1977. Interspecific hybridisation of black and white rhinoceroses has also been confirmed.

While the black rhinoceros has 84 chromosomes (diploid number, 2N, per cell), 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.

Anatomy

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,D.R. Prother
"Rhinocerotidae"
C.M. Janis, K.M. Scott, L. Jacobs (Eds.), Evolution of Tertiary Mammals of North America, Cambridge University Press, Cambridge (1998), pp. 595-605 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 (I1) and a tusk-like lower second incisor (i2), 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 (roughly textured) 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 (such as most members of the subfamily Aceratheriinae). The brains of rhinoceroses are relatively small compared to body size, around in an adult black rhinoceros. The limb bones tend to be robust (proportionally thick and stocky). 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.

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 (such as the white rhinoceros) to largely browsing (such as the black rhinoceros) to a mixture between both (the Sumatran and Javan rhinoceros). 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 polygnous 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 (''Ceratotherium simum simum'') and the northern white rhinoceros (''Ceratotherium simum cottoni''). 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 ' or the Dutch word (or its other possible spellings , , etc.,), 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 horn (anatomy), 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.

Black

The name "black rhinoceros" (''Diceros bicornis'') was chosen to distinguish this species from the white rhinoceros (''Ceratotherium simum''). This can be confusing, as the two species are not truly distinguishable by color. There are four subspecies of black rhino: South-central (''Diceros bicornis minor''), the most numerous, which once ranged from central Tanzania south through Zambia, Zimbabwe and Mozambique to northern and eastern South Africa; South-western (''Diceros bicornis occidentalis'') which are better adapted to the arid and semi-arid savannas of Namibia, southern Angola, western Botswana and western South Africa; East African (''Diceros bicornis michaeli''), primarily in Tanzania; and West African (''Diceros bicornis longipes'') which was declared extinct in November 2011. The native Tswana language, 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 horn (anatomy), 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.

Indian

The Indian rhinoceros, or greater one-horned rhinoceros, (''Rhinoceros unicornis'') has a single horn 20 to 60 cm long. It is nearly as large as the African white rhino. Its thick, silver-brown skin folds into the shoulder, back, and rump, giving it an armored appearance. Its upper legs and shoulders are covered in wart-like bumps, and it has very little body hair. Grown males are larger than females in the wild, weighing from . Shoulder height is . Females weigh about and are long. The record-sized specimen was approximately .

Indian rhinos once inhabited many areas ranging from Pakistan to Myanmar and maybe even parts of China. Because of humans, they now exist in only several protected areas of India (in Assam, West Bengal, and a few pairs in Uttar Pradesh) and Nepal, plus a pair in Lal Suhanra National Park in Pakistan reintroduced there from Nepal. They are confined to the tall grasslands and forests in the foothills of the Himalayas. Two-thirds of the world's Indian rhinoceroses are now confined to the Kaziranga National Park situated in the Golaghat district of Assam, India.

Javan

The Javan rhinoceros (''Rhinoceros sondaicus'') is one of the most endangered large mammals in the world. According to 2015 estimates, only about 60 remain, in Java, Indonesia, all in the wild. It is also the least known rhino species. Like the closely related, and larger, Indian rhinoceros, the Javan rhino has a single horn. Its hairless, hazy gray skin falls into folds into the shoulder, back, and rump, giving it an armored appearance. Its length reaches including the head, and its height . Adults are variously reported to weigh or . Male horns can reach in length, while in females they are knobs or altogether absent. These animals prefer dense lowland rain forest, tall grass and reed beds that are plentiful with large floodplains and mud wallows.

Though once widespread throughout Asia, by the 1930s, they were nearly hunted to extinction in Nepal, India, Burma, Peninsular Malaysia, and Sumatra for the supposed medical powers of their horns and blood. As of 2015, only 58–61 individuals remain in Ujung Kulon National Park, Java, Indonesia. The last known Javan rhino in Vietnam was reportedly killed for its horn in 2011 by Vietnamese poachers. Now only Java contains the last Javan rhinos.

Sumatran

The Sumatran rhinoceros (''Dicerorhinus sumatrensis'') is the smallest extant rhinoceros species, as well as the one with the most hair. It can be found at very high altitudes in Borneo and Sumatra. Because of habitat loss and poaching, their numbers have declined, and it has become the second most threatened rhinoceros. About 275 Sumatran rhinos are believed to remain. There are three subspecies of Sumatran rhinoceros: the Sumatran rhinoceros proper (''Dicerorhinus sumatrensis sumatrensis''), the Bornean rhinoceros (''Dicerorhinus sumatrensis harrissoni'') and the possibly extinction, extinct Northern Sumatran rhinoceros (''Dicerorhinus sumatrensis lasiotis'').

A mature rhino typically stands about high at the shoulder, has a length of and weighs around , though the largest individuals have been known to weigh as much as . Like the African species, it has two horns; the larger is the front (), with the smaller usually less than long. Males have much larger horns than the females. Hair can range from dense (the densest hair in young calves) to sparse. The color of these rhinos is reddish brown. The body is short and has stubby legs. The lip is prehensility, prehensile.

Sumatran rhinoceros once were spread across South-east Asia, but now are on the verge of extinction, confined to several parts of Indonesia and Malaysia by reproductive isolation. There were 320 ''D. sumatrensis'' in 1995, which, by 2011, had dwindled to 216. It has been found through DNA comparison that the Sumatran rhinoceros is the most ancient extant rhinoceros and related to the extinct Eurasian woolly rhino species, ''Coelodonta''. In 1994, Alan Rabinowitz publicly denounced governments, non-governmental organizations, and other institutions for lacking in their attempts to conserve the Sumatran rhinoceros. To conserve it, they would have to relocate them from small forests to breeding programs that could monitor their breeding success. To boost reproduction, the Malaysian and Indonesian governments could also agree to exchange the gametes of the Sumatran and (smaller) Bornean subspecies. The Indonesian and Malaysian governments have also proposed a single management unit for these two ancient subspecies.

Plantations for palm oil have taken out the living areas and led to the eradication of the rhino in Sumatra.

Evolution

The earliest representatives of Rhinocerotoidea appeared during the early-middle Eocene in Asia, around 54 million years ago. The family of modern rhinoceroses, Rhinocerotidae appeared during the middle-late Eocene around 39-40 million years ago, roughly at the same in North America and Asia, with rhinoceroses migrating into Europe at the Eocene-Oligocene boundary ~34 million years ago as part of the "Grande Coupure" along with many other Asian migrants. Rhinocerotids represented the only living family of rhinocerotoids following the end of the Oligocene epoch around 23 million and the extinction of other rhinocerotoid groups such as the giant Paraceratheriidae, paraceratheres. During the early Miocene epoch, around 20 million years ago rhinocerotids migrated into Africa following its connection to Eurasia. The last common ancestor of living rhinoceroses (which belong to the subgroup Rhinocerotina) is thought to have lived during the Miocene, at least 15-16 million years ago. Rhinocerotids reached maximum diversity during the Miocene epoch, with often 4-5 species of rhinoceros coexisting with each other at any location in Eurasia, up to 9 in South Asia, which include members of the living group Rhinocerotina, as well as the extinct groups Teleoceratini and Aceratheriinae, and Elasmotheriinae. Rhinocertoids declined in diversity during the late Miocene following unfavourable climatic change, becoming entirely extinct in North America at the beginning of the Pliocene

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