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Diversity of Living Rhinoceroses

The mammal families Rhinocerotidae (rhinoceroses), Equidae (horses), and Tapiridae (tapirs) together comprise the order Perissodactyla (the odd-toed ungulates). Dinerstein (2011) recognized five extant rhinoceros species in two subfamilies, Dicerotinae (including the two African species) and Rhinocerotinae (including the three Asian species) (see also Price and Bininda-Emonds 2001). Rhinoceroses are found in the tropics and subtropics of Africa and Asia in a range of open habitats; both African species sometimes ascend to montane forests.

The two African rhino species are the White Rhinoceros (Ceratotherium simum) and Black Rhinoceros (Diceros bicornis) (despite their common names, the White Rhinoceros is not white and the Black Rhinoceros is not black). Both of these species have two horns. The White Rhinoceros has a wide, square upper lip that facilitates grazing, allowing the animal to crop grasses close to the ground with its teeth. The Black Rhinoceros has a prehensile upper lip that facilitates browsing, allowing the animal to pluck leaves and grass stems.  A recent genetic and morphometric analysis (Groves et al. 2010) argued that the northern and southern forms of White Rhinoceroses should be recognized as distinct species. Populations of the southern form are relatively stable whereas just a handful of individuals of the northern form are alive today. 

Three rhino species occur in Asia. The Greater One-horned Rhinoceros (Rhinoceros unicornis) and Javan (or Lesser One-horned) Rhinoceros (Rhinoceros sondaicus) have just a single horn, but the Sumatran Rhinoceros (Dicerorhinus sumatrensis) has two horns. The horns of the Asian species are smaller than those of the African ones. Rhinoceros horns lack a bony core, containing only tiny keratin tubes dispersed in a keratinous matrix; a dark central core is strengthened by a composite of calcium and melanin. A close examination of rhino dentition reveals that only the three Asian species have tusks (enlarged incisors) in the lower jaw, especially striking in the Greater One-horned Rhinoceros. These tusks, rather than their horns, are used in fighting and defense (African rhinos use their horns).

Rhinoceros Ecology and Behavior

Rhinoceroses are massive animals. At the upper limit, White Rhinos can have a mass of as much as 3500 kg (despite their large size, rhinos can run quickly for brief periods, with most species able to reach a speed of 55 km/h). To obtain enough food to sustain their massive bodies, rhinos must often feed both day and night. Roughly speaking, the White Rhino and Greater One-horned Rhino are grazers (White Rhinos feed only on grass, but Greater One-horned Rhinoceroses eat a wide range of vegetation) whereas the Black, Sumatran, and Javan Rhinos are browsers.

Rhinos spend a substantial fraction of their time wallowing. The primary function of wallowing is presumed to be reducing heat stress, although escaping from biting insects may be another motivation. Rhinos are landscape engineers, with their grazing and browsing and defecation into latrines, and even wallowing, significantly modifying their habitat, shaping both its physical structure and species composition. In Asia, rhinoceroses play an important role in dispersing seeds of woody species into riverine grasslands.

All three Asian rhino species are excellent swimmers, but the African White and Black Rhinos are poor swimmers and can drown if they lose their footing in deep water. Greater One-horned Rhinoceroses are rarely found more than 2 km from water (this is suspected to be true also for Javan and Sumatran Rhinos although this is not actually known). In contrast, White and Black Rhinos can go several days without drinking and are less tied to water.

Rhinoceroses possess acute senses of hearing and smell, but have relatively poor vision. Their ears swivel independently. Diverse vocalizations are produced and at least the White Rhinoceros uses infrasonic communication, like elephants.

Intraspecific combat in Black Rhinos reportedly kills a substantial fraction of both male and female Black Rhinos.

In general, far more is known about Black Rhinos, White Rhinos, and Greater One-horned Rhinos than about the Javan and Sumatran Rhinos, which are rarely even seen.

Rhinoceroses and Humans

Until the last few hundred years, Black Rhinoceroses ranged widely across sub-Saharan Africa. During the colonial era in Africa and Asia, rhinos were hunted and all five species were apparently abundant. Subsequently, conflicts with human agriculture became more common and rhinos persisted mainly in regions unsuitable for farrming for one reason or another. The use of rhino horns in traditional medicine in Asia (especially Vietnam) has led to an illegal trade that has negatively impacted rhino populations. The horns, which are made entirely of densely appressed hairs (with no bony core as seen in the horns of cattle, sheep, and goats) have also been used for dagger handles in the Middle East.

The conservation status of rhinos varies considerably among species.

The Southern White Rhinoceros was abundant prior to the colonization of southern Africa, but the population dropped to 100 individuals around 1900. Today, however, there are more than 20,000 individuals, representing a great accomplishment resulting from concerted conservation efforts. The Northern White Rhino (which a 2010 study argued should be recognized as a distinct species rather than a subspecies) has had a very different trajectory. In the late nineteenth century, there may have been more Northern White Rhinos in Sudan, the Central African Republic, northern Uganda, and the DR Congo than there were Southern White Rhinos in South Africa. However, by the mid-1980s civil war had taken its toll; by the first decade of the 21st century, they were apparently extinct in the wild and the handful of remaining captive animals are closely related and likely insufficient for population recovery.

The Black Rhinoceros was once the most abundant of all five living rhino species and occurred across much of sub-Saharan Africa. Estimates of the Black Rhino population at the dawn of the 20th century range between around 300,000 and 1 million individuals. Subsequently, however, core populations were decimated across much of their range. Intensive conservation and translocation efforts, combined with locally based ecotourism efforts, have met with some, but not enough, success. In some areas, desperation in the face of poaching led to de-horning of Black Rhinos to make them unappealing to poachers, but this approach has been abandoned as ineffective, among other problems. 

In Asia, rhino populations are severely threatened. The total population of all three Asian species combined is less than 3000, with only two populations having more than 100 individuals. The loss of Asian rhinos has resulted from both poaching and habitat loss. In Africa, extensive habitat still remains, so controlling poaching would allow rhino populations to rapidly increase. In Asia, unfortunately, similar enormous blocks of habitat do not exist and habitat is threatened by deforestation, plantations to produce oil palm and pulp and paper, and human population growth adjacent to protected areas.

Greater One-horned Rhinoceros populations are estimated to have totaled at least 475,000 before the spread of agriculture to the Gangetic and Brahamputra River floodplains of South Asia around 1400 AD. A deadly strain of malaria largely protected their habitats from humans until malaria was controlled in the region in the late 1950s. Two core populations in Chitwan National Park in Nepal and Kaziranga National Park in Assam, India, dropped as low as 60-80 individuals in Nepal in the late 1950s or early 1960s and to less than 100 in Kaziranga National Park around 1900. Many smaller reserves in Assam and West Bengal had numbers in the single digits or less than 20 in the 1970s. Since 1986, however, Nepal and India have undertaken intensive conservation and translocation efforts.

The Javan Rhinoceros is believed to be the most endangered large mammal on Earth, with just a few dozen individuals surviving, all of them possibly restricted to a single population in western Java, although establishment of a second population on Java by translocation was being planned as of 2010. Proposals to try to establish a captive breeding population were rejected, given the dramatic failure of captive breeding efforts for the Sumatran Rhino.

At one time, the Sumatran Rhino was widely distributed across Southeast Asia, but it experienced a dramatic decline and so few individuals remain that today, together with the Javan Rhinoceros, it is considered the most endangered large mammal on Earth. Although common nowhere, very small Sumatran Rhino populations persist in Peninsular Malaysia, Sabah (on Borneo) and Sumatra. The Sumatran Rhino was the focus of an effort to capture individuals from vulnerable sites and use them in a captive breeding program, with the goal of eventual reintroduction. Tragically, the effort was a huge failure, with extremely high mortality in captivity. Many conservation biologists view this episode as an illustration of the fact that captive breeding is no substitute for the often politically and sociologically difficult challenges of in situ conservation efforts.

(Dinerstein 2011 and references therein)

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