The Sumatran rhinoceros (Dicerorhinus sumatrensis), also known as the Sumatran rhino, hairy rhinoceros or Asian two-horned rhinoceros, is a rare member of the family Rhinocerotidae and one of five extant species of rhinoceros; it is the only extant species of the genus Dicerorhinus. It is the smallest rhinoceros, although it is still a large mammal; it stands high at the shoulder, with a head-and-body length of and a tail of . The weight is reported to range from , averaging . Like both African species, it has two horns; the larger is the nasal horn, typically , while the other horn is typically a stub. A coat of reddish-brown hair covers most of the Sumatran rhino's body.
The Sumatran rhinoceros once inhabited rainforests, swamps and cloud forests in India, Bhutan, Bangladesh, Myanmar, Laos, Thailand, Malaysia, Indonesia and southwestern China, particularly in Sichuan.Schafer, Edward H. (1963) The Golden Peaches of Samarkand: A study of T'ang Exotics. University of California Press. Berkeley and Los Angeles. p. 83 It is now critically endangered, with only five substantial populations in the wild: four in Sumatra and one in Borneo, with an estimated total population of fewer than 80 mature individuals. The species was extirpated in Malaysia in 2019, with the last known bull and cow dying in May and November, respectively, and one of the Sumatran populations may already be extinct. In 2015, researchers announced that the Bornean rhinoceros had become extinct in the northern part of Borneo in Sabah, Malaysia. A tiny population was discovered in East Kalimantan in early 2016.
The Sumatran rhino is a mostly solitary animal except for courtship and offspring-rearing. It is the most vocal rhino species and also communicates through marking soil with its feet, twisting saplings into patterns, and leaving excrement. The species is much better studied than the similarly reclusive Javan rhinoceros, in part because of a program that brought 40 Sumatran rhinos into captivity with the goal of preserving the species. There was little or no information about procedures that would assist in ex situ breeding. Though a number of rhinos died once at the various destinations and no offspring were produced for nearly 20 years, the rhinos were all doomed in their soon-to-be-logged forest.Nardelli, F. 2014 The last chance for the Sumatran rhinoceros?. Pachyderm 55: 43–53 http://www.rhinoresourcecenter.com/index.php?s=1&act=refs&CODE=ref_detail&id=1411778068
The Indonesian ministry of Environment began an official counting of the Sumatran rhino in February 2019, planned to be completed in three years.
Taxonomy and naming
The first documented Sumatran rhinoceros was shot outside Fort Marlborough, near the west coast of Sumatra, in 1793. Drawings of the animal, and a written description, were sent to the naturalist Joseph Banks, then president of the Royal Society of London, who published a paper on the specimen that year. In 1814, the species was given a scientific name by Johann Fischer von Waldheim.
The specific epithet sumatrensis signifies "of Sumatra", the Indonesian island where the rhinos were first discovered. Carl Linnaeus originally classified all rhinos in the genus, Rhinoceros; therefore, the species was originally identified as Rhinoceros sumatrensis or sumatranus.. Joshua Brookes considered the Sumatran rhinoceros with its two horns a distinct genus from the one-horned Rhinoceros, and gave it the name Didermocerus in 1828. Constantin Wilhelm Lambert Gloger proposed the name Dicerorhinus in 1841. In 1868, John Edward Gray proposed the name Ceratorhinus. Normally, the oldest name would be used, but a 1977 ruling by the International Commission on Zoological Nomenclature established the proper genus name as Dicerorhinus. Dicerorhinus comes from the Greek terms (, meaning "two"), (, meaning "horn"), and (, meaning "nose").
The three subspecies are:
D. s. sumatrensis, known as the western Sumatran rhinoceros, which has only 75 to 85 rhinos remaining, mostly in the national parks of Bukit Barisan Selatan and Kerinci Seblat, Gunung Leuser in Sumatra, but also in Way Kambas National Park in small numbers. The subspecies became extinct in Malaysia in 2019. The main threats against this subspecies are habitat loss and poaching. A slight genetic difference is noted between the western Sumatran and Bornean rhinos. The rhinos in Peninsular Malaysia were once known as D. s. niger, but were later recognized to be a synonym of D. s. sumatrensis. Three bulls and five cows currently live in captivity at the Sumatran Rhino Sanctuary at Way Kambas, the youngest bull having been bred and born there in 2012.International_Rhino_Foundation#Sumatran_Rhino_Sanctuary Another calf, a female, was born at the sanctuary in May 2016. The sanctuary's two bulls were born at the Cincinnati Zoo and Botanical Garden. A third calf female was born in March 2022.
D. s. harrissoni, known as the Bornean rhinoceros or eastern Sumatran rhinoceros, which was once common throughout Borneo; now only about 15 individuals are estimated to survive. The known population lives in East Kalimantan, with them having recently gone extinct in Sabah. Reports of animals surviving in Sarawak are unconfirmed. This subspecies is named after Tom Harrisson, who worked extensively with Bornean zoology and anthropology in the 1960s. The Bornean subspecies is markedly smaller in body size than the other two subspecies. The captive population consisted of one bull and two cows at the Borneo Rhinoceros Sanctuary in Sabah; the bull died in 2019 and the cows died in 2017 and 2019 respectively.
D. s. lasiotis, known as the northern Sumatran rhinoceros or Chittagong rhinoceros, which once roamed India and Bangladesh, has been declared extinct in these countries. Unconfirmed reports suggest a small population may still survive in Myanmar, but the political situation in that country has prevented verification. The name lasiotis is derived from the Greek for "hairy-ears". Later studies showed that their ear hair was not longer than other Sumatran rhinos, but D. s. lasiotis remained a subspecies because it was significantly larger than the other subspecies.
Evolution
Ancestral rhinoceroses first diverged from other perissodactyls in the Early Eocene. Mitochondrial DNA comparison suggests the ancestors of modern rhinos split from the ancestors of Equidae around 50 million years ago. The extant family, the Rhinocerotidae, first appeared in the Late Eocene in Eurasia, and the ancestors of the extant rhino species dispersed from Asia beginning in the Miocene.
Although the relationships of modern rhinoceros species to each other were long controversial, modern genetic evidence has placed the Sumatran rhinoceros as more closely related to the Asian one-horned rhinoceroses (the Indian rhinoceros and Javan rhinoceros) belonging to the genus Rhinoceros than to living African rhinoceros species, with the split between Rhinoceros and Dicerorhinus estimated to have occurred around 14.8 million years ago, shortly after the split between the ancestors of Dicerorhinus and Rhinoceros and African rhinoceroses, which is placed around 15.6 million years ago.
Based on morphological and genetic evidence, the Sumatran rhinoceros is believed to be closely related to the extinct woolly rhinoceros (Coelodonta antiquitatis) and Stephanorhinus, with the split between their last common ancestors estimated to be around 9.5 million years ago. The woolly rhinoceros, so named for the coat of hair it shares with the Sumatran rhinoceros, first appeared in China; by the Upper Pleistocene, it ranged across the Eurasian continent from Korea to Spain. The woolly rhinoceros survived until its extinction near the end of the last ice age, around 14,000 years ago. Stephanorhinus species are well known in Europe from the Late Pliocene through the Pleistocene, and China from the Pleistocene, with two species, Stephanorhinus kirchbergensis and the Stephanorhinus hemitoechus surviving into the last glacial period, until at least 40,000 years ago and possibly later.
Although historically many fossil species were assigned to Dicerorhinus today only two fossil species are confidently placed in the genus. These include Dicerorhinus fusuiensis from the Early Pleistocene of South China, and Dicerorhinus gwebinensis from the Pliocene-Early Pleistocene of Myanmar. Fossils of the modern Sumatran rhinoceros are known from the Early Pleistocene onwards.
Pairwise sequential Markovian coalescent (PSMC) analysis of a complete nuclear genome of a Sumatran specimen suggested strong fluctuations in population size, with a general trend of decline over the course of the Middle to Late Pleistocene with an estimated peak effective population size of 57,800 individuals 950,000 years ago, declining to around 500–1,300 individuals at the start of the Holocene, with a slight rebound during the Eemian Interglacial. This was likely due to climate change causing limiting suitable habitat for the Rhinoceros, causing severe population fluctuations as well as population fragmentation due to the flooding of Sundaland. Human induced habitat change and hunting may have played a role in the Late Pleistocene. The study was later criticised for not including DNA from extinct mainland populations, which would have provided a holistic account. A Bayesian skyline plot of complete Mitochondrial genomes from multiple individuals from across the range of the species suggested that the population had been relatively stable with an effective population size of 40,000 individuals over the last 400,000 years, with a sharp decline starting around 25,000 years ago.
Cladogram showing the relationships of recent and Late Pleistocene rhinoceros species (minus Stephanorhinus hemitoechus) based on whole nuclear genomes, after Liu et al, 2021:
Description
A mature Sumatran rhino stands about high at the shoulder, has a body length of around , and weighs , though the largest individuals in zoos have been known to weigh as much as . Like the two African species, it has two horns. The larger is the nasal horn, typically only , though the longest recorded specimen was much longer at . The posterior horn is much smaller, usually less than long, and often little more than a knob. The larger nasal horn is also known as the anterior horn; the smaller posterior horn is known as the frontal horn. The horns are dark grey or black in color. The bulls have larger horns than the cows, though the species is not otherwise sexually dimorphic. The Sumatran rhino lives an estimated 30–45 years in the wild, while the record time in captivity is a female D. lasiotis, which lived for 32 years and 8 months before dying in the London Zoo in 1900.
Two thick folds of skin encircle the body behind the front legs and before the hind legs. The rhino has a smaller fold of skin around its neck. The skin itself is thin, , and in the wild, the rhino appears to have no subcutaneous fat. Hair can range from dense (the most dense hair in young calves) to scarce, and is usually a reddish brown. In the wild, this hair is hard to observe because the rhinos are often covered in mud. In captivity, however, the hair grows out and becomes much shaggier, likely because of less abrasion from walking through vegetation. The rhino has a patch of long hair around its ears and a thick clump of hair at the end of its tail. Like all rhinos, they have very poor vision. The Sumatran rhinoceros is fast and agile; it climbs mountains easily and comfortably traverses steep slopes and riverbanks.
Distribution and habitat