P. agris(Schreber, 1799) P. batangtuensis(Selenka, 1896) P. borneensisRöhrer-Ertl, 1983 P. borneo(Lacépède, 1799) P. brookei(Blyth, 1853) P. curtus(Blyth, 1855) P. dadappensis(Selenka, 1896) P. genepaiensis(Selenka, 1896) P. landakkensis(Selenka, 1896) P. morio(Owen, 1837) P. owenii(Blyth, 1853) P. rantaiensis(Selenka, 1896) P. rufus(Lesson, 1840) P. satyrus(Linnaeus, 1766) [in part] P. skalauensis(Selenka, 1896)> P. sumatranus(Mayer, 1856) P. tuakensis(Selenka, 1896) P. wallichii(Gray, 1871) P. wurmbii(Tiedemann, 1808)
The Bornean orangutan (Pongo pygmaeus) is a species of orangutan endemic to the island of Borneo. Together with the Sumatran orangutan (Pongo abelii) and Tapanuli orangutan (Pongo tapanuliensis), it belongs to the only genus of great apes native to Asia. It is the largest of the three species of orangutans. Like the other great apes, orangutans are highly intelligent, displaying tool use and distinct cultural patterns in the wild. Orangutans share approximately 97% of their DNA with humans.[2] Also called mias by the local population,[3] the Bornean orangutan is a critically endangered species, with deforestation, palm oil plantations, and hunting posing a serious threat to its continued existence.
The Bornean orangutan and the Sumatran orangutan diverged about 400,000 years ago,[4] with a continued low level of gene flow between them since then.[4] The two orangutan species were considered merely subspecies until 1996; they were elevated to species following sequencing of their mitochondrial DNA.
Central Bornean orangutan P. p. wurmbii – Southern West Kalimantan and Central Kalimantan (Indonesia).
Northeast Bornean orangutan P. p. morio – East Kalimantan (Indonesia) and Sabah (Malaysia).
There is some uncertainty about this, however. The population currently listed as P. p. wurmbii may be closer to the Sumatran orangutan (P. abelii) than to the Bornean orangutan. If this is confirmed, P. abelii would be a subspecies of P. wurmbii (Tiedeman, 1808).[5] In addition, the type locality of P. pygmaeus has not been established beyond doubt; it may be from the population currently listed as P. wurmbii (in which case P. wurmbii would be a junior synonym of P. pygmaeus, while one of the names currently considered a junior synonym of P. pygmaeus would take precedence for the taxon in Sarawak and northern West Kalimantan).[5] Bradon-Jones et al. considered P. morio to be a synonym of P. pygmaeus, and the population found in East Kalimantan and Sabah to be a potentially unnamed separate taxon.[5]
In early October 2014, researchers from domestic and foreign countries found about 50 orangutans in several groups in South Kalimantan Province, although previously there is no record that the province has orangutans.[6]
As a member of the family Hominidae, Bornean orangutans are one of the closest extant relatives to Homo sapiens.
This species was originally discovered by native Malaysians. There are several mentions of orangutans in Malaysian folklore. However, this species was originally named and described by the notable zoologist Carl Linnaeus in 1799. Its original name was Simia satyrus, meaning "satyr monkey", but was changed when scientists discovered that not all orangutans are one species. The holotype of this organism is located in the British Museum in London.[7]
The current species name P. pygmaeus is not Latin unlike most other Linnean classifications. The genus name Pongo is derived from the Bantu word mpongo used to indicate a large primate. It was originally used to describe chimpanzees in Western African dialects.[8] The species name pygmaeus is derived from the Greek word "pygmy" meaning dwarf.[9]
Physical description
The skullThe skeleton
The Bornean orangutan is the third-largest ape after the western gorilla, and the largest truly arboreal (or tree-dwelling) extant ape.[10][11] Body weights broadly overlap with the considerably taller Homo sapiens, but the latter is considerably more variable in size.[12] By comparison, the Sumatran orangutan is similar in size but, on average, is marginally lighter in weight.[13][14] A survey of wild orangutans found that males weigh on average 75kg (165lb), ranging from 50–100kg (110–220lb), and 1.2–1.7m (3.9–5.6ft) long; females average 38.5kg (85lb), ranging from 30–50kg (66–110lb), and 1–1.2m (3.3–3.9ft) long.[15][16] While in captivity, orangutans can grow considerably overweight, up to more than 165kg (364lb).[17] The heaviest known male orangutan in captivity was an obese male named "Andy", who weighed 204kg (450lb) in 1959 when he was 13 years old.[18]
The Bornean orangutan has a distinctive body shape with very long arms that may reach up to 1.5 metres in length. It has grey skin, a coarse, shaggy, reddish coat[19] and prehensile, grasping hands and feet.[20] Its coat does not cover its face unlike most mammals, although Bornean orangutans do have some hair on their faces including a beard and mustache. It also has large, fatty cheek pads known as flanges as well as a pendulous throat sac.[21]
A male orangutan at Moscow Zoo. The male's face pad widens as he grows older.
Bornean orangutans are highly sexually dimorphic and have several features that differ between males and females. Males have much larger cheek pads, or flanges, that are composed of muscle and large amounts of fat. In females, the flanges are mostly composed of muscle. Males have relatively larger canines and premolars. Males have a more pronounced beard and mustache. The throat sac in males is also considerably larger. There are two body types for sexually mature males: smaller or larger. Larger males are more dominant but smaller males still breed successfully. There is little sexual dimorphism at birth.[21]
Habitat and distribution
The Bornean orangutan lives in tropical rain forests in the Bornean lowlands, as well as montane rain forests in mountainous areas up to 1,500 metres (4,900ft) above sea level.[22] This species lives throughout the canopy of primary and secondary forest, and moves large distances to find trees bearing fruit.[22]
It is found in the two Malaysian states of Sabah and Sarawak, and four of the five Indonesian Provinces of Kalimantan.[1] Due to habitat destruction, the species distribution is now highly patchy throughout the island, the species has become rare in the southeast of the island, as well as in the forest between the Rajang River in central Sarawak and the Padas River in western Sabah. Its presence in Brunei is uncertain and unconfirmed.[1]
The first complete orangutan skeleton that was discovered was in the Hoa Binh province in Vietnam and thought to be from the late Pleistocene epoch. It differed from modern orangutans only in that its body was proportionately smaller compared to its head. This fossil and others confirm that orangutans once inhabited continental Southeast Asia even though currently, Bornean orangutans are only found in Malaysia and Indonesia.[23]
Behavior and ecology
Bornean orangutan in its nest
In history, orangutans ranged throughout Southeast Asia and into southern China, as well as on the island of Java and in southern Sumatra. They primarily inhabit peat swamp forest, tropical heath forest, and mixed dipterocarp forest.[2] Bornean orangutan are more solitary than their Sumatran relatives. Two or three orangutans with overlapping territories may interact, but only for short periods of time.[24] Although orangutans are not territorial, adult males will display threatening behaviors upon meeting other males, and only socialize with females to mate.[25] Males are considered the most solitary of the orangutans. The Bornean orangutan has a lifespan of 35–45 years in the wild;[4] in captivity it can live to be about 60.[26]
Despite being arboreal, the Bornean orangutan travels on the ground more than its Sumatran counterpart. This may be in part because no large terrestrial predators could threaten an orangutan in Borneo. In Sumatra, orangutans must face predation by the fierce Sumatran tiger.[27]
The Bornean orangutan exhibits nest-building behavior. Nests are built for use at night or during the day. Young orangutans learn by observing their mother's nest-building behaviour. This skill is practiced by juvenile orangutans. Nests may be elaborate and involve a foundation and mattress made by intertwining leaves and branches and adding broken leafy branches. Additional features such as shade, waterproof roof, "pillow", and "blanket", all of which are made from branches, twigs and leaves, may also be added.[28] Nest-building in primates is considered as an example of tool use and not animal architecture.[29]
Diet
The Bornean orangutan diet is composed of over 400 types of food, including wild figs, durians (Durio zibethinus and D. graveolens),[30]leaves, seeds, bird eggs, flowers, sap, vines,[31]honey, fungi, spider webs,[31] insects, and, to a lesser extent than the Sumatran orangutan, bark.[2][16] They have also been known to consume the inner shoots of plants and vines.[2] They will also occasionally eat nutrient rich soil.[31] They get the necessary quantities of water from both fruit and from tree holes.[22]
An orangutan peeling a banana with its hand and foot.
Bornean orangutans have been sighted using spears to attempt (unsuccessfully) to catch fish.[32] The species has been observed using tools such as leaves to wipe off faeces, a pad of leaves for holding spiny durian fruit, a leafy branch for a bee swatter, a bunch of leafy branches held together as an "umbrella" while traveling in the rain, a single stick as backscratcher, and a branch or tree trunk as a missile.[33] In some regions, orangutans occasionally eat soil to get minerals that may neutralize the toxins and acids they consume in their primarily vegetarian diets.[2] On rare occasions, orangutans will prey upon other, smaller primates, such as slow lorises.[2]
Males and females generally come together only to mate. Subadult males (unflanged) will try to mate with any female and will be successful around half the time.[24] Dominant flanged males will call and advertise their position to receptive females, who prefer mating with flanged males.[24] Adult males will often target females with weaned infants as mating partners because the female is likely to be fertile.[34]
Females reach sexual maturity and experience their first ovulatory cycle between about six and 11 years of age, although females with more body fat may experience this at an earlier age.[24] The estrous cycle lasts between 22 and 30 days and menopause has been reported in captive orangutans at about age 48.[24] Females tend to give birth at about 14–15 years of age. Newborn orangutans nurse every three to four hours, and begin to take soft food from their mothers' lips by four months. During the first year of its life, the young clings to its mother's abdomen by entwining its fingers in and gripping her hair. Offspring are weaned at about four years, but this could be much longer, and soon after they start their adolescent stage of exploring, but always within sight of their mother.[24] During this period, they will also actively seek other young orangutans to play with and travel with. On average, juveniles do not become completely independent until they are about seven years of age. The birth rate for orangutans has been decreasing largely due to a lack of sufficient nutrients as a result of habitat loss.
A 2011 study on female orangutans in free-ranging rehabilitation programs found that individuals that were supplemented with food resources had shorter interbirth intervals, as well as a reduced age, at first birth.[35]
Conservation status
The Bornean orangutan is more common than the Sumatran, with about 104,700 individuals in the wild, whereas just under 14,000 Sumatran orangutans are left in the wild.[1][36] Orangutans are becoming increasingly endangered due to habitat destruction and the bushmeat trade, and young orangutans are captured to be sold as pets, usually entailing the killing of their mothers.[37]
The Bornean orangutan is critically endangered[1][38] according to the IUCN Red List of mammals, and is listed on Appendix I of CITES. The total number of Bornean orangutans is estimated to be less than 14% of what it was in the recent past (from around 10,000 years ago until the middle of the 20th century), and this sharp decline has occurred mostly over the past few decades due to human activities and development.[1] Species distribution is now highly patchy throughout Borneo; it is apparently absent or uncommon in the southeast of the island, as well as in the forest between the Rajang River in central Sarawak and the Padas River in western Sabah (including the Sultanate of Brunei).[1] A population of around 6,900 is found in Sabangau National Park, but this environment is at risk.[39]
This view is also supported by the United Nations Environment Programme, which stated in its 2007 report that, due to illegal logging, fire and the extensive development of palm oil plantations, orangutans are critically endangered, and if the current trend continues, they will become extinct.[40] When forest is burned down to clear room for palm oil plantations, not only does the Bornean orangutan suffer from habitat loss, but several individuals have been burned and killed in fires. Palm oil accounts for over one tenth of Indonesia's export earnings. It is in high demand because it is used in several packaged foods, deodorants, shampoos, soaps, candies, and baked goods.[41]
Climate change is another threat to Bornean orangutan conservation. The effects that human activity have had on Indonesian rainfall have made food less abundant and so Bornean orangutans are less likely to receive full nutrients so that they can be sufficiently healthy to breed.
A November 2011 survey, based on interviews with 6,983 respondents in 687 villages across Kalimantan in 2008 to 2009, gave estimated orangutan killing rates of between 750 and 1800 in the year leading up to April 2008.[42] These killing rates were higher than previously thought and confirm that the continued existence of the orangutan in Kalimantan is under serious threat. The survey did not quantify the additional threat to the species due to habitat loss from deforestation and expanding palm-oil plantations. The survey found that 73% of respondents knew orangutans were protected by Indonesian law.[42]
A young captive orangutan sleeping.
However, the Indonesian government rarely prosecutes or punishes perpetrators.[43] In a rare prosecution in November 2011, two men were arrested for killing at least 20 orangutans and a number of long-nosed proboscis monkeys. They were ordered to conduct the killings by the supervisor of a palm oil plantation, to protect the crop, with a payment of $100 for a dead orangutan and $22 for a monkey.[44]
Rescue and rehabilitation centers
Bornean orangutan in the Hong Kong Zoological and Botanical Gardens.
A number of orangutan rescue and rehabilitation projects operate in Borneo.
Orangutan Foundation International, founded by Birutė Galdikas, rescues and rehabilitates orangutans, preparing them for release back into protected areas of the Indonesian rain forest. In addition, it promotes the preservation of the rain forest for them.
Orangutan Foundation, founded by Ashley Leiman, operates programmes in Central Kalimantan, Indonesian Borneo. The Foundation rescues orphaned orangutans and enters them into their soft-release programme, allowing them to develop the skills necessary to survive in the wild. When old enough, orangutans are released into the protected Lamandau Wildlife Reserve. Orangutan Foundation works to protect orangutans by focusing on habitat protection and capacity building, especially in local communities.[46]
A seven-year longitudinal study published in 2011 looked at whether the lifespan of zoo-housed orangutans was related to a subjective assessment of well-being, with the intent of applying such measures to assess the welfare of orangutans in captivity. Of the subjects, 100 were Sumatran (Pongo abelii), 54 Bornean (Pongo pygmaeus) and 30 were hybrid orangutans. 113 zoo employees, who were highly familiar with the typical behavior of the orangutans, used a four-item questionnaire to assess their subjective well-being. The results indicated that orangutans in higher subjective well-being were less likely to die during the follow-up period. The study concluded that happiness was related to longer life in orangutans.[47]
In late 2014, Nyaru Menteng veterinarians failed to rescue the life of a female orangutan. An operation was performed in which 40 air-rifle pellets were removed from her body. The orangutan was found at a palm oil plantation in Indonesian Borneo.[48]
Orangutans and humans diverged lineages approximately 14-18 million years ago. About 17,000 years ago, there was a migration of the Bornean orangutans as they eventually went to Sumatra, effectively trading places with the Sumatra orangutans that were there at the time. These two species of orangutans have been closely related throughout their evolutionary history due to the fact that they were so close in physical proximity. Therefore, their genomes and demographic history are similar. The two species themselves are estimated to have split about 3.5 million years ago.[49] Although these two species have officially diverged, it is speculated that the reason as to why they are genetically similar is because the males of each respective species tend to migrate between the two islands and breed with the females from their sister species. As a result, both the Bornean orangutans and the Sumatran orangutans have been studied closely as a pair, and thus much genome findings attribute evolutionary changes to this relationship. In addition, the Bornean orangutans, as compared to the Sumatran orangutans, have lower autosomal gene diversity. This is attributed to the fact that they have a much smaller population size. Also, the Bornean orangutans have lower nucleotide diversity.[50]
As the Bornean orangutans and Sumatra orangutans both exist within the same species, they exhibit similar cultural behaviors that have been found to exist amongst most orangutan populations. The fact that orangutans tend to showcase similar cultural traditions is due to the fact that they typically live in similar environments and are adept at learning from one another from their early stages of life.[51]
The Bornean orangutan has been linked to the fact that it has gone through a deep divergence in relation to its relatives and ancestors.[52] During the Middle Pleistocene, there were low levels of gene flow, which was determined through the analysis of Y-chromosomal data. One reason as to why this may have occurred is because of the Sunda shelf, which is where the island of Borneo is located.[49] During this time, this event's dry climate during the Late Pleistocene attributed to a more abundant genetic exchange. As a result, there were many early divergences of gene pools between the Bornean orangutans, as well as the Sumatran orangutans. Relating back to the Middle Pleistocene, the Bornean orangutan lineage went through a dramatic population decline. This is likely attributed to the fact that they had been isolated from their ancestral populations.[50] Therefore, natural geographic barriers are attributed to be the reason as to why the Bornean orangutans were eventually isolated and ended up colonizing other regions. In addition, this geographic isolation also indicates that the Bornean orangutans did not undergo a severe genetic bottleneck.[52] With the Borneo orangutan, selection was found to have been found through physiological adaptations – most of which has to do with being able to adapt to the ever-changing climate on the Borneo island.[49]
Orangutans are great apes native to the rainforests of Indonesia and Malaysia. They are now found only in parts of Borneo and Sumatra, but during the Pleistocene they ranged throughout Southeast Asia and South China. Classified in the genus Pongo, orangutans were originally considered to be one species. From 1996, they were divided into two species: the Bornean orangutan and the Sumatran orangutan. A third species, the Tapanuli orangutan, was identified definitively in 2017. The orangutans are the only surviving species of the subfamily Ponginae, which diverged genetically from the other hominids between 19.3 and 15.7 million years ago.
Ponginae, also known as the Asian hominids, is a subfamily in the family Hominidae. Once a diverse lineage of Eurasian apes, the subfamily has only one extant genus, Pongo (orangutans), which contains three extant species; the Sumatran orangutan, the Tapanuli orangutan and the Bornean orangutan. All three species are listed as critically endangered by the International Union for Conservation of Nature (IUCN).
Gunung Leuser National Park is a national park covering 7,927 km2 in northern Sumatra, Indonesia, straddling the border of Aceh and North Sumatra provinces, a fourth portion and three-fourths portion, respectively. The national park, settled in the Barisan mountain range, is named after Mount Leuser (3,119 m), and protects a wide range of ecosystems. An orangutan sanctuary at Bukit Lawang is located within the park. Together with Bukit Barisan Selatan and Kerinci Seblat National Parks, it forms a World Heritage Site, the Tropical Rainforest Heritage of Sumatra.
The Sumatran orangutan is one of the three species of orangutans. Critically endangered, and found only in the north of the Indonesian island of Sumatra, it is rarer than the Bornean orangutan but more common than the recently identified Tapanuli orangutan, also found in Sumatra. Its common name is based on two separate local words, "orang" and "hutan" ("forest"), derived from Malay, and translates as 'person of the forest'.
The fauna of Indonesia is characterised by high levels of biodiversity and endemicity due to its distribution over a vast tropical archipelago. Indonesia divides into two ecological regions; western Indonesia which is more influenced by Asian fauna, and the east which is more influenced by Australasian species.
The Hominidae, whose members are known as the great apes or hominids, are a taxonomic family of primates that includes eight extant species in four genera: Pongo ; Gorilla ; Pan ; and Homo, of which only modern humans remain.
The Borneo Orangutan Survival (BOS) Foundation is an Indonesian non-profit non-governmental organization founded by Dr. Willie Smits in 1991 and dedicated to the conservation of the endangered Bornean orangutan and its habitat through the involvement of local people. It is audited by an external auditor company and operates under the formal agreement with the Indonesian Ministry of Forestry to conserve and rehabilitate orangutans. The BOS Foundation manages orangutan rescue, rehabilitation and re-introduction programmes in East and Central Kalimantan. With more than 400 orangutans in its care and employing more than 440 people at a 10 sites BOS Foundation is the biggest non-human primate conservation non-governmental organization worldwide. Nyaru Menteng and Samboja Lestari are the BOS Foundation sites that have received most extensive media coverage. Nyaru Menteng, founded by Lone Drøscher Nielsen, has been the subject of a number of TV series, including Orangutan Diary, Orangutan Island and the series Orangutan Jungle School, airing since 2018.
The Borneo lowland rain forests is an ecoregion, within the tropical and subtropical moist broadleaf forests biome, of the large island of Borneo in Southeast Asia. It supports approximately 15,000 plant species, 380 bird species and several mammal species. The Borneo lowland rain forests is diminishing due to logging, hunting and conversion to commercial land use.
Tanjung Puting National Park is a national park in Indonesia located in the southeast part of West Kotawaringin Regency in the Indonesian province of Central Kalimantan. The nearest main town is the capital of the Regency, Pangkalan Bun. The park is famous for its orangutan conservation.
Palm oil, produced from the oil palm, is a basic source of income for many farmers in South East Asia, Central and West Africa, and Central America. It is locally used as cooking oil, exported for use in much commercial food and personal care products and is converted into biofuel. It produces up to 10 times more oil per unit area than soybeans, rapeseed or sunflowers.
Samboja Lestari is a Bornean orangutan rescue and rehabilitation centre, tropical rainforest restoration project, sun bear sanctuary, and eco-lodge located in the district of Samboja in Kutai Kartanegara Regency, East Kalimantan, Indonesia, owned and operated by the Borneo Orangutan Survival (BOS) Foundation. According to its founder, Willie Smits, Samboja Lestari uses the principles of People, Planet, Profit, attempting to provide incomes for local people using conservation. It is located about 38 kilometres from Balikpapan.
The Borneo montane rain forests is an ecoregion on the island of Borneo in Southeast Asia. It includes montane tropical and subtropical moist broadleaf forests, also known as a cloud forests. The ecoregion is partly in East Malaysia and Indonesia (Kalimantan).
Anne E. Russon is a Canadian psychologist and primatologist. She is a researcher and Professor of Psychology at Glendon College, York University, Toronto, Ontario, Canada whose research focuses on learning and intelligence in ex-captive Bornean orangutans. Russon is widely published in the fields of primate behavior and ecology, is executive director of the Borneo Orangutan Society of Canada, and is the author of several popular press books dealing with Great Apes including Orangutans: Wizards of the Rainforest, Reaching into Thought: The Minds of the Great Apes, and The Evolution of Thought: Evolution of Great Ape Intelligence.
Pawan River is a major river of West Kalimantan, Indonesia. It has a length of 197 kilometres (122 mi). Tributaries include the Keriau River.
Lamandau River is a river of Kalimantan, southern-central Borneo, Indonesia. Downstream after passing the village of Nanga Bulik it becomes the Kotawaringin River.
The Bornean rhinoceros, also known as the eastern Sumatran rhinoceros or eastern hairy rhinoceros, is one of three subspecies of Sumatran rhinoceros. The subspecies was feared to be functionally extinct, with only one individual, a female named Pahu, surviving in captivity, and held in the state of Sabah. In April 2015, the Malaysian government declared the Bornean rhinoceros to be extinct in the wild in the Malaysian portion of Borneo. However, in March 2016, a young female rhino was captured in East Kalimantan, providing evidence of their continued existence. The International Union for Conservation of Nature (IUCN) classifies the subspecies as critically endangered.
The Sumatran Orangutan Conservation Programme (SOCP) is a collaborative project involving Indonesian NGO Yayasan Ekosistem Lestari (YEL) - as the main implementer in Indonesia, its Swiss partner the PanEco Foundation, and the Indonesian Ministry of Environment and Forestry’s Directorate General of Natural Resource and Ecosystem Conservation, under several Memoranda of Understanding (MoU) starting in 1999.
A hybrid orangutan or cocktail orangutan is usually an orangutan derived from interbreeding between any of the three Orangutan species: Bornean, Sumatran and Tapanuli, but the term "hybrid orangutan" could also refer to hybrids of the three known Bornean subspecies. As of 2015, there are approximately 134 living Bornean x Sumatran hybrid orangutans in captivity.
The Tapanuli orangutan is a species of orangutan restricted to South Tapanuli in the island of Sumatra in Indonesia. It is one of three known species of orangutan, alongside the Sumatran orangutan, found farther northwest on the island, and the Bornean orangutan. It was described as a distinct species in 2017. As of 2018, there are roughly 800 individuals of this species and it is currently on the critically endangered species list.
There are three species of orangutan. The Bornean orangutan, the most common, can be found in Kalimantan, Indonesia and Sarawak and Sabah in Malaysia. The Sumatran orangutan and the Tapanuli orangutan are both only found in Sumatra, Indonesia. The conservation status of all three of these species is critically endangered, according to the International Union for Conservation of Nature (IUCN) Red List.
↑ Bacon, A. M.; The Long, V. (September 2001). "The first discovery of a complete skeleton of a fossil orang-utan in a cave of the Hoa Binh Province, Vietnam". Journal of Human Evolution. 41 (3): 227–241. doi:10.1006/jhev.2001.0496. ISSN0047-2484. PMID11535001.
↑ Fox, E.A. (2002). "Female tactics to reduce sexual harassment in the sumatran orangutan (Pongo pygmaeus abelii)". Behavioral Ecology and Sociobiology. 52 (2): 93–101. doi:10.1007/s00265-002-0495-x. S2CID13583879.
↑ Kuze, Noko; Pratje, Peter; Banes, Graham L.; Pratje, Peter; Tajima, Tomoyuki; Russon, Anne E. (2011). "Factors affecting reproduction in rehabilitant female orangutans: young age at first birth and short inter-birth. interval". Primates. 53 (2): 181–92. doi:10.1007/s10329-011-0285-z. PMID22109351. S2CID17214597.
↑ Singleton, Ian; Wich, Serge A.; Nowak, Matthew G.; Usher, Graham; Utami-Atmoko, Sri Suchi (2018) [errata version of 2017 assessment]. "Pongo abelii". IUCN Red List of Threatened Species. 2017: e.T121097935A123797627.
Russon, Anne E.; Compost, Alain; Kuncoro, Purwo; Ferisa, Agnes (December 2014). "Orangutan fish eating, primate aquatic fauna eating, and their implications for the origins of ancestral hominin fish eating". Journal of Human Evolution. 77: 50–63. doi:10.1016/j.jhevol.2014.06.007. PMID25038033.
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