|Scientific Name:||Pongo abelii|
|Species Authority:||Lesson, 1827|
|Taxonomic Notes:||Historically, the Sumatran and Bornean Orangutans were both considered to be subspecies of Pongo pygmaeus (e.g., Rijksen and Meijaard 1997). Recent taxonomic reviews (Groves 2001, Brandon-Jones et al. 2004) support the acceptance of the Sumatran Orangutan (Pongo abelii) as distinct from its Bornean relative (Pongo pygmaeus). This classification has since been widely adopted (e.g., Singleton et al. 2004).|
|Red List Category & Criteria:||Critically Endangered A4cd ver 3.1|
|Assessor(s):||Singleton, I., Wich , S.A., Nowak, M. & Usher, G.|
|Reviewer(s):||Williamson, E.A. & Mittermeier, R.A.|
Due to high levels of habitat conversion and fragmentation, and illegal killing, Pongo abelii is estimated to have experienced a significant population reduction in recent years. Forest loss data indicate that key Sumatran Orangutan forest habitat (i.e., below 500 m asl) was reduced by 60% of its area between 1985 and 2007 (Wich et al. 2008, 2011). It is thought that this reduction will continue as forests within the species’ range remain under considerable threat (Wich et al. 2016). When relative stability returned to Aceh in 2005 after several years of civil conflict, pressure on natural habitats increased dramatically (Wich et al. 2011). Significant areas of the Orangutan’s range are seriously threatened by logging, mining concessions and agricultural plantations, while new roads are continuously being cut through the habitat. Even in formally protected areas, Orangutans remain under threat from forest conversion to plantations, illegal settlement and encroachment (Wich et al. 2008, 2011, 2016), and in some areas poaching (Wich et al. 2012). Furthermore, an illegal spatial land-use plan being implemented by the Government of Aceh Province ignores the Leuser Ecosystem’s status as a 'National Strategic Area', designated for its environmental function. Moreover, modelling based on different land-use scenarios and their likely impacts predicts that an additional 4,500 Sumatran Orangutans could be lost by 2030 as a direct consequence of this spatial plan and related developments (Wich et al. 2016).
Due to their slow life history, with a generation time of at least 25 years (Wich et al. 2004, 2009), Sumatran Orangutan populations are unable to sustain substantial and continual loss of individuals. If the rate of decline observed since 1985 and predicted continues unabated, the population decline will exceed 80% over a three-generation period (i.e., 75 years from 1985 to 2060), hence qualifying Pongo abelii as Critically Endangered under criterion A.
|Previously published Red List assessments:|
|Range Description:||Pongo abelii is endemic to the island of Sumatra, Indonesia. It is restricted to the north of the island, with its southern limit being the Batang Toru River on the west coast of North Sumatra Province and its northern limit coinciding with the northern boundary of the Leuser Ecosystem in Aceh Province (Wich et al. 2003, 2008, 2016). It was once far more widespread, occurring as far south as Jambi and West Sumatra provinces until at least the mid-1800s (see Rijksen 1978). There were in fact reports of Sumatran Orangutans in some parts of West Sumatra Province as recently as the 1960s, but surveys by Wich et al. (2003) found no evidence of their continuing survival south of the Batang Toru River catchment.|
Today the majority of Sumatran Orangutans (78.6%) are found in Aceh Province at the northernmost tip of the island. There are populations in North Sumatra Province but the largest of these, in the southern and eastern regions of the Leuser Ecosystem, straddles the border with Aceh. Despite a few smaller patches of forest south of the Leuser Ecosystem appearing to still host Orangutan populations, only two entirely North Sumatran populations are considered viable in the long term, namely the West Batang Toru and Pakpak Barat populations (for precise locations, see Wich et al. 2008, 2016).
Sumatran Orangutan densities decline with increasing altitude and few, if any, reproducing populations are thought to reside in forests above 1,500 m asl (Wich et al. 2016).
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||The most recent population estimate for the Sumatran Orangutan is 14,613 individuals, in a total area of 17,797 km² of forest (Wich et al. 2016). Excluding populations of fewer than 250 individuals (i.e., considering only populations that are potentially viable over the long term) leaves just 13,835 individuals. The vast majority (90.2%) occur in the Leuser Ecosystem, while other populations are found in the Sidiangkat, Pakpak and Batang Toru forests. The 2016 estimate is higher than the previous estimate of around 6,600 individuals remaining (Wich et al. 2008), as it takes into account three factors: a) Orangutans were found in greater numbers at higher altitudes than previously supposed (i.e., up to 1,500 m asl not just to 1,000 m asl), b) they were found to be more widely distributed in selectively-logged forests than previously assumed, and c) Orangutans were found in some previously unsurveyed forest patches. The new estimate does not, therefore, reflect a real increase in Sumatran Orangutan numbers. On the contrary, it reflects only much improved survey techniques and coverage, and hence more accurate data. It is extremely important to note, therefore, that overall numbers continue to decline dramatically.|
Since Sumatran Orangutans have been found up to 1,500 m asl in many areas, there is probably better connectivity among subpopulations in the mountainous Leuser Ecosystem than was previously considered the case (Wich et al. 2008). Nevertheless, recent studies found genetic differentiation between subpopulations that is at least partially due to geographic barriers, such as rivers and high mountain ridges, even within the Leuser Ecosystem (Nater et al. 2013).
In addition to the wild populations, two entirely new Sumatran Orangutan populations are gradually being established via the reintroduction of confiscated illegal pets; one in and around the Bukit Tigapuluh National Park (Jambi and Riau provinces) and one in and around the Jantho Pine Forest Nature Reserve, in the far north of Aceh. To date, more than 260 individuals have been reintroduced. The goal of these efforts is to eventually establish new, genetically-viable, fully-reproducing and self-sustaining wild populations as a safety net against catastrophe elsewhere in the species’ range.
|Current Population Trend:||Decreasing|
|Habitat and Ecology:||Sumatran Orangutans inhabit moist lowland forest, montane forest and peat swamps. They are diurnal and almost exclusively arboreal; females virtually never travel on the ground and adult males do so only rarely. This contrasts somewhat with Bornean Orangutans (especially adult males), which descend and travel on the ground more often (Ancrenaz et al. 2014). While both species depend on high-quality primary forests, Bornean Orangutans appear better able to tolerate limited habitat disturbance (Husson et al. 2009). In Sumatra densities plummet by up to 60%, even if logging is highly selective (see Rao and van Schaik 1997) and Orangutan behavioural ecology differs in primary and logged forests (Hardus et al. 2012a).|
Sumatran Orangutans are primarily frugivores, but also eat young and mature leaves, seeds, shoots, pith, flowers, insects (termites and ants), bark and, on occasion, the meat of slow loris (Fox et al. 2004, Wich et al. 2006, Morrogh-Bernard et al. 2009, Hardus et al. 2012b). Female home ranges vary in size from 0.8–1.5 km². The true extent of male home range size is not fully known, although ranges well in excess of 30 km² are inferred (Singleton and van Schaik 2001, Singleton et al. 2009).
Females first give birth at about 15 years of age (Wich et al. 2004, 2009). Sumatran Orangutans have the longest interbirth intervals of any mammalian species, ranging from 8.2 to 9.3 years (compared with 6.1 to 7.7 years for P. pygmaeus; Wich et al. 2004, 2009, van Noordwijk and van Schaik 2005). Gestation lasts approximately 254 days (Kingsley 1981). Infants are weaned at ca 7 years. On average, a female bears 4–5 offspring during her lifetime. Males mature at about about 14 years (Wich et al. 2004). They exhibit bimaturism, whereby fully flanged adult males and smaller unflanged males are both capable of reproducing, but employ differing mating strategies (Utami Atmoko et al. 2002, 2009). Longevity in the wild has been estimated at 53 years for females and 58 years for males (Wich et al. 2004). Generation length is at least 25 years (Wich et al. 2004, 2009).
|Generation Length (years):||25|
|Movement patterns:||Not a Migrant|
|Use and Trade:||It is illegal to capture, injure, kill, own, keep, transport, or trade a Sumatran Orangutan.|
The Sumatran Orangutan's survival is seriously threatened by habitat loss and fragmentation (Wich et al. 2008, 2011, 2016). Forests continue to be cleared at the large and medium scale for oil-palm plantations that can each cover hundreds of square kilometres. On a smaller scale, logging for timber (both legal and illegal) remains a threat, as does the creation of new roads, which fragment populations and gives access to illegal settlements and further encroachment for agriculture and plantations (also frequently illegal), and to wildlife poachers. When industrial plantations are established, the resident orangutans are forced to seek refuge in adjacent forest patches, if any remain, but in the long term they are likely to succumb to malnutrition and starvation due to competition and limited resources. Such forest fragments are often subsequently cleared as well.
Sumatran Orangutans are frequently killed deliberately, completely illegally, and surviving infants end up in an illegal pet trade. This trade tends to be a by-product of habitat conversion, for example, if an Orangutan is found in an isolated patch of trees during the conversion process, there is a high probability it will be killed. Sumatran Orangutans are also regularly killed in human-wildlife conflict situations, for example, if raiding fruit crops on farmland at the forest edge. In parts of North Sumatra, orangutans are still occasionally hunted as food (Wich et al. 2012).
By far the largest single current threat to the Sumatran Orangutan comes from a spatial land-use plan ratified by the government of Aceh Province in 2013. Conservation of the Leuser Ecosystem being obligated by Aceh’s own special autonomy law (Law No 11 2006) due to its designation in 2007/8 as a National Strategic Area for its environmental function (which requires its inclusion and special consideration at all levels of spatial planning). But alarmingly the current Aceh spatial plan completely ignores the Leuser Ecosystem’s existence. Both the provincial and national governments have publicly acknowledged the illegality of the Aceh spatial plan, but to date it remains officially ratified at the provincial level and has not been formally cancelled by the national government. As of early 2016, legal challenges are attempting to rectify this situation and to have the current Aceh spatial plan replaced with one that complies fully with all relevant laws, is based on sound environmental sensitivity analyses, and affords appropriate protection to the Leuser Ecosystem in accordance with existing legislation. However, whilst the status quo persists, the Aceh Provincial Spatial Plan of 2013 allows huge tracts of Sumatran Orangutan habitat to be designated for new plantations, and timber and mining concessions, and will lead to many more Sumatran Orangutans being lost in the ensuing years (Wich et al. 2016). The existing plan also effectively legitimises numerous roads cut illegally through the forest, which further fragment Orangutan populations and provide yet more access to hunting and encroachment.
Pongo abelii is strictly protected by Indonesian National Law No. 5/1990 on the Conservation of Natural Resources and Ecosystems, under which it is illegal to capture, injure, kill, own, keep, transport, or trade a Sumatran Orangutan. Sumatran Orangutans are also protected by international legislation, and listed on CITES Appendix I.
Protection of large areas of primary forest below 1,500 m asl is needed to secure their long-term future. The species’ major stronghold is the Leuser Ecosystem – an area of 26,000 km² mostly contiguous forest that supports circa 90.2% of the Sumatran Orangutans remaining in the wild. Conservation of the Leuser Ecosystem was called for under Indonesian National Law No. 11/2006 concerning Governance in Aceh, and it was inaugurated by Presidential Decree in 1998. Designated a National Strategic Area for its environmental function, the Leuser Ecosystem must be fully recognised and its integrity ensured at all levels of spatial land-use planning. According to these laws, management of the Leuser Ecosystem does not exclude non-forest uses, but stresses the importance of sustainable management with conservation of natural resources as the primary goal.
Within the Leuser Ecosystem is the 9,000 km² Gunung Leuser National Park, also designated a Man and Biosphere Reserve and a part of the Tropical Rainforest Heritage of Sumatra World Heritage Cluster Site by UNESCO. The park alone, comprising mostly high mountains, supports only 34.6% of Sumatra’s Orangutans. Much of Sumatra’s dense lowland forest is outside the National Park’s boundaries, but is part of the larger Leuser Ecosystem. Also within the Leuser Ecosystem, but not a part of the World Heritage Cluster Site, is the 1,025 km² Singkil Swamps Wildlife Reserve.
Outside the Leuser Ecosystem, no other large, formally-established conservation areas of note harbour this species. However, recent land-use status changes have brought more Orangutans under protection, as a large swathe of former ‘production forest’ in west Batang Toru is now designated as ‘protection forest’. This status is not the same as a formally-recognised 'Conservation Area’, but does mean that the area is no longer unavailable to non-forest uses, such as extractive industry or forest conversion. Even though Batang Toru is relatively small, it is a critical area because of its behaviourally- and genetically-distinct Orangutan population (Nater et al. 2011, 2013, 2015, Wich et al. 2014).
|Errata reason:||This is an errata version of the 2016 assessment to correct some minor typos and grammatical errors in the Habitat & Ecology and Conservation Actions sections, and to remove a duplicate reference from the Bibliography section.|
Ancrenaz, M., Sollmann, R., Meijaard, E., Hearn, A.J., Ross, J., Samejima, H., Loken, B., Cheyne, S.M., Stark, D.J., Gardner, P.C., Goossens, B., Mohamed, A., Bohm, T., Matsuda, I., Nakabayasi, M., Lee, S.K., Bernard, H., Brodie, J., Wich, S., Fredriksson, G., Hanya, G., Harrison, M.E., Kanamori, T., Kretzschmar, P., Macdonald, D.W., Riger, P., Spehar, S., Ambu, L.N. and Wilting, A. 2014. Coming down the trees: is terrestrial activity in orangutans natural or disturbance-driven? Nature Scientific Reports 4(4024): 1–4.
Brandon-Jones, D., Eudey, A.A., Geissman, T., Groves, C.P., Melnick, D.J., Morales, J.C., Shekelle, M. and Stewart, C.B. 2004. Asian primate classification. International Journal of Primatology 25(1): 97–164.
Courtney, J., Groves, C. and Andrews, P. 1988. Inter- and intra-island variation? An assessment of the differences between Bornean and Sumatran orangutans. In: Schwartz, J.H. (ed.), Orangutan Biology, pp. 19–29. Oxford University Press, Oxford, UK.
Fox, E.A., van Schaik, C.P., Sitompul, A. and Wright, D.N. 2004. Intra- and interpopulational differences in orangutan (Pongo pygmaeus) activity and diet: implications for the invention of tool use. American Journal of Physical Anthropology 125: 162–174.
Goossens, B., Chikhi, L. Ancrenaz, M., Lackman-Ancrenaz, I., Audau, P. and Bruford, M.W. 2006. Genetic signature of anthropogenic population collapse in orangutans. PLoS Biology 4: 285–291.
Goossens, B., Chikki, L., Jalil, F., James, S., Ancrenaz, M., Lackman-Ancrenaz, I. and Bruford, M.W. 2009. Taxonomy, geographic variation and population genetics of Bornean and Sumatran orangutans. In: S.A. Wich, S.S. Utami Atmoko, T. Mitra Setia and C.P. van Schaik (eds), Orangutans: Geographic Variation in Behavioral Ecology and Conservation, pp. 1–13. Oxford University Press, Oxford.
Gregory, S.D., Brook, B.W., Goossens, B., Ancrenaz, M., Alfred, R., Ambu, L.N. and Fordham, D.A. 2012. Long-term field data and climate-habitat models show that orangutan persistence depends on effective forest management and greenhouse gas mitigation. PLoS One 7(9): e43846.
Groves C. 2001. Primate Taxonomy. Smithsonian Institution Press, Washington, DC, USA.
Hardus, M.E., Lameira, A.R., Menken, S.B.J. and Wich, S.A. 2012a. Effects of logging on orangutan behavior. Biological Conservation 146: 177–187.
Hardus, M.E., Lameira, A.R., Zulfa, A., Utami Atmoko, S., de Vries, J. and Wich, S.A. 2012b. Behavioral, ecological and evolutionary aspects of meat-eating by Sumatran orangutans (Pongo abelii). International Journal of Primatology 33: 287–304.
Husson, S.J., Wich, S.A., Marshall, A.J., Dennis, R.D., Ancrenaz, M., Brassey, R., Gumal, M., Hearn, A.J., Meijaard, E., Simorangkir, T. and Singleton, I. 2009. Orangutan distribution, density, abundance and impacts of disturbance. In: S.A. Wich, S.S. Utami Atmoko, T. Mitra Setia and C.P. van Schaik (eds), Orangutans: Geographic Variation in Behavioral Ecology and Conservation, pp. 77–96. Oxford University Press, Oxford, UK.
IUCN. 2016. The IUCN Red List of Threatened Species. Version 2016-2. Available at: www.iucnredlist.org. (Accessed: 04 September 2016).
IUCN. 2016. The IUCN Red List of Threatened Species. Version 2016-3. Available at: www.iucnredlist.org. (Accessed: 07 December 2016).
Kingsley, S. 1981. The reproductive physiology and behaviour of captive orangutans (Pongo pygmaeus). University of London.
Marshall, A.J., Lacy, R., Ancrenaz, M., Byers, O., Husson S.J., Leighton, M., Meijaard, E., Rosen, N., Singleton, I., Stephens, S., Traylor-Holzer, K., Utami Atmoko, S.S., van Schaik, C.P. and Wich, S.A. 2009. Orangutan population biology, life history, and conservation. Perspectives from population viability analysis models. In: S.A. Wich, S.S. Utami Atmoko, T. Mitra Setia and C.P. van Schaik (eds), Orangutans: Geographic Variation in Behavioral Ecology and Conservation, pp. 311–326. Oxford University Press, Oxford, UK.
Meijaard, E., Wich, S., Ancrenaz, M. and Marshall, A.J. 2012. Not by science alone: why orangutan conservationists must think outside the box. Annals of the New York Academy of Science 1249: 29–44.
Morrogh-Bernard, H. C., Husson, S.J., Knott, C.D., Wich, S.A., van Schaik, C.P., van Noordwijk, M.A., Lackman-Ancrenaz, I., Marshall, A.J., Kanamori, T., Kuze, N. and bin Sakong, R. 2009. Orangutan activity budgets and diet. In: Wich, S.A., Utami Atmoko, S.S., Mitra Setia, T. and van Schaik, C.P. (eds), Orangutans: Geographic Variation in Behavioral Ecology and Conservation, pp. 119–134. Oxford University Press, Oxford, UK.
Nater, A., Arora, N., Greminger, M.P., van Schaik, C.P., Singleton, I., Wich, S.A., Fredriksson, G., Perwitasari-Farajallah, D., Pamungkas, J. and Krützen, M. 2013. Marked population structure and recent migration in the critically endangered Sumatran orangutan (Pongo abelii). Journal of Heredity 104: 213.
Nater, A., Greminger, M.P., Arora, N., van Schaik, C.P., Goossens, B., Singleton, I., Verschoor, E.J., Warren, K.S. and Krützen, M. 2015. Reconstructing the demographic history of orangutans using approximate Bayesian computation. Molecular Ecology 24: 310–327.
Nater, A., Nietlisbach, P., Arora, N., van Schaik, C.P., van Noordwijk, M.A., Willems, E.P., Singleton, I., Wich, S.A., Goossens, B., Warren, K.S., Verschoor, E.J., Perwitasari-Farajallah, D., Pamungkas, J. and Krützen, M. 2011. Sex-biased dispersal and volcanic activities shaped phylogeographic patterns of extant orangutans (genus: Pongo). Molecular Biology and Evolution 28: 2275–2288.
Rao, M. and van Schaik, C.P. 1997. The behavioural ecology of Sumatran orangutans in logged and unlogged forest. Tropical Biodiversity 4: 173–185.
Rijksen, H.D. 1978. A Field Study on Sumatran Orangutans (Pongo pygmaeus abelii, Lesson 1827): Ecology, Behavior, and Conservation. H. Veenman and Zonen, Wageningen, Netherlands.
Rijksen, H.D. and Meijaard, E. 1999. Our Vanishing Relative: The Status of Wild Orangutans at the Close of the Twentieth Century. Kluwer, Dordrecht, The Netherlands.
Russon, A.E., Wich, S.A., Ancrenaz, M., Kanamori, T., Knott, C.D., Kuze N., Morrogh-Bernard, H.C., Pratje, P., Ramlee, H., Rodman, P., Sawang, A., Sidiyasa, K., Singleton, I. and van Schaik, C.P. 2009. Geographic variation in orangutan diets. In: S.A. Wich, S.S. Utami Atmoko, T. Mitra Setia and C.P. van Schaik (eds), Orangutans: Geographic Variation in Behavioral Ecology and Conservation, pp. 135–156. Oxford University Press, Oxford, UK.
Santika, T., Meijaard, E. and Wilson, K.A. 2015. Designing multifunctional landscapes for forest conservation. Environmental Research Letters 10: 114012–114020.
Singleton, I. and van Schaik, C.P. 2001. Orangutan home range size and its determinants in a Sumatran swamp forest. International Journal of Primatology 22: 877–911.
Singleton, I., Knott, C.D., Morrogh-Bernard, H.C., Wich, S.A. and van Schaik, C.P. 2009. Ranging behavior of orangutan females and social organization. In: Wich, S.A., Utami Atmoko, S.S., Mitra Setia, T. and van Schaik, C.P. (eds), Orangutans: Geographic Variation in Behavioral Ecology and Conservation, pp. 205–214. Oxford University Press, Oxford, UK.
Singleton, I., Wich, S., Husson, S., Stephens, S., Utami-Atmoko, S.S., Leighton, M., Rossen, N., Traylor-Holzer, K., Lacy, R. and Byers, O. 2004. Orangutan Population and Habitat Viability Assessment: Final Report. IUCN/SSC Conservation Breeding Specialist Group, Jakarta, Indonesia.
Utami Atmoko, S.S., Mitra Setia, T., Goossens, B., James, S.S., Knott, C.D., Morrogh-Bernard, H.C., van Schaik, C.P. and van Noordwijk, M.A. 2009. Orangutan mating behaviour and strategies. In: Wich, S.A., Utami Atmoko, S.S., Mitra Setia, T. and van Schaik, C.P. (eds), Orangutans: Geographic Variation in Behavioral Ecology and Conservation, pp. 235–245. Oxford University Press, Oxford, UK.
Utami, S., Goossens, B., Bruford, M.W., de Ruiter, J.R. and van Hooff, J.A.R.A.M. 2002. Male bimaturism and reproductive success in Sumatran orangutans. Behavioral Ecology 13: 643–652.
van Noordwijk, M.A. and van Schaik, C.P. 2005. Development of ecological competence in Sumatran orangutans. American Journal of Physical Anthropology 127: 79–94.
Wich, S.A., de Vries, H., Ancrenaz, M., Perkins, L., Shumaker, R.W., Suzuki A. and van Schaik, C.P. 2009. Orangutan life history variation. In: S.A. Wich, S.S. Utami Atmoko, T. Mitra Setia and C.P. van Schaik (eds), Orangutans: Geographic Variation in Behavioral Ecology and Conservation, pp. 65–75. Oxford University Press, Oxford, UK.
Wich, S.A., Fredriksson, G.M., Usher, G.F., Peters, H.H., Priatna, D., Basalamah, F., Susanto, W.W. and Kühl, H. 2012. Hunting of Sumatran orangutans and its importance in determining distribution and density. Biological Conservation 146: 163–169.
Wich, S.A., Meijaard, E., Marshall, A., Husson, S., Ancrenaz, M., Lacy, R., van Schaik, C., Sugardjito, J., Simorangkir, T., Traylor-Holzer, K., Doughty, M., Supriatna, J., Dennis, R., Gumal, M., Knott, C. and Singleton, I. 2008. Distribution and conservation status of the orangutan (Pongo spp.) on Borneo and Sumatra: how many remain? Oryx 42: 1–11.
Wich, S.A., Riswan., Jenson, J., Refisch, J. and Nelleman, C. 2011. Orangutans and the Economics of Sustainable Forest Management in Sumatra. UNEP/GRASP/PanEco/YEL/ICRAF/GRID-Arendal.
Wich, S.A., Singleton, I., Nowak, M.G., Utami Atmoko, S.S., Nisam, G., Arif, S.M., Putra, R.H., Ardi, R., Fredriksson, G., Usher, G., Gaveau, D.L.A and Kühl, H.S. 2016. Land-cover changes predict steep declines for the Sumatran orangutan (Pongo abelii). Science Advances 2(3): e1500789.
Wich, S.A., Singleton, I., Utami-Atmoko, S.S., Geurts, M.L., Rijksen, H.D. and van Schaik, C.P. 2003. The status of the Sumatran orang-utan Pongo abelii: an update. Oryx 37: 49–54.
Wich, S.A., Usher, G., Peters, H.H., Khakim, M.F.R., Nowak, M.G. and Fredriksson, G.M. 2014. Preliminary data on the highland Sumatran orangutans (Pongo abelii) of Batang Toru. In: Grow, N.B., Gursky-Doyen, S. and Krzton, A. (eds), High Altitude Primates, pp. 265–283. Springer Press, Cambridge, UK.
Wich, S.A., Utami-Atmoko, S.S., Setia, T.M., Djojosudharmo, S. and Geurts, M.L. 2006. Dietary and energetic responses of (Pongo abelii) to fruit availability fluctuations. International Journal of Primatology 27: 1535–1550.
Wich, S.A., Utami-Atmoko, S.S., Setia, T.M., Rijksen, H.D., Schurmann, C., van Hooff, J.A.R.A.M. and van Schaik, C.P. 2004. Life history of wild Sumatran orangutans (Pongo abelii). Journal of Human Evolution 47: 385–398.
|Citation:||Singleton, I., Wich , S.A., Nowak, M. & Usher, G. 2016. Pongo abelii. (errata version published in 2016) The IUCN Red List of Threatened Species 2016: e.T39780A102329901.Downloaded on 19 August 2017.|
|Feedback:||If you see any errors or have any questions or suggestions on what is shown on this page, please provide us with feedback so that we can correct or extend the information provided|