This species was first described in 2017 (Nater et al. 2017). The Nater et al. study showed that an isolated orangutan population in the Batang Toru region, which is the southernmost range of extant Sumatran Orangutans (Pongo abelii), south of Lake Toba, is distinct from other Sumatran and Bornean populations. Through a comparison of cranio-mandibular and dental characters from an orangutan killed during human-orangutan conflict to 33 adult male orangutans of similar developmental stage, the study found consistent differences between the Batang Toru individual and other extant Ponginae. Model-based approaches based on the analyses of 37 orangutan genomes supported the morphological results by revealing that the deepest split in the evolutionary history of extant orangutans occurred ~3.38 Ma ago between the Batang Toru population and those to the north of Lake Toba, while both currently recognized species separated much later at about 674 ka. The analyses show that there was a small gene flow between P. abelii and P. tapanuliensis until 10–20 ka. The combined analyses support a new classification of orangutans into three extant species.

Due to high levels of habitat conversion and fragmentation, and illegal killing, Pongo tapanuliensis is estimated to have experienced a significant population reduction in recent decades. Forest loss data indicate that orangutan habitat below 500 m asl of both P. tapanuliensis and P. abelii was reduced by 60% between 1985 and 2007 (Wich et al. 2008, 2011). It is thought that this reduction will continue for the Tapanuli Orangutan as forests within its range remain under considerable threat (Wich et al. 2016). Significant areas of the Tapanuli Orangutan’s range are seriously threatened by habitat conversion for small-scale agriculture, mining exploration and exploitation, a large-scale hydroelectric scheme, geothermal development, and agricultural plantations. Approximately 14% of the geographic range of the Tapanuli Orangutan is not protected, nor even allocated as forest estate, and even the protected areas are not immune from the above threats (Wich et al. 2008, 2011, 2016). The Tapanuli Orangutan is also hunted (Wich et al. 2012). Due to their slow life history, with a generation time of at least 25 years (Wich et al. 2004, 2009), orangutans on Sumatra are unable to sustain substantial and continual loss of individuals. A quantitative population viability analysis estimated that in 1985 there were ~1,489 individuals of the Tapanuli Orangutan, and that these would decline to only 257 over a 75-year period to 2060 (Sumatran Orangutan Conservation Programme unpublished data). With current management, the key threats of loss of lowland habitat and hunting are not being effectively reduced, and we therefore predict an 83% decline over the course of three generations. The species therefore qualifies as Critically Endangered under criterion A4.

For much of the 20th century, orangutans on Sumatra were thought to be restricted to the north and west of Lake Toba, in and around the Leuser Ecosystem. Populations south of Lake Toba were overlooked, even though a 1939 review of the species’ range mentioned that orangutans had been reported in several forest areas in that region (Nederlandsch-Indische Vereeniging tot Natuurbescherming 1939). It was not until 1997 that these isolated orangutan populations were rediscovered (Meijaard 1997, Rijksen and Meijaard 1999). As a consequence of further sightings, published in 2003 (Wich et al. 2003), a study of the southern populations started in 2005 when the Sumatran Orangutan Conservation Programme (SOCP) began a conservation project in the mountainous Batang Toru region with the intent of conserving the orangutan population and their remaining habitat south of Lake Toba.

The only known population of Pongo tapanuliensis occurs in the uplands of the Batang Toru Ecosystem, an area of roughly 1,500 km² consisting of three forest blocks, of which 1,022 km² is suitable orangutan habitat (Wich et al. 2016). Pongo tapanuliensis is the least numerous of all great ape species.

Pongo tapanuliensis was until relatively recently more widespread, with sightings further south in the lowland peat swamp forests in the Lumut area (Wich et al. 2003) and several nests encountered during a rapid survey in 2010 (G. Fredriksson pers. obs.). The forests in the Lumut area have in recent years almost completely been converted to oil-palm plantations. In 2008, a captive Tapanuli Orangutan was confiscated in Lumut. Observation were also made of a male orangutan in the Dolok Ginjang area, just north of the Batang Toru West forest block in the Adiankoting subdistrict in North Tapanuli, during a human conflict situation where an orangutan was shot at with an air rifle when it was found foraging on durian fruits (G. Fredriksson pers. obs.). Recent surveys in this area did not find any orangutan nests (Nowak et al. unpublished data).

A hunting trip report from 1879 described orangutan encounters near the small town of Mantinggi, south of the current population (0°58’ N, 100°0’ E) (Kramm 1879). In 1902, orangutan signs and reports were noted along several small rivers on the south side of the Sibolga Bay, west of Batang Toru (Miller 1903). Unsubstantiated reports suggest that orangutans, presumably P. tapanuliensis, may have occurred even further south in Sumatra in historic periods. A number of museum specimens are claimed to have come from Padang, Palembang and Jambi (Schlegel and Müller 1839–1844, Beccari 1904, Hooijer 1948, Veracini et al. 2010), and an orangutan kill was reported in 1930 near Palembang and along the Musi River toward the base of the Barisan Mountain range (Delmont 1930). These accounts suggest that isolated individuals or small populations of P. tapanuliensis may have survived outside the current range well into the 1900s, although without studying specimens from these areas, we cannot exclude that they could be P. abelii. Despite the distance to the south and their rarity, such reports are perhaps not surprising, as orangutans are long lived and isolated individuals or populations could potentially exist in fragmented landscapes for >50 years, provided that food resources were available and stochastic processes were rare.

See the Supplementary Information for details of how the distribution map for this species was updated for the amended version of this assessment.

With a population estimate of fewer than 800 individuals (Wich et al. 2016), Pongo tapanuliensis is the least numerous of all great ape species. Its distribution is separated by around 100 km from the closest population of P. abelii to the north. A combination of small population size and geographic isolation is of particularly high conservation concern, as it may lead to inbreeding depression (Hedrick and Kalinowski 2000) and threaten population persistence (Allendorf et al. 2013). Highlighting this, a recent study (Nater et al. 2017) discovered extensive runs of homozygosity in the genomes of two P. tapanuliensis individuals, pointing at the occurrence of recent inbreeding.

Only about 10% of the geographic range is in an area recognized by the World Database of Protected Areas. Another 76% is in Hutan Lindung (Protection Forest), and 14% does not have any 'forest status' in the spatial plans, despite the fact that it consists of rugged primary forest with the highest densities of Tapanuli Orangutans in the Batang Toru Ecosystem (SOCP unpublished data). This roughly translates to 8% of the total distribution located in nationally recognized conservation area (Cagar Alam, Strict Nature Reserve), 78% in Protection Forest (Hutan Lindung), and the remaining 14% of the range is entirely unprotected.

Despite a complex geological history, the underlying parent material within the geographic range of Pongo tapanuliensis consists primarily of recent (i.e., Quaternary, <2.588 Ma) igneous rock, most of which is acidic volcanic rock from the Toba super-eruption of ~74 ka (Subardja et al. 1990, Oppenheimer 2002, Barber et al. 2005). This is in stark contrast to the north Sumatran Orangutan population that persists in forests atop older (i.e., Tertiary and above, >2.588 Ma) igneous and sedimentary rock (Darul Sukma et al. 1990a, 1990b, 1990c, 1990d, Hidayat et al. 1990, Hikmatullah et al. 1990, Wahyunto et al. 1990, Barber et al. 2005). While comparative data in the format analysed for Sumatra are lacking for Borneo, previously published reconstructions of Borneo’s geology highlighted a complex geological history that was in general different from that of Sumatra. This includes large swaths of landscape with pre-Cenozoic and Cenozoic sedimentary rock, with comparatively fewer areas characterized by igneous rock formations (Hall et al. 2008).

Using freely available WorldClim data (Hijmans et al. 2005), P. tapanuliensis can be differentiated from P. abelii by a highly narrow ecological niche, including a restricted upland elevation range (P. abelii: 701.7±454.8 m asl; P. tapanuliensis: 834.4±219.3 m asl), cooler average temperatures (P. abelii: 23.3±2.5°C; P. tapanuliensis: 22.1±1.2°C), and relatively high levels of average yearly rainfall (P. abelii: 2,435.1±460.5 mm/year; P. tapanuliensis: 2,607.5±231.2 mm/year). Similarly, compared to P. pygmaeus, P. tapanuliensis occurs at much higher elevation (P. pygmaeus: 170.6±187.0 m asl; P. tapanuliensis: 834.4±219.3 m asl), and experiences far lower average temperatures (P. pygmaeus: 26.0±1.0°C; P. tapanuliensis: 22.1±1.2°C), and lower levels of average rainfall (P. pygmaeus: 2,827.3±420.9 mm/year; P. tapanuliensis: 2,607.5±231.2 mm/year). At SOCP’s long-term monitoring station, average monthly temperature from 2009–2015 was recorded as 22.0°C (±0.89°C, n=83 months), with a monthly range of 20.5–24.2°C, whereas average total rainfall from 2009–2015 was recorded as 3,654 mm per year (±390.0 mm, n=7 years), with a yearly range of 3,186-4,210 mm (SOCP unpublished data).

Provided that climate, parent material, and time are key factors in soil formation/quality, and thus forest productivity/quality (Tan 2008, Marshall et al. 2009, Wich et al. 2011, Bockheim et al. 2014), the aforementioned factors are key distinctions between the P. tapanuliensis and most Bornean and north Sumatran Orangutan populations. A recent study showed that forest compositions experienced by P. tapanuliensis are at present mainly known from upland areas of southern Sumatra and Borneo, areas that notably do not currently maintain orangutan populations (Slik et al. 2011). They are largely restricted to primary forest, though they do venture into disturbed habitat at forest edges.

As noted by Wich and colleagues (Wich et al. 2014), P. tapanuliensis was observed feeding on a number of tree species that had not previously been recorded as orangutan food species. These unique species include Agathis borneensis from the Araucariaceae family, Gymnostoma sumatranum from the Casuarinaceae family, and Dacrycarpus imbricatus, Dacrydium beccarii, Dacrydium comosum, and Podocarpus neriifolius from the Podocarpaceae family. To our knowledge, occasional feeding on species from these families had only been documented in Gunung Palung, Borneo (fruits of an unidentified Agathis species and vegetation of an unknown Casuarina species [Casuarinaceae]), and Ketambe, Sumatra (bark of Sundacarpus amara [Podocarpaceae]) (Russon et al. 2009). In contrast, the five aforementioned conifer species (Araucariaceae and Podocarpaceae) and one non-conifer evergreen species (Casuarinaceae) represent 21.9% of all feeding observations (n=58,558 two-minute instantaneous samples) recorded between 2011 and 2015 at SOCP’s long-term monitoring station in the Batang Toru Ecosystem (SOCP unpublished data). Thus, a significant proportion of the dietary profile of Tapanuli Orangutans is markedly different from that of previously studied orangutan populations.

Due to the extremely rugged terrain, external threats have been primarily limited to illegal clearing of protected forests, hunting and killing during crop conflict, and trade in young orangutans (Wich et al. 2012, Wich et al. 2016). Encroachment and hunting have increased in recent decades, due to an influx of migrants from Nias Island, west of Sumatra, who settle on protected forest land on Batang Toru’s forest edge where no land claims exist at present (Wich et al. 2012). In addition, despite land status changes from production forest to that of protection forest in 2014 (Ministry of Forestry of the Republic of Indonesia 2014), one company still maintains a controversial 300 km² logging permit located in primary forest within the current range of Pongo tapanuliensis. In the southwest corner of the Batang Toru Forest Complex a gold and silver mine is located, which has recently converted ca 3 km² of P. tapanuliensis habitat and retains controversial mining permits overlapping parts of the remaining P. tapanuliensis range. Uncontested land speculation related to the company’s exploration is further threatening the primary forest. More recently, a hydro-electric development has been proposed in the area of highest orangutan density, which could impact roughly 100 km² of P. tapanuliensis habitat, or nearly 10% of the entire species population. It could also jeopardize the chances of maintaining habitat corridors between the western and eastern P. tapanuliensis ranges, and with two smaller strict nature reserves, which could also maintain small populations of P. tapanuliensis.

It is illegal to capture, injure, kill, own, keep, transport, or trade an orangutan.

To ensure the long-term survival of Pongo tapanuliensis, conservation measures need to be implemented swiftly. Due to the rugged terrain, external threats have been primarily limited to illegal clearing of forests, hunting, killings during crop conflict and trade in orangutans (Wich et al. 2012, Wich et al. 2016). A hydroelectric development has been proposed recently in the area of highest orangutan density, which could impact nearly 10% of P. tapanuliensis’ population. This project might lead to further genetic impoverishment and inbreeding, as it would jeopardize chances of maintaining habitat corridors between the western and eastern range, and smaller nature reserves, all of which maintain small populations of P. tapanuliensis.

The Tapanuli Orangutan is also protected by international legislation by default because it is included within the old concept of P. abelii which is listed on CITES Appendix I.