The new orangutan species, Pongo tapanuliensis, or Tapanuli orangutan, is endemic to the three Tapanuli districts of North Sumatra, Indonesia and occurs in roughly 1,100 km² of upland forest in the Batang Toru Ecosystem. Despite nearly 50 years of orangutan research in Sumatra, the Batang Toru population was only ‘rediscovered’ in 1997, during a series of field surveys by co-author Prof. Erik Meijaard, director of Borneo Futures, who carried out the initial survey south of Lake Toba looking for orangutan populations. In 2005, the Sumatran Orangutan Conservation Programme (SOCP) and other non-governmental organisations intensified previous research efforts on the orangutans in the Batang Toru Ecosystem, together with several universities, and Indonesian authorities. As a part of this effort, a monitoring station was established in 2006 by SOCP, allowing for a more detailed look at their behavioural ecology and genetics.

First author Alexander Nater (University of Zurich) and fellow researchers described their genomic research on orangutans in 2011. To their surprise they discovered that, with respect to mitochondrial DNA, the Tapanuli orangutans were more closely related to orangutans in Borneo than to the Sumatran orangutans found more to the north. It was not until 2013, however, when skeletal material from an adult male orangutan killed in a human-animal conflict became available, that researchers around Matt Nowak from SOCP realised the uniqueness of the Batang Toru population. The researchers conducted morphological research on the skull and were completely surprised to find that it was quite different in some characteristics from anything they had seen before. While this suggested that the Batang Toru population was potentially unique, much stronger evidence was required to actually assign species status to the Batang Toru orangutans. This was achieved by the largest genomic study of wild orangutans to date.

10,000 to 20,000 years in isolation

The researchers subsequently spent a considerable amount of time working on genomic data to investigate the genetic structure and evolutionary history of all existing orangutan populations. They consistently identified three very old evolutionary lineages among all orangutans, despite only having two species currently described. The pieces of the puzzle finally fell into place after the researchers realised that Batang Toru orangutangs are morphologically different from all other orangutans. The oldest evolutionary line in the genus Pongo is actually found in Batang Toru orangutans, which appear to be direct descendants of the first Sumatran population in the Sunda archipelago. Extensive computer modelling reconstructing the population history of orangutans revealed that the Batang Toru population appears to have been isolated from all other Sumatran populations for at least 10-20’000 years.

Anthropogenic pressure

It is fascinating that this species of orangutans is so different from orangutans in the north of Sumatra and that we have discovered a new species of great ape in the 21st century’, says co-author Serge Wich, professor by special appointment of Conservation of the Great Apes at the UvA, which is a special chair in partnership with the World Wildlife Fund (WWF). ‘But we need to take decisive action to ensure the survival of this species, which is found in a small patch of forest and faced with all kinds of threats.’ A recent study led by Wich shows that only about 800 individuals are currently alive. There is strong anthropogenic pressure on the Tapanuli orangutan due to conversion of pristine forest for mining, plans to build a hydro-electric dam, and general human encroachment. ‘If steps are not taken quickly to reduce current and future threats to conserve every last remaining bit of forest’ the scientists point out, ‘we may see the discovery and extinction of a great ape species within our lifetime.’

Publication details

Alexander Nater, Maja P. Mattle-Greminger, Anton Nurcahyo, Matthew G. Nowak, Marc de Manuel, Tariq Desai, Colin Groves, Marc Pybus, Tugce Bilgin Sonay, Christian Roos, Adriano R. Lameira, Serge A. Wich, James Askew, Marina Davila-Ross, Gabriella Fredriksson, Guillem de Valles, Ferran Casals, Javier Prado-Martinez, Benoit Goossens, Ernst J. Verschoor, Kristin S. Warren, Ian Singleton, David A. Marques, Joko Pamungkas, Dyah Perwitasari-Farajallah, Puji Rianti, Augustine Tuuga, Ivo G. Gut, Marta Gut, Pablo Orozco-ter Wengel, Carel P. van Schaik, Jaume Bertranpetit, Maria Anisimova, Aylwyn Scally, Tomas Marques-Bonet, Erik Meijaard & Michael Krützen: ‘Morphometric, Behavioral, and Genomic evidence for a New Orangutan Species’, in Current Biology, 2 November 2017. Doi.org/10.1016/j.cub.2017.09.047

Also see

Serge Wich et al.: ‘Land-cover changes predict steep declines for the Sumatran orangutan (Pongo abelii)’, in Science Advances (4 maart 2016). DOI: 10.1126/sciadv.1500789.