Meet The Tapanuli Orangutan, The Rarest Great Ape On Earth, Just Now Classified As A New Species

A small population of orangutans living on the Indonesian island of Sumatra has just been identified as a new species, following a spectacular breakthrough in a laborious research project spanning two decades. Found exclusively in the Batang Toru forest, up in the remote mountainous region of Tapanuli, the new species was named Pongo tapanuliensis, or the Tapanuli orangutan.

Up until now, there were only two species of orangutans known to science: the Sumatran orangutan, and the Bornean orangutan. Yet a new study published on Thursday in the journal Current Biology provides conclusive evidence of another species, the Tapanuli orangutan, showing it is genetically different from the other two species.

The study, an international collaboration between the University of Zurich (UZH), Liverpool John Moores University, and the Sumatran Orangutan Conservation Programme (SOCP), involved a collective research of genetic, anatomical, and acoustic data, and culminated with the designation of Tapanuli orangutans as a separate species.

“Based on diverse sources of evidence, we describe a new orangutan species, Pongo tapanuliensis, that encompasses a geographically and genetically isolated population found in the Batang Toru area at the southernmost range limit of extant Sumatran orangutans, south of Lake Toba, Indonesia,” the researchers explain in their paper.

Previously regarded as Pongo abelii, or the Sumatran orangutans, Pongo tapanuliensis has now become the third recognized species of orangutan in the world, and the eighth species of great ape — alongside humans, eastern and western gorillas, chimpanzees, bonobos, and Sumatran and Bornean orangutans.

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This tiny populace of orangutans, currently numbering fewer than 800, has long bewildered scientists due to their unique appearance and the peculiarity of their genome. In a surprising turn of events, the orangutans in Batang Toru have been found to be more closely related to the Bornean orangutans living across the sea than to their Sumatran counterparts inhabiting the same island. However, there was insufficient proof to categorically classify the Batang Toru orangutans as a separate species. That is, until now.

The world was first introduced to this specific type of orangutans back in 1997, when a research team led by Dr. Erik Meijaard, a conservation scientist affiliated with Australian National University and one of the study authors, discovered them during an expedition in the isolated mountain forests of Sumatra.

His team originally began by conducting research in Borneo, and had been there mapping the areas inhabited by orangutans over the course of three years. His work then took him to Sumatra, where he found accounts of an orangutan population in the Tapanuli districts, located south to the swamp forests inhabited by Sumatran orangutans, notes ABC News.

Given the notoriously shy and elusive nature of orangutans in general, it was quite some time before these mysterious creatures were eventually spotted in the wild. After considerable effort, Dr. Meijaard’s team eventually tracked them down in the forest and was able to see how this group of animals was different from other orangutans.

Unlike their Sumatran and Bornean counterparts, the Tapanuli orangutans have frizzier body hair. The newly-discovered species also has subtly different facial features, particularly a “prominent mustache,” shows the study. Moreover, a third distinctive physical trait of the Batang Toru orangutans is that the females can also grow a beard.

Sixteen years after the Tapanuli orangutans were first sighted, another important, albeit unfortunate, find allowed researchers to gain more insight into the animals’ unique features. In 2013, SOCP found Raya, a Batang Toru orangutan that had been shot by local villagers. In spite of all the attempts to save Raya’s life, the orangutan eventually died, leaving its body to science as the first ever recovered skeleton of the new species.

An analysis of Raya’s remains uncovered significant differences in the measurements and overall shape of the animal’s skull, jaw, and teeth. When compared with 33 other skeletons belonging to Sumatran and Bornean orangutans, this unique and singular skeleton exhibited a smaller skull, as well as a flatter face and wider canine teeth.

An analysis of the sole existing Tapanuli skeleton revealed evidence that Pongo tapanuliensis is a completely distinct species from the Sumatran and Bornean orangutan
Photo of the only recovered Tapanuli skull, showing significant differences in skeletal morphology between the newly-discovered species and its two other cousins [Image by Nater et al./ Current Biology]

Aside from the examination of the sole existing Pongo tapanuliensis skeleton, an additional investigation compared the genome of two Tapanuli orangutans (obtained from blood samples taken early on in the study) with those of their Sumatran and Bornean counterparts. A team of geneticists led by Dr. Michael Krützen, professor at UZH and co-author of the study, sequenced a total of 37 orangutan genomes and found irrefutable proof that the Batang Toru orangutans constitute a separate species.

“When we realized that Batang Toru orangutans are morphologically different from all other orangutans, the pieces of the puzzle fell into place,” Dr. Krützen said in a news release issued by UZH.

This stage of study, comprising the most comprehensive genomic research of wild orangutans so far, allowed the scientists to reconstruct the apes’ evolutionary history through their genetic code. The findings showed Tapanuli orangutans split from their Sumatran cousins about a million years ago, and that they are in fact the oldest lineage of the three species. According to this analysis, Pongo tapanuliensis are the direct descendants of the orangutan ancestors that migrated from Asia to what is now Indonesia and Malaysia.

Although there is evidence of interbreeding between Sumatran and Tapanuli orangutans, the two species have evolved separately for the last 15,000 years, Krützen explains. At the same time, research suggests Tapanuli orangutans separated from their Bornean relatives less than 700,000 years ago, which accounts for their genetic similarities — greater than between Pongo tapanuliensis and Pongo abelii.

“Orangutans are ancient creatures, as old as the very first members of our own genus Homo,” points out Dr. Meijaard in a statement for the New York Times.

“I discovered the population south of Lake Toba in 1997, but it has taken us 20 years to get the genetic and morphological data together that shows how distinct the species is.”

Another thing that sets Tapanuli orangutans apart from their other two cousins is the peculiarity of their calls. According to the study, each orangutan species has its own signature call. Observations of Pongo tapanuliensis in the wild revealed “subtle differences between these and other populations” of orangutans when it comes to the loud sounds the male apes make to announce their presence, Prof. Serge Wich, from Liverpool John Moores University and co-author of the study, told BBC.

National Geographic credits the discovery of this particular disparity among the three orangutan species to James Askew, a graduate student from University of Southern California, who recorded male orangutans both in Batang Toru and other areas of Sumatra and Borneo. After analyzing these recordings, Askew was able to ascertain that Tapanuli orangutans produce a very distinct type of call, which is long and high-pitched. By comparison, Sumatran orangutans have long but low-pitched calls, whereas Bornean orangutans produce short, high-pitched calls.

Yet the news of a newly-discovered orangutan species is bitter-sweet. Pongo tapanuliensis are restricted to just a few hundred surviving individuals, and their numbers and still dwindling. The approximately 800 Tapanuli orangutans living in Batang Toru occupy a forest area of no more than 425 square miles — roughly the size of Dallas, Texas.

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The species is facing loss of habitat due to agricultural exploits, particularly palm oil plantations. Plans for the construction of a hydropower plant in the area, which is expected to be finalized in 2022, could flood up to 8 percent of the Tapanuli orangutans’ habitat. This project could also result in the disappearance of forest corridors the animals use to move between populations, paving the way for further isolation and inbreeding.

The already low numbers of Tapanuli orangutans, coupled with the continuous threat posed by human hunters, make Pongo tapanuliensis the rarest and most endangered orangutan species. Since orangutans have one of the slowest mammalian reproductive rates and give birth only every six to nine years, their declining numbers take a lot of time to replenish.

“If even eight out of 800 animals were killed each year, on top of the normal background death rate, the species will disappear,” cautions Dr. Meijaard.

Their two other cousins aren’t doing much better either. Both Bornean and Sumatran orangutans are listed as critically endangered by the International Union for the Conservation of Nature, numbering 54,000 and around 7,000 individuals, respectively.

[Featured Image by Tatan Syuflana/AP Images]