Photo by Maxime Aliaga.

Scientists have identified a new species of orangutan in Indonesia, but there are so few individuals in the population that the newly named Tapanuli orangutan is already being described as one of the most endangered great ape species in the world.

The Tapanuli orangutans (Pongo tapanuliensis) live in the Batang Toru forest, south of Lake Toba on the island of Sumatra.

Fewer than eight hundred of the primates remain and they are surviving in 1,100 square kilometres of remaining habitat, which is divided into three blocks of forest, separated by roads and agricultural land.

There are two main blocks of forest (east and west), plus an area in the Sibuali-Buali Nature Reserve, located next to the west block, where there is a third, smaller population.

There is no great ape species that has a smaller population than the Tapanuli orangutan.

Matthew Nowak, who is one of the 37 co-authors of the paper about the new species, says that, despite only just being described, “with so few individuals left, the Tapanuli orangutan is already the most endangered great ape species in the world”.

Serge Wich from the IUCN/SSC Primate Specialist Group’s section on great apes, who is also a co-author of the new paper, said: “Orangutans reproduce extremely slowly, and if more than 1 percent of the population is lost annually this will spiral them to extinction.”

Female Tapanuli orangutans have their first offspring when they are about 15 years old, and the interbirth interval is then about 8 to 9 years.

The Tapanuli orangutans can live until fifty to sixty years of age.

Eighty-five percent of the Batang Toru forest has protected status, but two potentially devastating industrial projects – a hydroelectric dam and an expansion of gold mining – are proposed in a sensitive area of connectivity inside the unprotected area.

Photo by Maxime Aliaga.

The international team of scientists who identified the Tapanuli orangutan came to their conclusion after comparing its skeleton and genomes with the two other orangutan species, the Bornean orangutan (Pongo pygmaeus) and the Sumatran orangutan (Pongo abelii).

The researchers, whose paper has just been published in the journal Current Biology, say that the Tapanuli orangutan is genetically and morphologically distinct from the other two species. The new species, they say, is more closely related to the Bornean orangutan, whose home is on an island more than one thousand kilometres away, than it is to other orangutans in Sumatra, who live further north on that island.

The oldest lineage belongs to the newly discovered species.

When the scientists compared the cranio-mandibular and dental characteristics of an orangutan who died from wounds inflicted by local villagers in November 2013 with those of 33 adult male orangutans at a similar stage of developmental, they found consistent differences.

The Tapanuli orangutan’s skull was found to be smaller than that of Sumatran and Bornean orangutans. Its face was flatter and its canine teeth were wider.

“Our analyses of 37 orangutan genomes provided a second line of evidence,” the researchers said.

The researchers also found some subtle differences between the call of the Tapanuli orangutan and that of other orangutan populations.

The Tapanuli orangutans have thicker, frizzier hair than their Bornean and Sumatran relatives, and, unlike the Bornean orangutans, the females grow beards. The dominant males have a prominent moustache and flat flanges covered in downy hair, whereas the flanges of older males are more like those of Bornean males.

The newly described species is similar to the Sumatran orangutan in its linear body build and has a more cinnamon-coloured pelage than that of its Bornean relative.

Researchers have seen the Tapanuli orangutans eat plant species that have not been observed in the diet of other orangutan species. These include aturmangan (Casuarinaceae) seeds, sampinur (Podocarpaceae) fruits and flowers, and agatis (Araucariaceae).

The director of the Sumatran Orangutan Conservation Programme (SOCP), Ian Singleton, said: “It is fascinating that this population of orangutans differs so much from the orangutans in the north of Sumatra, and that even in the 21st century a new species of great ape has been discovered.”

Photo by Andrew Walmsley.

Another of the authors of the new report, conservation scientist Erik Meijaard, says he discovered the population of orangutans south of Lake Toba in 1997, but it has taken twenty years to compile the data that shows that the Tapanuli orangutan is a distinct species.

The Tapanuli orangutan, which is named after the area in which it lives, is the first new great ape to be described for almost a century.

The scientists’ genome analyses revealed that the deepest split in the evolutionary history of extant orangutans – between the Batang Toru population and those living to the north of Lake Toba – occurred about three million years ago.

The Bornean and Sumatran orangutan species separated much later – about 674,000 years ago.

The authors of the new study say that the orangutan populations north and south of Lake Toba were in genetic contact for most of the time after their split, but there was a marked reduction in gene flow after about 100,000 years.

This, the researchers say, was consistent with habitat destruction caused by the massive eruption of the Toba supervolcano 73,000 years ago.

Gene flow between the Tapanuli orangutans and the other orangutans in Sumatra ceased completely betweeen 10,000 and 20,000 years ago, and is now impossible because of habitat loss in areas between the species’ ranges.

In 2016, the International Union for Conservation of Nature (IUCN) changed the classification of the Bornean orangutan (Pongo pygmaeus) from endangered to critically endangered, citing the main causes of its population decline as habitat loss and fragmentation, primarily for logging and oil palm plantations, along with illegal hunting and fires.

The Sumatran orangutan (Pongo abelii) is also listed as critically endangered. There are now only about 14,600 left in the wild.

There are estimated to be between 55,000 and 62,000 Bornean orangutans living in the wild, split into three distinct subspecies.

The IUCN says the population trend is a decrease for both species.

It is expected that the Tapanuli orangutan will also be listed as critically endangered.

Photo by Andrew Walmsley.

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The discovery of a new orangutan species means that there are now seven recognised extant species of non-human great apes: the Sumatran, Bornean and Tapanuli orangutans, eastern and western gorillas, and chimpanzees and bonobos.

The scientists who have identified the Tapanuli orangutan say that there are fewer individuals than in any other great ape species.

“A combination of small population size and geographic isolation is of particularly high conservation concern, as it may lead to inbreeding depression and threaten population persistence.”

The researchers say that, to ensure the Tapanuli orangutan’s long-term survival, conservation measures need to be implemented swiftly.

They say that, because of the rugged terrain in the Batang Toru forest, external threats to the primates have been primarily limited to road construction, illegal clearing of forests, hunting, killings during conflict over crops, and trade in orangutans.

However, they say, the proposed new hydroelectric development in the area of highest orangutan density could impact up to 8 percent of the animal’s habitat.

“This project might lead to further genetic impoverishment and inbreeding, as it would jeopardise chances of maintaining habitat corridors between the western and eastern range, as well as smaller nature reserves, all of which maintain small populations of P. tapanuliensis.” 

The chairman of the Sumatran Orangutan Conservation Forum (FOKUS), Kusnadi, says the focus now needs to be on reconnecting the three Tapanuli orangutan populations by creating corridors.

“The most critical habitat area for the species, with the highest densities of orangutans, is not currently protected in any way.” 

Primates expert Gabriella Fredriksson, who has coordinated the SOCP’s conservation efforts in Tapanuli since 2006, who is also one of the co-authors of the paper about the Tapanuli orangutan, says there is a lot of work to be done to make sure that the species does not go extinct in the same century in which it was first described.

“But I am confident that, with close collaboration with the Indonesian government, and especially with local stakeholders, we can make this joyful news a conservation success story.”

Photo by James Askew for the SOCP.

Photo by Andrew Walmsley.