Maxime Aliaga

The hominid family just got a little bigger. A new orangutan species has been found hiding in the forests of Sumatra. The Tapanuli orangutan is only the third orangutan species, and the seventh non-human great ape. But they may not be around for long: there are only 800 of them and they live in an area smaller than London.

For years, researchers have recognised two species of orangutan living in Indonesia: the Bornean orangutan (Pongo pygmaeus) and the Sumatran orangutan (P. abelii). Both are critically endangered.

In the late 1930s there were reports of a population of orangutans in Tapanuli, in the Batang Toru area, south of the range of Sumatran orangutans, but the claims were never fully investigated. The Tapanuli population was only rediscovered in 1997 by Erik Meijaard at the Australian National University in Canberra.


Initial genetic studies suggested the population was unique, perhaps unsurprisingly, given the nearest orangutan neighbours were 100 kilometres away. Then in 2013, a male Tapanuli orangutan called Raya died of his wounds after a conflict with local villagers. Finally, scientists were able to study a Tapanuli specimen and compare it to its Sumatran and Bornean brethren.

An orangutan like no other

Michael Krützen at the University of Zürich in Switzerland helped assemble a team of nearly 40 people to study Raya. They compared Raya’s skull and teeth with those of 33 other orangutans. Raya’s skull was smaller than that of the other two species at comparable developmental stages, and his upper and lower canines were significantly wider than Sumatran orangutans’.

The Tapanuli orangutans also behave differently. Male orangutans make “long calls” that can be heard over a kilometre away, which repel rivals and attract females. Tapanuli males’ long calls were 21 seconds longer than those of Bornean ones, and their roars were delivered at a higher maximum frequency than those of Sumatran orangutans.

Finally, the researchers analysed 37 genomes from various orangutan populations. Once again, the Tapanuli orangutans were significantly different from their Bornean and Sumatran cousins.

“Scientific serendipity brought everything together and it dawned on us that this is a new species,” says Krützen. They named it Pongo tapanuliensis.

“Orangutans must be among the most intensively studied tropical species, and they are also a famous conservation icon,” says Meijaard. “How could this distinct species slip through and stay hidden to science for such a long time?

“The justification for this classification is as strong as the justification for splitting Gorilla or Pan [chimpanzees],” says Erin Vogel at Rutgers University in New Brunswick, New Jersey. However, she says the anatomical data should be taken with caution until more skeletons are examined.

Nater et al./Current Biology

The genetic analysis also offers a glimpse at the evolutionary history of orangutans.

Fossils and studies of genomes had already revealed that orangutans split from the other great apes around 14 million years ago. The new study reveals that the orangutan lineage first split around 3.38 million years ago, when the Tapanuli orangutans split off from the rest. That main population would then split in Sumatran and Bornean orangutans much later, around 674,000 years ago.

That makes the Tapanuli orangutans the oldest of the three species. Krützen says they may be the descendants of orangutans that migrated from mainland Asia to what is now Indonesia more than 3 million years ago.

It is unclear why the three orangutan species formed when they did. However, shifting climates may be a factor.

Throughout much of the last 2.6 million years, sea levels were lower than today, because Earth was in an ice age and much water was locked up in the poles. As a result, the Indonesian islands of Borneo, Java and Sumatra were often connected to mainland Asia, forming a landmass the size of Europe known as Sundaland. When the ice periodically melted and seas rose, the islands became cut off as they are today. Conceivably, the three orangutan species diverged when populations became isolated on these islands.

But the Tapanuli orangutans’ history may be coming to an end. They are confined to an area of about 1000 square kilometres. Modelling suggests that if more than 1 per cent of the adult population – that is, eight of the 800 animals remaining – is removed per year, the species will go extinct, Meijaard says.

Just based on population numbers, Tapanuli orangutans may be the most endangered of all great apes, says Vincent Nijman at Oxford Brookes University in the UK. Most other great apes have populations in the tens of thousands.

The species is primarily threatened by hunting and poaching, and forest loss due to agriculture plantations and mining exploration, says co-author Matthew Nowak at the Sumatran Orangutan Conservation Programme. What’s more, a proposed hydroelectric project threatens 8 per cent of the orangutans’ habitat.

Krützen and Nowak are also concerned that their habitat is becoming increasingly fragmented. The Tapanuli occupy three areas in the Batang Toru region. Forested corridors enable the apes to travel between the three habitats, but these are disappearing, trapping the Tapanuli in the smaller habitats. This is causing inbreeding, which poses an additional threat by allowing harmful mutations to build up. Krützen says Tapanuli orangutans are more inbred than both Sumatran and Bornean orangutans.

For now, the researchers hope to galvanise conservation efforts to protect the Tapanuli orangutans’ habitat. ”If we preserve the Batang Toru forest for these orangutans, we also protect numerous other species that we might not even know existed and don’t have this flagship status,” Krützen says.

Journal reference: Current Biology, DOI: 10.1016/j.cub.2017.09.047

Read more: Majority of primate species may vanish in next 25 to 50 years

We have corrected the credit for the second photograph