Climate-induced landslides kill 58 rare Tapanuli orangutans in Sumatra

A catastrophic event for a critically endangered species

In November 2025, Cyclone Senyar unleashed torrential rains across Indonesia's Sumatra island, triggering massive landslides and floods. Among the casualties were at least 58 Tapanuli orangutans, a species with fewer than 800 individuals remaining in the wild. The loss represents approximately 7% of the total global population of this great ape, described as the world's rarest.

The Tapanuli orangutan was only scientifically classified as a distinct species in 2017. Confined to a small area of the Batang Toru forest in North Sumatra, its survival is already under pressure from habitat destruction and human encroachment.

The loss of an estimated 58 Tapanuli orangutans to a single climate-induced landslide event is a devastating demographic shock to the world's rarest great ape.

Measuring the loss

The study, published in the journal Current Biology, was a collaborative effort by Borneo Futures, World Weather Attribution and Liverpool John Moores University. Researchers analysed satellite images of landslide scars in the western block of Batang Toru, home to the largest remaining population of the apes. They found that about 8,300 hectares of forest, more than 11% of the area, were damaged. By overlaying this data with orangutan density maps, they calculated the death toll.

Importantly, the survey only covered the western block, meaning the actual number of orangutan deaths could be higher. The flooding also killed between 1,000 and 1,200 people and damaged around 300,000 homes.

Climate change and deforestation as drivers

The study concluded that human-induced climate change has increased the intensity and frequency of extreme rainfall in the Malacca Strait region, making such devastating events more likely. The rapid deforestation of Sumatra has compounded the problem. Between 2001 and 2024, the island lost 4.4 million hectares of forest, an area larger than Switzerland, according to the conservation group Mighty Earth. This loss of tree cover destabilises hillsides, leaving them vulnerable to landslides during heavy rains.

The heavy rain soaked the soil so much that large parts of hillsides in the primary forests collapsed in fast-moving landslides. If you get caught as an orangutan... if anything comes down at great speeds, survival chances are going to be very minimal.

The need for urgent conservation action

Environmentalists have long campaigned against industrial activities in Batang Toru, particularly a hydroelectric dam and a gold mine that drive orangutans into higher, less suitable habitats. The highland homes inhabited by Tapanulis are not their preferred habitat, but it is where remaining orangutans have been pushed by human development elsewhere.

We can minimise the poaching or hunting and then the number probably can be stabilised.

Lead author Erik Meijaard stressed that the loss is substantial for a species with such a small total population and that ongoing pressures require immediate implementation of a coordinated species action plan. Jatna Supriatna urged international partners to provide biodiversity-recovery financing to prevent the first modern extinction of a great ape species.