Rainfall and landslides last year in Indonesia killed 7% of world’s rarest great apes – study

Critically endangered Tapanuli orangutan population falls after extreme weather – fuelled by climate crisis – in North Sumatra

Extreme rainfall and landslides fuelled by the climate crisis killed 7% of the remaining population of the world’s rarest great ape, a study has found, prompting fears for the species’ survival.

The research suggests 58 out of the remaining 800 critically endangered Tapanuli orangutans ( Pongo tapanuliensis ) were killed after more than 1,000mm (39in) of rain fell over four days in Indonesia’s North Sumatra province in November 2025. This equates to 11% of the local population and 7% of the entire species.

“It is tragic to lose so many apes in this way. In landscapes where populations are small and fragmented, this type of weather or climate event can have population-level consequences. It is extremely worrying for the future of this ape,” said Prof Serge Wich, a primatologist at Liverpool John Moores University and co-author of the study , published in the journal Current Biology.

The scientists overlaid new analysis of satellite imagery with estimates of ape density to work out the impacts of Cyclone Senyar on the orangutan population in its primary habitat in the West Block of the Batang Toru ecosystem, already threatened by mining , palm oil plantations and a large hydropower project.

Satellite imagery also revealed that approximately 8,300 hectares (20,510 acres) – 11.7% – of this key forest habitat were wiped out by landslides caused by extreme rainfall . The scientists said human-induced climate change caused by the burning of fossil fuels had increased the rainfall intensity by up to 50%.

“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,” said Prof Jatna Supriatna, a conservation biologist at Universitas Indonesia . “To prevent the first modern extinction of a great ape species, Indonesia must permanently protect the Batang Toru ecosystem, but our international partners must also meet their global commitments by providing immediate biodiversity-recovery financing.”

Previous research has suggested annual Tapanuli orangutan population losses of just 1% of the population would be sufficient to lead to eventual extinction.