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Climate change intensified rainfall from Cyclone Senyar may have killed 58 Tapanuli orangutans, the world’s rarest great ape, a new study says.
The suspected deaths represent about 7% of the species’ total wild population, which is estimated at fewer than 800 individuals.
Satellite imagery showed 8,303 hectares of landslide scars in the West Block of the Batang Toru ecosystem in Sumatra, the species’ largest remaining stronghold.
News reports in December 2025 had already warned that floods in the region may have caused catastrophic wildlife losses.
Researchers say the disaster could affect the population for up to a decade because Tapanuli orangutans reproduce slowly, with females giving birth only once every six to nine years.
Climate change intensified rainfall that may have killed 58 Tapanuli orangutans, the world’s rarest great ape, in just four days, according to a new study.
The findings, published in the journal Current Biology on June 10, 2026, say landslides triggered by Cyclone Senyar destroyed large parts of the Batang Toru ecosystem in Sumatra, Indonesia, home to the critically endangered Tapanuli orangutan (Pongo tapanuliensis).
News reports in December 2025 had already warned that the floods could have devastating consequences for the species. Satellite imagery and accounts from local residents suggested severe damage in the West Block, the most densely populated of the three known Tapanuli orangutan habitats, which was estimated to hold 581 individuals before the disaster. Erik Meijaard, the study’s lead author and chief scientist at Borneo Futures, had told news agency AFP at the time that 6 per cent to 11 per cent of orangutans in the area may have been killed.
Fewer than 800 Tapanuli orangutans remain in the wild. Scientists said the suspected deaths represent about 7 per cent of the total population, a severe blow to a species already under pressure from habitat fragmentation and degradation.
The Tapanuli orangutan was recognised as a separate species only in 2017. It survives in three isolated populations in the West, East and South blocks of Batang Toru. Researchers said sustained additional losses of more than 1 per cent a year could push the species further towards extinction.
Cyclone Senyar brought more than 500 millimetres of rain to the Batang Toru ecosystem in five days in late November 2025, the study said. Between November 23 and 28, rainfall ranged from 103 mm in Padang Lawas Utara to 1,003 mm in West Sumatra. In the affected West Block, 564 mm fell over the same period, including two days of more than 150 mm of rain.
The extreme rainfall triggered landslides across the West Block, the largest remaining stronghold for the Tapanuli orangutan. Satellite imagery showed 8,303 hectares of landslide scars, amounting to an 11.7 per cent loss of forest cover in the West Block, the study said.
The damage was far greater than in previous years. The area affected by the 2025 landslides was about 200 times larger than landslide scars observed in the same region in 2019. Researchers said the orangutans, which live mostly in trees, would have had little chance of escaping fast-moving debris flows. They may have been killed by falling trees, burial or drowning.
“These are high-velocity, shallow landslides triggered by intense rainfall,” David Petley, a landslide researcher at Nottingham Trent University and one of the authors of the paper, said in a statement.
“Because the debris flows are directly connected to the channel system, the failure is rapid and exceptionally destructive, leaving those in the path with little warning or chance of escape,” he said.
Tapanuli orangutans reproduce extremely slowly. Like other great apes, they have slow life histories, but orangutans are among the slowest, with females giving birth only once every six to nine years. That means even a relatively small population loss can have long-term consequences.
The study said the deaths, combined with the sudden loss of habitat, could affect the population for up to a decade. Landslides stripped away topsoil and biomass, leaving affected areas with almost no food resources for at least five to 10 years, until pioneer forest begins to regrow.
Extreme rainfall can also reduce the growth and fruit production of trees that survive. The researchers said surviving orangutans may be forced into poorer habitats, including higher elevations where food is scarcer, or into smaller remaining forest areas where competition for food is greater.
Canopy gaps can also make movement more difficult. Orangutans may have to climb up and down more often or travel on the ground, both of which increase energy costs and risk. Because orangutans already live on low daily energy budgets, even small increases in movement costs can reduce feeding time, weaken reproduction and increase mortality, the study said.
The authors said there were still uncertainties about the long-term effects of the landslides, changing food availability and increased movement costs on surviving orangutans in the West Block. But they said the event showed how climate change can intensify risks for species already close to extinction.
An attribution analysis found that human-induced climate change increased the rainfall intensity of Cyclone Senyar by between 9 per cent and 50 per cent.
Friederike Otto, professor of climate science at Imperial College London, in the statement said the study showed how closely linked the climate and biodiversity crises have become. “By increasing the rainfall intensity of Cyclone Senyar by up to 50 per cent, human-induced climate change turned a severe tropical storm into a catastrophic trigger for these tragic landslides,” she said.
Meijaard in the press statement said the loss was substantial for a species with such a small population. “When combined with ongoing pressures such as habitat degradation and human–wildlife conflict, it further increases the urgency of implementing and adequately resourcing a coordinated species action plan,” he said.
