Four days of extreme rain in Indonesia triggered devastating landslides, killing dozens of critically endangered Tapanuli orangutans
A cyclone that brought catastrophic flooding and devastating landslides to Indonesia wiped out more than 7% of the global population of the world’s rarest great apes, a new study has found.
Nearly 60 of the 800 Tapanuli orangutans remaining in the wild were killed when Cyclone Senyar slammed into the Indonesian island of Sumatra last November, according to the study published this month in the journal Current Biology. The cyclone has pushed these critically endangered orangutans closer to extinction, the study scientists said.
And it’s a sign that climate change-fueled extreme weather is adding to the risks the orangutans already face as the forests they live in are cleared for roads, farming and industry. Cyclone Senyar brought more than 16 inches of rain to Indonesia, making it one of the region’s most intense rain events in recent years, according to the Tsunami and Disaster Mitigation and Research Center.
Climate change fueled the tropical storm, increasing the rain’s intensity by 9% to 50%, according to one analysis. The cyclone killed more than 1,000 people and displaced over a million. It also caused devastation in the orangutans’ main habitat on Sumatra - the West Block of the Batang Toru ecosystem in North Sumatra.
The researchers used satellite data to identify more than 20,000 acres of landslide scars — visible patches of rock, soil, and debris — which wiped out nearly 12% of forest cover in this region. Satellite view of the widespread flooding and landslides across the Aceh province of Indonesia on November 29, 2025. Researchers say the landslides happened as extremely heavy rainfall saturated the ground, causing parts of the hillside to suddenly break away.
Because these kinds of collapses often happen with little or no warning, any orangutans had very little time to escape. The analysis found 58 Tapanuli orangutans were killed, accounting for 11% of the local population and 7% of the total global population. These numbers may be conservative, the scientists said, as they used estimates of population density. The study also did not account for other cyclone-related threats such as reduced food availability for surviving orangutans or destruction of forest canopy.
“It could well be that up to 120 animals died during the landslide events,” said Erik Meijaard, the study’s lead author and the managing director of Borneo Futures in Brunei. The numbers could also be lower than 58, he added. The orangutans’ deaths are a huge blow to a very vulnerable species. Tapanuli orangutans recover extremely slowly, as females tend to only give birth every six to nine years, making it difficult for the population to rebound.
The apes live in isolated populations across Sumatra, meaning that even a single cyclone or landslide can have long-lasting consequences. Researchers warn the damage may increase if surviving orangutans face reduced breeding success after the disaster. The Tapanuli orangutan struggle to withstand even small increases in death rates and annual losses of more than 1% a year likely put the species on a path toward extinction, Meijaard said.
The aftermath of the devastating flash floods triggered by Tropical Cyclone Senyar in Aceh, Indonesia, on December 29, 2025. The findings have prompted calls for the Indonesian government to enforce stronger conservation measures to protect the endangered species.
“Whether these orangutans can recover will depend on what happens next,” said Friederike Otto, a climate science professor at Imperial College London. “If this tragedy serves as a wake-up call to really halt deforestation, reconnect some of fragmented forest habitats where that is possible, and bring governments, local communities, and industries together to support conservation efforts, the species may still have a chance,” she told CNN.
But, she warned, another heavy rainfall event, which climate change is making more likely, poses a huge risk to their recovery. ⛅ Get CNN Weather in your inbox Adriano Lameira, a primatologist from the University of Warwick who studies Sumatran orangutans and was also not involved in the research, said the Indonesian government must better protect the orangutans’ habitat from both people and industry.
“After decades of sustained conservation efforts, it is clear the current system is not working and cannot guarantee the protection of the country’s unique natural resources,” he said. The orangutans have huge value, he added, including furthering our understanding of humanity: “As one of the last few remaining great ape species, the Tapanuli orangutan also provides unique insights into the potential lives of ancient human ancestors and why and how humans came to become who we are today. ”
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