Deadly floods and landslides on the Indonesian island of Sumatra in 2025 wiped out at least seven per cent of the critically endangered Tapanuli orangutan population, a new report has revealed.

At least 58 of these rare primates, which are endemic to north Sumatra's Batang Toru forest, perished in the disaster.

The report, a joint study by Brunei-based Borneo Futures, World Weather Attribution and Liverpool John Moores University, surveyed only the western block of the forest, home to the majority of the estimated 800 remaining Tapanuli orangutans.

This suggests that the actual death toll could be significantly higher.

The cyclone-induced floods and landslides also claimed at least 1,200 human lives and damaged approximately 300,000 homes.

Environmental groups have attributed the extensive damage to rapid deforestation across the island of Sumatra.

The findings were derived from analysing satellite images of the damage to the West Block of Batang Toru and historical records of the orangutan population there.

Human-induced climate change has likely increased the intensity and frequency of extreme rainfall around Malacca Strait, putting the Tapanuli orangutan's habitat at greater risk, the study said.

Erik Meijaard from Borneo Futures, who was the lead author of the study, said 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, so it became a real concern,” he said.

“This level of loss is substantial for a species with such a small total 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.”

Tapanuli orangutans are described as the Orangutan Foundation as the world’s most endangered great ape.

Panut Hadisiswoyo, another researcher, urged the Indonesian government to work together with NGOs and researchers to prevent the further decline of orangutan population.

“We can minimise the poaching or hunting and then the number probably can be stabilised,” he said, adding that all parties must pay attention to the poor use of land, which also contributes to the declining population.