Borneo's wildfires could have a lasting impact on orangutans

Indonesia is battling one of its most severe wildfire seasons in over a decade, as intense El Niño conditions fuel widespread drought. Some of the most severely affected areas are on the island of Borneo. Peatlands across West, Central and South Kalimantan are smoldering underground, releasing smoke haze that threatens human health and the environment.

But the damage doesn't end when the flames are extinguished or the smoke clears. Fires can transform the forests that wildlife depends on, with consequences that last for years.

Scientists studying wild orangutans in Borneo have seen this firsthand. At the Tuanan Orangutan Research Station, researchers witnessed the devastation caused by the 2015 fires. In the years that followed, they documented how the orangutans adapted to life in their altered habitat: travelling shorter distances, resting more, and spending less time with others.

The researchers had an advantage. Since 2003, they had been monitoring a wild orangutan population, meticulously recording the animals' movements alongside changes in the forest's health and productivity. Their long-term records allowed them to trace the effects of the fires long after the immediate crisis had passed.

After the fire

The threat from this year’s fires peaked in October, when emergency workers at Tuanan spent long days and nights cutting firebreaks and pumping water onto active fires. “Nobody is following orangutans during the peak fire risk,” says Dr. Maria van Noordwijk, a scientist at the Max Planck Institute of Animal Behavior who helped establish Tuanan.

Van Noordwijk remembers what it was like returning to Tuanan after the 2015 fires to assess what had happened to the forest and its inhabitants. “The haze is absolutely everywhere,” she remembers of the dense smog that blanketed large parts of Southeast Asia for months.

At the research site, the haze persisted for three months, accompanied by troubling signs. Orangutans appeared to be suffering from respiratory problems. “You hear them coughing,” van Noordwijk says. They were also spending more time resting and travelling less, often staying lower in the canopy.

At the time, the researchers didn't know whether these behavioral changes would persist once the haze lifted or, if they did, what was causing them. Their observations over the following years would reveal some answers.

More crowding, less food

An important discovery was what happened to the food. Following the fires, the researchers documented a sharp decline in fruit production that lasted for about 2.5 years. The prolonged fruit shortage may have resulted partly from haze restricting photosynthesis, and possibly from a loss of pollinators.

For orangutans, which depend heavily on fruit when it is available, the consequences were easy to spot. With their preferred food scarce, females adopted energy-conserving strategies: they travelled shorter distances, rested more and relied on lower-quality fallback foods. Research also found that orangutans lost lean body mass, suggesting that they were drawing on their own body tissue to meet their energy needs.

The forest itself had also changed. Around 15% of Tuanan's forest was lost in the 2015 fires, forcing some orangutans out of their home ranges. Displaced animals had to move into areas occupied by others, bringing more individuals into closer proximity and intensifying competition for resources.

“Animals who lost their homes had to move, and those that didn’t had to accept more crowding,” says van Noordwijk. “Everybody was affected.”

With food in short supply and more animals sharing the remaining habitat, social relationships began to fray. Adult females spent less time foraging together with female relatives and even their own independent young. Among unrelated adult females, competition for scarce resources was associated with greater social intolerance and aggression.

The consequences may have extended to reproduction and survival. No pregnancies were recorded among females during the two and a half years after the fires, and three of the study's oldest females had disappeared by late 2017. The researchers cannot establish that the fires caused the females' disappearance, but the timing was striking.

“After the fires we lost three older females, far more than expected based on the extremely high survival rates of adult orangutan females established during the long-term study. What are the chances?” van Noordwijk says.

What might the current fires mean for orangutans?

The findings from the 2015 fires and their aftermath offer a unique perspective on the fires burning in Borneo today. The immediate dangers of flames and smoke are only part of the story. Forest loss can reduce food availability, disrupt social relationships, and force orangutans to spend energy adapting to a changed environment. "So, fires impact the animals for months or even years after the flames have been put out,” van Noordwijk says.

The 2026 fires have so far destroyed an estimated 30% of the study area. “That loss will translate into many of the same pressures documented after the 2015 fires,” says van Noordwijk. “At the same time, I’m afraid that the toll will be worse than last time.”

That’s why long-term monitoring sites like Tuanan are invaluable. They provide a record of what the forest and its orangutans were like before the fires began. By continuing to monitor the population, scientists can compare the aftermath of the current fires with what they observed a decade ago.

“Only field sites like Tuanan can answer the crucial question of what impact fires have, and how long the orangutans will take to recover,” says Prof. Meg Crofoot, director of MPI-AB.