JAKARTA — The fires that raze vast swaths of Indonesian Borneo every year are having a lasting health impact on the region’s critically endangered orangutans that threatens them with extinction, a preliminary study has found.

The fires, which in nearly all cases are started to clear land for plantations, such as oil palm, reduce the availability of food for the orangutans, pushing them into what researchers say is starvation mode. Meanwhile, the smoke from the fires weakens their immune system and damages their DNA.

The researchers, from Rutgers University–New Brunswick and the National University in Jakarta, have for years been studying Bornean orangutans (Pongo pygmaeus) from the Tuanan research station, inside the Mawas conservation area that’s home to an estimated 3,500 of the great apes. In 2015, 90 hectares (220 acres) of peat forest in the area was burned; in 2019, 160 ha (395 acres) was burned.

“What we can do is take advantage of this situation and see how these forest fires are affecting the behavior and the health of the orangutans,” Erin Vogel, a professor of anthropology at Rutgers and co-director of the Tuanan research station, said at a presentation of the new study in Jakarta.

After a painstaking process of collecting thousands of urine samples using a bag at the end of a stick and comparing the orangutans’ behaviors before, during and after the fires, the researchers concluded that the apes were negatively affected by the burning and smoke.

For one, the orangutans were found to travel less as they tried to conserve their energy.

“So after the fires, they’re using an energy-conserving strategy and did not move as much,” Vogel said.

She added this might be caused by the declining availability of fruit during the fire season. This fruit shortage forces the orangutans to resort to eating bark.

“And bark is very low in energy and it takes a lot of time to eat the bark,” Vogel said. “So we’re finding that they’re feeding on much lower-quality resources in terms of nutritional intake after the fires.”

This means that even if they don’t move as much, they’re still expending more energy than they can take in, Vogel said, and thus entering a state of negative energy balance where they’re drawing on their body fat for energy.

“[T]hey’re not getting enough nutrients, and they’re starting to use any source of energy that they have to survive,” Vogel said.

‘Hollowed out’

The researchers confirmed the orangutans’ altered metabolic state by detecting significantly higher levels of the compound ketone in the animals’ urine — a sign that their bodies are experiencing low glucose intake and hence having to burn fat.

“They’re very energetically stressed,” Vogel said.

In some cases, they starve so much that they start losing muscle mass.

“We’ve seen this in Tuanan, when food availability was extremely low and lasted for a long stretch of time, over six months,” Vogel said.

She recalled a particularly bad fire episode, in 2015, when some of the orangutans that lived near Tuanan fled from their burning habitat.

“The orangutans that lived along that forest edge were pushed to the edge of the river and had very little to eat,” Vogel said. “And they started to go into starvation and had to be rescued by the Borneo Orangutan Survival Foundation.”

When they were rescued, she said, it was apparent that they had suffered from the lack of nutrition.

“It was very clear that they were hollowed out, kind of like when humans are starving,” she said. “You can see their limbs get much thinner. They have big bellies because they eat leaves and they need big guts to digest those leaves. But when they go into starvation mode, they lose those big bellies.”

Vicious cycle

Extreme cases like this could happen again if fire seasons grow more intense and more frequent, Vogel warned. This is already happening due to the combination of a changing climate and sustained deforestation and drying of peatlands.

“This repeated burning creates more and more fuel, so every year we get more and more fires,” Vogel said. That cycle perpetuates another: More frequent fires lead to less food availability and smaller windows of opportunity for orangutans to build up their body fat.

“During high fruit season, they are building body fat and they can more than quadruple the caloric consumption,” Vogel said. “But if the fires happen more and more, there’s going to be lower and lower fruit availability and they might not be able to have that period where they put on fat. And that can become a very dangerous situation where they start to break down skeletal muscle.”

The smoke might also affect the orangutans’ immune system, the researchers found. When one of Vogel’s students looked at nitrous oxide emissions during the 2015 fires, they detected a slight increase in a substance that serves as marker of oxidative stress.

“Oxidative stress is what results in DNA damage,” Vogel said. “So what’s happening during these smog periods, these animals are affected. Their immune systems are likely affected and they’re experiencing damage to their DNA. We haven’t tested it fully yet but it’s likely.”

Increased aggression

The smoke also seems to affect male and female orangutans differently. The females tend to stay in the area where they’re born because they need to stay close to their families, as opposed to their male counterparts.

When their habitat is burned, the females are pushed even closer to the other orangutans because they don’t tend to flee far from their habitat. As a result, their living space gets more crowded, resulting in the females becoming more aggressive.

“We have much higher level of aggression among females which is really rare in orangutans, up to 10 times more aggression than we see before fires in these animals,” Vogel said.

This heightened aggression risks sparking clashes among the orangutans, according to Rutgers doctoral student Didik Prasetyo, who chairs the Indonesian Primatological Society.

“Orangutans fight and many of them die maybe because there’s too many of them in one place,” he said. “That’s an impact of the fires.”

The reduced food availability as a result of the fires might also affect the reproduction cycle of female orangutans. Bornean orangutans usually reproduce every six to eight years; over the course of a lifetime, a female will give birth to no more than four or five offspring.

“We don’t see clear effect during low fruit period, but from 19 conceptions that we’ve documented, only one conception is during a low fruit period,” Vogel said. “So it seems that females are conceiving less during low fruit period.”

Falling population

With Bornean orangutans already listed as critically endangered due to habitat loss from logging and large-scale farming — their population has plunged by 60% since 1950 — the health impacts from forest fires could push the apes further toward the brink of extinction.

New projections anticipate their numbers will fall by another 20% by the year 2025, to an estimated 47,000 apes across the island.

“[T]he area where we work has 3,500 orangutans in the region and these animals are affected by fires that keep popping up,” Vogel said. “So these fires are really threatening the world’s remaining orangutans and they are really vulnerable to fires.”

Clarification 1/27/2020: This story has been updated to further reflect Erin Vogel’s view.

Banner image: A Bornean Orangutan in Indonesia. Image by Rhett A. Butler/Mongabay.