Bornean orangutans are one of three orangutan species, all critically endangered. They thrive in carbon-rich peat swamp forests on the Indonesian island of Borneo. These habitats are also the sites of massive wildfires.

Indonesian wildfires in 2015 caused some of the worst fire-driven air pollution ever recorded. The fires were driven by an El Niño climatic cycle, which caused especially dry weather in the region.

Compared to other wildfires, peatland fires smolder underground and produce exceptionally high levels of hazardous gases and particulate matter – a leading cause of global pollution-related deaths and illnesses.

Orangutans are well known as an “indicator species” – one that can serve as a proxy for the health of an ecosystem. Changes in their environments often cause conspicuous changes in the apes’ health and behavior. Frequent and persistent exposure to toxic smoke could have severe consequences for orangutans and other wildlife.

Toxic air pollution also poses serious health and safety risks for researchers. However, remote sensing techniques, such as satellite images, GPS data and acoustic monitoring, are increasingly popular ways to track wildlife populations and see how creatures respond to changes in their environments.

I have studied the behavior, ecology and acoustic communication of wild primates in Indonesia since 2005. In a new study, my co-authors and I investigated how wild orangutans in Borneo were affected by toxic emissions from Indonesia’s 2015 peatland wildfires – by studying their voices.

Smoke exposure poses long-term risks

Around the world, wildfires are on the rise. They often produce a thick blanket of haze that contains diverse hazardous gases and particulate matter, or PM. Most recently, smoke from Canadian wildfires blanketed the U.S. East Coast and Midwest in early June 2023, turning skies orange and triggering public health alerts.

Studies have shown that human health risks from wildfire smoke include respiratory and cardiovascular illnesses, systemic inflammation and premature death. Much less is known about how smoke affects wildlife, but in a pair of studies published in 2021 and 2022, scientists at the California National Primate Research Center reported alarming findings.

After less than two weeks of exposure to high concentrations of particulate matter – in particular, ultrafine particles measuring less than 2.5 microns in diameter, which are known as PM2.5 – captive rhesus macaques suffered a spike in pregnancy loss. What’s more, surviving fetuses and infants suffered long-term effects on lung capacity, immune responses, inflammation, cortisol levels, behavior and memory.

During Indonesia’s 2015 fires, Borneo’s air had particulate matter concentrations nearly an order of magnitude higher than the levels in these studies. This made the potential implications for people and wildlife who gasped through Indonesia’s wildfire smoke for nearly two months extremely worrying.

Orangutans in the haze

I was studying wild orangutans in the forests of Indonesian Borneo when the 2015 fires started. My colleagues and I at the Tuanan Orangutan Research Station tracked local fires and patrolled nearby hot spots to assess the risk of fire spreading to our research area.

Wearing N-95 masks, we continued to monitor orangutans in hopes of learning how the animals were coping with encroaching fires and thick smoke. A few weeks into the fire season, I noticed a difference in the sound of the males’ “long call,” which was the focus of my research.

Long calls are booming vocalizations that can be heard over distances of more than half a mile (1 kilometer). Orangutans are semi-solitary and live in dispersed communities, so these calls serve an important social role. Adult males make them to advertise their prowess to listening females in the area and to scare off any eavesdropping rival males. A couple of weeks after the smoke had appeared, I thought these males sounded raggedy – a little like humans who smoke a lot.

We observed the orangutans for 44 days during the fires, until large blazes encroached on our study area. At that point, we stopped the study to help extinguish the blazes with local firefighting teams and other government and nonprofit groups. Fires burned in our study area for three weeks.

Using data that we collected before, during and after the fires, I led an analysis of this Bornean orangutan population’s behavior and health. My co-authors and I found that in the weeks after the fires, the apes reduced their activities – resting more and traveling shorter distances – and consumed more calories than normal.

But although they were eating more and moving less, we found by collecting and testing the apes’ urine that they were still burning stored fat – a sign that they somehow were using up more energy. We hypothesized that the cause might be inflammation – the swelling, fever, pain and fatigue that human and animal bodies experience in response to infection or injury.

Sentinel sounds

Studies have shown that when humans are exposed to particulate matter, they can experience inflammation, both in their respiratory tracts and throughout their bodies. We wanted to know whether inhaling wildfire smoke would cause vocal changes in orangutans, just as inhaling cigarette smoke does in humans.

For this study, my co-authors and I carefully analyzed more than 100 sound recordings of four male orangutans that we followed before and during the fires to measure their vocal responses to wildfire smoke. Research has shown that a suite of vocal features – including pitch, vocal harshness or hoarseness, and shaky voice – reflects the underlying health and condition of both human and nonhuman animals. We were looking for acoustic clues about how this toxic air might be affecting the orangutans.

During the fires and for several weeks after the smoke cleared, these males called less frequently than usual. Normally, orangutans call about six times a day. But during the fires, their call rate was cut in half. Their voices dropped in pitch, showing more vocal harshness and irregularities.

Collectively, these features of vocal quality have been linked to inflammation, stress and disease – including COVID-19 – in human and nonhuman animals.

Listening to vocal species

Increasingly frequent and prolonged exposure to toxic smoke could have severe consequences for orangutans and other animals. Our research highlights the urgent need to understand the long-term and far-ranging effects of peatland fires in Indonesia, which is one of the most biodiverse countries in the world.

By uncovering the linkages between acoustic, behavioral and energetic shifts in orangutans, our study highlights a way for scientists and wildlife managers to safely monitor the health of orangutans and other animals. Using passive acoustic monitoring to study vocally active indicator species, like orangutans, could unlock critical insights into wildfire smoke’s effects on wildlife populations worldwide.

Wendy M. Erb is affiliated with the American Society of Primatologists; the Borneo Nature Foundation; and Primate Conservation, Inc. She has received research funding from the American Association of University Women; the American Institute for Indonesian Studies; the American Association of Biological Anthropologists; the American Society of Primatologists; the British Academy; the Conservation, Food, and Health Foundation; Conservation International; Cornell University; Disney Conservation Fund; the Fulbright Program; the International Society of Primatologists; and the National Science Foundation.