Messages To Friends

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.

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 N95 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 coauthors 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.

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 coauthors 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.

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.