Traffic Noise Silences Gibbons' Morning Songs in Malaysia

Traffic noise silences gibbons. Their morning songs are gone. In a stretch of Malaysian forest, the resident white-handed gibbons, a pair named Mambang and Walinong, have fallen largely quiet at dawn, and it's a silence that stretches through the trees where they're supposed to sing. The air there is cool and still, bordered by housing and roads. And the only sound rising through the trees is the hum of a nearby highway.

That silence is not a fluke. Research conducted in 2023 and 2024 across three sites found that the louder the background, the less likely white-handed gibbons were to call at all. The group living beside the highway called in 36% of recordings. The seaside group, furthest from human activity, called in 71.4%. The hiking trail group managed 84.6%.

Forest Sopranos Go Quiet

White-handed gibbons are arboreal animals. Their calls can travel more than half a mile through dense forest canopies. Their songs are complex, built on vocal techniques that have been likened to those of soprano singers. Dawn is normally when they erupt into song. They wa-oo and wa from the highest branches. And they fill the tree tops with rich melodies.

Nadine Ruppert, an associate professor at Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, and a supervisor of the research, puts the stakes plainly.

“Their songs are not just background noise or a way of announcing their presence. They play an important role in communication between groups, territorial interactions and social relationships. So when we introduce persistent human-generated noise into their habitat, we are potentially interfering with a very important part of their lives.”

Benjamin Galea recorded morning calls for a week. He's a final-year PhD student at Universiti Sains Malaysia and the author of the work, and he analysed those calls in both the wet and dry seasons at each location. Three spots. They varied in background noise. The highway site carried the highest level of continual noise pollution. A popular hiking trail sat in the middle. And a secluded beachfront location lay furthest from human activity, which means it's the quietest of the three, though that's not a claim he makes in the paper. But the contrast is clear.

What the Recordings Showed

The pattern held within sites as well as between them. As background sound increased, the estimated odds of calling decreased. Galea also found that the hiking trail group sang in more recordings than the gibbons at the quieter beach.

Several explanations are possible. The hiking trail gibbons lived near another gibbon group, which may have pushed them to sing more often to demarcate territory. That group also sat toward the forest interior, so noise posed less of a problem.

"They seem to have been able to sing despite the acute noise from humans, but we found that they widen their bandwidth when vehicle noise becomes loud," Galea says.

Earlier Songs, New Problems

There is another possibility for the highway pair. They may have avoided singing because the traffic was too loud, or they may have been singing earlier than the monitoring devices, which recorded from 6am until 1pm. On some days the gibbons were already midway through their songs when recording began.

Move a chorus earlier. That's the fix. It could help individuals overcome the problem of noisy cars, but it might introduce other costs. Galea says it could interfere with resting, foraging, coordination between partners, or communication with neighbouring groups that aren't active at the same time. And those costs don't vanish.

If background noise is driving gibbons to skip singing, that could affect their ability to hold territory, attract mates or warn of predators.

The Solo Song Puzzle

When the highway gibbons did sing, something stood out. Most of their phrases, 93.3%, occurred in the context of a solo song. It is not clear whether that reflects normal differences between groups.

“We can only guess,” Galea says. “It’s possible that traffic noise may have made it harder for the pair to hear and coordinate with each other.”

A Wider Pattern of Noise

The findings add to a growing body of research on how human-generated noise disrupts animal routines. Noise pollution has been found to reduce the ability of mammals to hear their prey, with species such as the greater mouse-eared bat less able to forage effectively when exposed to background noise. Birds have shown effects too. One paper found that noise reduced the survival of passerine birds by masking parent-offspring communication, while another study in Brazil found it reduced the diversity of bird species in urban parks.

Not every consequence of quieter gibbons is obviously bad. Ulrich H Reichard, a professor of anthropology at Southern Illinois University Carbondale who was not involved in the Malaysia research, points out that fewer alarm calls might not put gibbons at high risk of predation, given how fast they swing through the tree tops.

The trouble may land elsewhere. Squirrels and birds appear to recognise and respond to gibbons’ warnings.

“If you have interspecies communication, even if one species shifts their common behaviour, then that will impact the others,” Reichard says.

He also raises a question the data cannot yet answer. “One important question that we have to ask is how much calling is necessary for successful communication. Even with sustained noise, do they have to call every day? Do they have to call every other day? Do they have to call for 20 minutes or 10 minutes or in order to achieve their goal?”

An Endangered Species in Fragments

The white-handed gibbon is listed as endangered by the IUCN, which warns that its population is believed to have declined more than 50% over the past 45 years, driven by habitat destruction and the threat of hunting and capture for the pet and photo-prop trade. Malaysia has no recent population estimate, though numbers are believed to have fallen over recent decades.

Malaysia's situation is unusual. Gibbons live on isolated plots inside urban centres, including Kuala Lumpur, the capital. And a forest fragment can still look like perfectly good gibbon habitat from a purely physical perspective, even though it's cut off, surrounded by concrete, and sitting right there in the middle of a city. It's strange. They're not where you'd expect.

“It may contain trees, food and sleeping sites,” Ruppert says, “but it may also be surrounded by roads, housing, industry or other sources of chronic noise.”

That gap matters. It's where the conservation argument is heading. As urban and peri-urban forests gain recognition for their conservation value, Ruppert argues for a broader view of habitat quality, one that weighs what these places actually experience against what they merely appear to be.

“It is not only about how much forest is left, but also about what the animals experience within that forest.”