It’s at about this time that gibbons usually erupt into song, wa-oo-ing and wa-ing from the highest branches and filling the tree tops with rich melodies. But in this corner of Malaysian forest, bordered by housing and roads, the resident white-handed gibbons are quiet. All that can be heard is the hum of traffic from a nearby highway.
This isn’t a rare occurrence for this group of gibbons, according to research, which found these particular gibbons, named Mambang and Walinong, appeared to be producing far fewer morning songs than others that are not exposed to the same level of traffic noise. White-handed gibbons, arboreal animals whose calls can travel more than half a mile through dense forest canopies, are known for their complex songs and vocal techniques that have been likened to those of soprano singers. But as their habitats are increasingly fragmented by roads, industry and urban development, researchers say there is a growing need to understand how human-generated noise is affecting when and how they sing.
They play an important role in communication between groups, territorial interactions and social relationships,” says Nadine Ruppert, associate professor at Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, and a supervisor of the research. In 2023 and 2024, for a week in both wet and dry seasons, Benjamin Galea, a final-year PhD student at Universiti Sains Malaysia, and the author of the study, recorded and analysed the morning calls of white-handed gibbons living in three different locations that had varying levels of background noise. They included the spot near the highway, which had the highest level of continual noise pollution, a popular hiking trail, and a secluded beachfront location that was the furthest from human activity.
Galea’s research, which is yet to be peer reviewed, found that the greater the background noise, the less likely gibbons were to call. The group living close to the highway called in 36% of recordings – far less than the seaside group (71.4%), and the hiking trail group (84.6%). Within sites, the estimated odds of calling also decreased as background sound increased.
Galea found that the hiking trail group sang in more recordings than the gibbons located at the beach, a quieter spot. Various possible reasons exist for this, including the hiking trail gibbons’ proximity to another gibbon group, which may have prompted them to sing more to demarcate their territory. The hiking trail group was also located towards the forest interior, so noise posed less of a problem, he says.
It is possible that the highway group avoided singing because the traffic was too loud, or that they were singing at an earlier time – before the study’s monitoring devices began recording, from 6am until 1pm, says Galea. On some days, the gibbons were already midway through their songs when the recordings started at 6am. Singing earlier could help individuals overcome the problem of noisy cars, but it might introduce other problems, and potentially interfere “with resting, foraging, coordination between partners, or communication with neighbouring groups that are not active at the same time”, he says.
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