In the forests of Malaysia, the distinctive morning songs of white-handed gibbons are increasingly being silenced, as human-generated noise disrupts their vocal behavior. Researchers monitoring gibbon populations near a highway, a hiking trail, and a remote beachfront location found that increased background noise correlates with a significant reduction in their morning calls.

White-handed gibbons (Hylobates lar), known for their elaborate duets that can carry over half a mile through dense forest canopies, use these vocalizations for communication, territorial defense, and social bonding. However, rising noise pollution from roads and urban development appears to interfere with these functions.

Over two weeks spanning the wet and dry seasons of 2023 and 2024, Benjamin Galea, a PhD student at Universiti Sains Malaysia, recorded gibbon song activity across the three sites. The group near the highway—characterized by persistent, high levels of traffic noise—vocalized in only 36% of recordings, compared with 71.4% for gibbons near the quieter seashore and 84.6% for those at the hiking trail site. Within each location, increased ambient noise corresponded to a decreased likelihood of vocalization.

Interestingly, the hiking trail group sang more frequently than the more isolated beach group, possibly due to factors including proximity to neighboring gibbon groups and reduced impact of human noise deeper within the forest. Galea noted that the hiking trail gibbons appeared to adapt their vocalizations by increasing bandwidth when exposed to vehicle noise, suggesting some behavioral plasticity.

The highway gibbons' diminished singing could result from avoidance of loud traffic noise or shifting their calls to earlier pre-dawn hours, a time when recording equipment was not yet active. Adjusting call timing might mitigate interference but could disrupt essential activities like resting, foraging, and coordinated communication with mates and neighboring groups.

Reduced vocalization may carry broader ecological consequences. Gibbon songs are integral to maintaining territory boundaries and attracting mates; interruptions may thus affect reproductive success. Solo songs predominated among the highway group’s vocalizations, potentially reflecting difficulty in duet coordination caused by traffic sounds, though more research is needed to clarify this pattern.

This study contributes to a growing body of evidence linking noise pollution to disruptions in animal communication. Comparable effects have been documented in species such as bats, whose foraging efficiency declines amid noise, and birds, where survival rates and species diversity can be diminished due to masking of critical signals.

Experts emphasize the need for further investigation to determine how much vocal activity is essential for effective communication in gibbons. Ulrich H Reichard, professor of anthropology at Southern Illinois University Carbondale, notes that even if gibbons reduce calling frequency or duration, the implications for territory defense and predator warnings remain uncertain. Additionally, shifts in gibbon behavior could impact other animals that rely on their alarm calls, highlighting potential cascading effects within the ecosystem.

The white-handed gibbon is classified as endangered, with populations having decreased by more than 50% over the past four decades due to habitat loss, hunting, and capture for the pet trade. As human development continues to encroach on forest habitats, understanding how noise pollution affects gibbon communication will be vital to conservation efforts aimed at preserving these arboreal primates and their ecological communities.