Even for New Zealand’s elusive bats, silence is golden when it comes to dodging noisy roads.
Threatened bat species cut their activity in half near the din of simulated traffic, and those that stick around may crank up their calls to be heard over the rumble, researchers report in a bioRxiv preprint. The findings highlight how road noise could shrink usable habitat for these already declining animals, potentially harming their survival.
Anthropogenic sound, like the constant hum from cars and trucks, is a growing pollutant in natural areas, but its impact on bats—who rely on echoes and sounds for hunting, navigating, and chatting — has been understudied. In this experiment, scientists focused on New Zealand’s two remaining native bats: the long-tailed bat and the lesser short-tailed bat, both listed as threatened due to habitat loss, predators, and other pressures.
To test traffic noise without the dangers of real roads, the team set up a “phantom road” in Pureora Forest Park, a protected area in the central North Island where both species still hang on. They lined speakers along a forest edge to blast recordings from a busy highway, mimicking the roar of passing vehicles at levels around 65 to 80 decibels — much louder than the quiet nighttime background of about 36 decibels.
Over 26 nights in early 2022, acoustic detectors captured bat calls before, during, and after the playback sessions. The setup included spots right near the speakers and farther away, up to 80 meters into the forest or along the edge, to see how distance played a role.
The results showed a sharp drop in bat presence close to the noise. Both species slashed their overall call sequences by about 50 percent during playback, suggesting they were steering clear of the area. For long-tailed bats, foraging calls — those rapid bursts used when zeroing in on insect prey — also dipped by roughly the same amount, pointing to real avoidance rather than just quieter behavior.
Short-tailed bats showed a similar overall dip in activity, but their foraging calls held steady, hinting they might switch to more aerial hunting in noisy spots instead of ground-based searching, where they listen for rustling prey that could get drowned out.
Farther out, at 30 to 80 meters, the noise didn’t seem to faze them — activity levels stayed the same whether playback was on or off. This could be because sound drops off quickly with distance, especially in dense forest, though the team notes that real highways might cast a wider shadow of disturbance.
For the long-tailed bats that did venture near during playback, their calls changed in intriguing ways. Based on a subset of detailed recordings, these bats belted out shorter, louder calls at higher pitches, with wider frequency ranges. This could be a way to cut through the background racket, similar to how people raise their voices in a crowded room—a phenomenon called the Lombard effect. But such tweaks might burn extra energy, adding hidden costs.
The avoidance alone acts like invisible habitat loss, the researchers suggest, potentially barring bats from prime foraging or roosting spots and fragmenting their world. Roads could become barriers, especially for ground-foragers like short-tailed bats, who might struggle more with low-frequency noise overlapping prey sounds.
With human expansion ramping up noise in wild places, these insights underscore the need for quieter infrastructure — think noise barriers or rerouted roads — to safeguard bats and the pest control they provide ecosystems. As bat populations dwindle, even subtle pollutants like sound could tip the balance.
Journal References: Threatened bat species reduce their activity in the presence of traffic noise playback, and may shout louder. DOI: 10.1101/2025.11.21.689748
