Threatened bat species reduce their activity in presence of traffic noise playback, and may shout louder

Anthropogenic sound is a prevalent pollutant with broad effects on wildlife. As human populations grow, exposure of natural environments to anthropogenic noise will increase. Automobile traffic is a notably pervasive source of noise pollution; transportation networks often cross otherwise relatively undisturbed habitats and the sound they produce is typically chronic and of high intensity. How this noise interferes with the production and reception of animal acoustic signals for communication and navigation remains largely unknown. In particular, bats (order Chiroptera) have received limited attention in anthropogenic noise studies, despite being the second most diverse mammalian group and relying strongly on acoustic signals. We used a ‘phantom road’ paradigm to experimentally isolate and assess the effect of traffic sound playback on the behaviour of the two extant bat species in Aotearoa New Zealand. To begin to understand how bats might behaviourally respond to a noisy environment, we also investigated whether bats’ call sequence and structure differed with and without sound playback. We found an approximate 50% reduction in the mean number of sequences per night of both bat species during sound playback, suggesting bats avoid noisy areas. We also found that calls from the bats that were present were emitted at higher frequencies and increased intensities (loudness) during playback. As avoidance of areas reduces the amount of quality habitat accessible and call modulation can impose metabolic costs, chronic noise may impose fitness costs on declining populations of native bats. With increasing areas of natural habitat exposed to anthropogenic noise, untangling the effects of high sound levels is crucial for conservation of bats and the ecosystem services they provide.