Bats and ambient noise : From chatty neighbours to disturbing humans

Abstract

Living in the dark, many species of bats rely strongly on sounds for crucial activities such as communication, spatial navigation, or foraging. On the other hand, acoustic signals are particularly venerable to ambient noise. For bats, typical sources of ambient noise include insects, rustling leaves, and chatty neighbours. However, this original acoustic world of bats is under continual and intensified modification by a novel type of noise, namely anthropogenic (human-made) noise. So far, only a few studies have investigated the effects of anthropogenic noise on bats. Hence, it is largely unknown how anthropogenic noise affects bats, and how bats in turn may cope with anthropogenic noise. In this thesis, I aimed to answer these two fundamental questions experimentally.To answer the first question about how anthropogenic noise may affect bats, I conducted two experiments examining the foraging behaviour of the Daubenton’s bat (Myotis daubentonii) and the roosting behaviour of the Greater mouse-eared bat (Myotis myotis). Results on foraging Daubenton’s bats revealed that traffic noise decreased the foraging efficiency in three out of four individuals. Unexpectedly, this effect was present even if the playback noise did not overlap in frequency with the prey echoes. Neither overlapping nor non-overlapping noise influenced the search effort required for a successful prey capture. Hence, noise did not mask prey echoes or reduce the attention of bats. Instead, noise acted as an aversive stimulus that caused avoidance response, thereby reducing foraging efficiency. Results on roosting Greater mouse-eared bats revealed that torpid bats were less sensitive to traffic noise than to vegetation or colony noise. Moreover, I found evidence that torpid bats rapidly habituated to repeated and prolonged noise exposure. These two experiments showed that low-frequency anthropogenic noise can affect high-frequency echolocating bats via a mechanism other than acoustic masking, and behavioural context (e.g. foraging vs. roosting) needs to be considered when assessing the anthropogenic noise effects on bats.To answer the second question about how bats may cope with anthropogenic noise or ambient noise in general, I conducted two experiments examining the vocal behaviour of the Pale spear-nosed bat (Phyllostomus discolor). Through these two experiments, I show that bats are capable of adjusting a set of sound parameters to reduce noise interference. These include increase of signal amplitude (the Lombard effect), increase of signal duration, and decrease of signal frequencies in broadband noise. Particularly, I was able to show that the Lombard effect was more effective, by contributing about 80% to the overall signal detectability, than adjusting signal duration or signal redundancy (signal repetition). Furthermore, I showed that decreases in signal frequencies were mainly achieved by bats adjusting the vocal tract, instead of the laryngeal source signals.In an additional experiment, I showed that the Lombard effect arose in infant bats at an age of a few days. Taken together, these data have implications for understanding animals’ signalling strategies in noisy environments.In summary, this thesis provides experimental evidence on how anthropogenic noise affects echolocating bats and in turn how bats maintain signal transmission in noisy environments by adjusting the vocal production. Thus, data of this thesis have implications for wildlife conservationists, behavioural ecologists, and physiologists.