Ambient noise induces independent shifts in call frequency and amplitude within the Lombard effect in echolocating bats

Abstract

The Lombard effect, an involuntary rise in call amplitude in response to masking ambient noise, represents one of the most efficient mechanisms to optimize signal-to-noise ratio. The Lombard effect occurs in birds and mammals, including humans, and is often associated with several other vocal changes, such as call frequency and duration. Most studies, however, have focused on noisedependent changes in call amplitude. It is therefore still largely unknown how the adaptive changes in call amplitude relate to associated vocal changes such as frequency shifts, how the underlying mechanisms are linked, and if auditory feedback from the changing vocal output is needed. Here, we examined the Lombard effect and the associated changes in call frequency in a highly vocal mammal, echolocating horseshoe bats. We analyzed how bandpassfiltered noise (BFN; bandwidth 20 kHz) affected their echolocation behavior when BFN was centered on different frequencies within their hearing range. Call amplitudes increased only when BFN was centered on the dominant frequency component of the bats' calls. In contrast, call frequencies increased for all but one BFN center frequency tested. Both amplitude and frequency rises were extremely fast and occurred in the first call uttered after noise onset, suggesting that no auditory feedback was required. The different effects that varying the BFN center frequency had on amplitude and frequency rises indicate different neural circuits and/or mechanisms underlying these changes.A ny transmission of signals between sender and receiver faces the challenge of being subjected to masking by noise. For acoustic signals, for example, animals have evolved several strategies that aid in increasing the signal-to-noise ratio, thus facilitating signal transmission. One of the most efficient mechanisms is the socalled Lombard effect, i.e., the involuntary rise in call amplitude in response to masking ambient noise (1). This effect was first described in human communication a century ago (2) and has since been found in several species of birds (3-11) and various mammals (12-17), including bats (18). In human speech, several vocal changes, such as a rise in fundamental frequency (19) or lengthening in word duration (20), are often accompanied with the Lombard effect; combined, these changes are referred to as Lombard speech (21). Vocal changes associated with the Lombard effect have rarely been analyzed in animal species. So far, noisedependent changes in call frequency have been observed in birds (8,11,22), and changes in call duration in birds and monkeys (8,12).Most animal studies, however, have focused on noise-dependent changes in call amplitude and do not examine other possible vocal changes (2-5, 7, 9, 10, 15-17). It is therefore still largely unknown how the adaptive changes in call amplitude relate to frequency shifts or call elongation and how the underlying mechanisms are linked. It is also unknown if auditory feedback from the changing vocal output is needed to drive the Lombard effect in general.Over...

Sensorimotor integration on a rapid time scale

“…Thus, the Lombard effect is fundamentally different from the Fletcher effect and does not require a comparison of internal forward model predictions with the sensory feedback. Our results are in line with the data from an earlier study on another bat species (Rhinolophus ferrumequinum) that exhibited the Lombard effect in the first call after noise onset (26). However, the term reflex should not be interpreted as that the Lombard effect is a fixed response to background noise.…”

“…The Lombard effect was first described over a century ago and is among the most widely studied audio-vocal integration phenomena. The response latency of the Lombard effect was estimated to be about 150-175 ms for humans (23,24), 150 ms for birds (25), and less than 150 ms for bats (26). There are at least two possible explanations for the overestimation.…”

Changes in bottlenose dolphin whistle parameters related to vessel presence, surface behaviour and group composition

“…In species that display vocal plasticity, such as birds, bats and cetaceans, shifts in call frequency can help achieve this and geographical variation in these call parameters has been linked to natural variation in ambient noise levels from biotic and abiotic factors (Brumm & Slabbekoorn, 2005;Foote & Nystuen, 2008;Hage, Jiang, Berquist, Feng, & Metzner, 2013;Mossbridge, Shedd, & Thomas, 1999). Elevated anthropogenic noise levels may also promote these frequency shifts (Ansmann et al, 2007;Lesage, Barrette, Kingsley, & Sjare, 1999;Luís, most noise in the low-to mid-frequency ranges (Jensen et al, 2009;Lemon et al, 2006;Lesage et al, 1999) and several studies focused on cetacean vocal parameters have identified an upward frequency shift or increase in minimum or mean frequency associated with boat presence (Lesage et al, 1999;Luís et al, 2014;Parks et al, 2007).…”

Environmental correlates of geographic divergence in a phenotypic trait: A case study using bat echolocation

“…One potential limitation to plasticity is the tight coupling between RF and the acoustic fovea in high duty cycle bats, like R. damarensis , that use Doppler shift compensation (Neuweiler, 1984; Schnitzler & Denzinger, 2011). However, HDC bats are able to shift their RFs (up to 3.9 kHz) in response to both neighboring conspecifics and different ambient noise conditions (Hage, Jiang, Berquist, Feng, & Metzner, 2013; Hiryu et al., 2006). Although small, such shifts in frequency encompass the differences between the mean RFs of neighboring populations of R. damarensis and may allow bats to optimize their detection range under the local climatic conditions without a change in allele frequency.…”