Echolocating bats exhibit differential amplitude compensation for noise interference at a sub-call level

Call production and wingbeat coupling is flexible and species-specific in echolocating bats

Echolocation and flight are two key behavioral innovations that contribute to the evolutionary success and diversification of bats, which are classified phylogenetically into two suborders: Yinpterochiroptera and Yangochiroptera. Considerable research has identified a coupling between call production and wingbeat in flying bats, although only a few have quantified the relationship and all were restricted to bats from the suborder Yangochiroptera. Here, we quantitatively compared the coupling between call production and wingbeat in two representative species of bats, Hipposideros pratti of the suborder Yinpterochiroptera and Myotis pilosus of the suborder Yangochiroptera, under identical experimental settings. We found that (1) both species exhibited the temporal coupling of call production and wingbeat; (2) the degree of coupling is species-specific, with M. pilosus showing a tighter coupling between call timing and wingbeat cycle than H. pratti; (3) the coupling is a plastic trait, as evidenced by the effect of environmental clutter in H. pratti; and (4) there is no evidence that the coupling of call production and wingbeat limits the source level control in either species. We suggest that the coupling between call production and wingbeat is flexible and species-specific, which may not compromise precise echolocation control in bats.

Echolocating Bats Have Evolved Decreased Susceptibility to Noise-Induced Temporary Hearing Losses

Glenis Long championed the application of quantitative psychophysical methods to understand comparative hearing abilities across species. She contributed the first psychophysical studies of absolute and masked hearing sensitivities in an auditory specialist, the echolocating horseshoe bat. Her data demonstrated that this bat has hyperacute frequency discrimination in the 83-kHz range of its echolocation broadcast. This specialization facilitates the bat's use of Doppler shift compensation to separate echoes of fluttering insects from concurrent echoes of non-moving objects. In this review, we discuss another specialization for hearing in a species of echolocating bat that contributes to perception of echoes within a complex auditory scene. Psychophysical and behavioral studies with big brown bats show that exposures to long duration, intense wideband or narrowband ultrasonic noise do not induce significant increases in their thresholds to echoes and do not impair their ability to orient through a naturalistic sonar scene containing multiple distracting echoes. Thresholds of auditory brainstem responses also remain low after intense noise exposures. These data indicate that big brown bats are not susceptible to temporary threshold shifts as measured in comparable paradigms used with other mammals, at least within the range of stimulus parameters that have been tested so far. We hypothesize that echolocating bats have evolved a decreased susceptibility to noise-induced hearing losses as a specialization for echolocation in noisy environments.

Doppler detection triggers instantaneous escape behavior in scanning bats

Animals must instantaneously escape from predators for survival, which requires quick detection of approaching threats. Although the neural mechanisms underlying the perception of looming objects have been extensively studied in the visual system, little is known about their auditory counterparts. Echolocating bats use their auditory senses to perceive not only the soundscape, but also the physical environment through active sensing. Although object movement induces both echo delay changes and Doppler shifts, the actual information required to perceive movement has been unclear. Herein, we addressed this question by playing back phantom echoes mimicking an approaching target to horseshoe bats and found that they relied only on Doppler shifts. This suggests that the bats do not perceive object motion in the spatiotemporal dimension (i.e., positional variation), as in vision, but rather take advantage of acoustic sensing by directly detecting velocity, thereby enabling them to respond instantaneously to approaching threats.

Effects of insect pursuit on the Doppler shift compensation in a hipposiderid bat

Doppler shift compensation (DSC) is a unique feature observed in certain species of echolocating bats and is hypothesized to be an adaptation to detecting fluttering insects. However, current research on DSC has primarily focused on bats that are not engaged in foraging activities. In this study, we investigated the DSC performance of Pratt's roundleaf bat, Hipposideros pratti, which was trained to pursue insects in various motion states within a laboratory setting. Our study yielded three main results. First, H. pratti demonstrated highly precise DSC during insect pursuit, aligning with previous findings of other flutter-detecting foragers during orientation or landing tasks. Second, we found that the motion state of the insect prey had little effect on the DSC performance of H. pratti. Third, we observed variations in the DSC performance of H. pratti throughout the course of insect pursuit. The bats exhibited the highest DSC performance during the phase of maximum flight speed but decreased performance during the phase of insect capture. These findings of high precision overall and the time-dependent performance of DSC during insect pursuit support the hypothesis that DSC is an adaptation to detecting fluttering insects.

Giant panda (Ailuropoda melanoleuca) neonates use broadband calls to communicate with their mothers

Infant call structure should have evolved to elicit maximum maternal attention and investment. Neonates of giant pandas produce three types of vocalizations reported to be vitally important in the context of mother-infant communications. However, how cubs, 0-15 days old, communicate with their mothers to elicit maternal care remains unknown. We analyzed 12 different call parameters of 3475 squawks, 1355 squalls, and 491 croaks from 11 captive giant panda (Ailuropoda melanoleuca) neonates from age 0 to 15 days. In playback experiments, we also tested whether mothers could detect ultrasound. Our results show that neonates use broadband calls with ultrasonic frequencies up to 65 kHz to convey information about their physiological needs and to attract maternal care. In playback experiments, we tested if mothers reacted differently to broadband calls (BBC) than to artificially altered calls that included only frequencies <20 kHz (AUDC) or calls that included only frequencies >20 kHz (USC). Playback confirmed that, although adult females responded significantly less often to USC, BBC than to or AUDC, they could detect USC, BBC and generally made appropriate behavioral responses, indicating a potential benefit for neonates to utilize ultrasonic and broadband frequencies. Our findings provide a new insight into mother-infant communication in giant pandas and will be helpful for reducing the mortality of cubs, younger than 1 month old, in captivity.

Spectrally non-overlapping background noise disturbs echolocation via acoustic masking in the CF-FM bat, Hipposideros pratti

The environment noise may disturb animal behavior and echolocation via three potential mechanisms: acoustic masking, reduced attention and noise avoidance. Compared with the mechanisms of reduced attention and noise avoidance, acoustic masking is thought to occur only when the signal and background noise overlap spectrally and temporally. In this study, we investigated the effects of spectrally non-overlapping noise on echolocation pulses and electrophysiological responses of a constant frequency-frequency modulation (CF-FM) bat, Hipposideros pratti. We found that H. pratti called at higher intensities while keeping the CFs of their echolocation pulses consistent. Electrophysiological tests indicated that the noise could decrease auditory sensitivity and sharp intensity tuning, suggesting that spectrally non-overlapping noise imparts an acoustic masking effect. Because anthropogenic noises are usually concentrated at low frequencies and are spectrally non-overlapping with the bat's echolocation pulses, our results provide further evidence of negative consequences of anthropogenic noise. On this basis, we sound a warning against noise in the foraging habitats of echolocating bats.

Perceptual hearing sensitivity during vocal production

Vocalization, such as speaking, inevitably generates sensory feedback that can cause self-generated masking. However, perceptual hearing sensitivity during vocal production is poorly understood. Using an adaptive psychophysical method, we measured the perceptual hearing sensitivity of an echolocating bat, Hipposideros pratti, in a passive listening (PL) task to detect pure tones, an active listening (AL) task to detect pure tones triggered by its vocalization, and a phantom echo task. We found that hanging H. pratti had the best hearing sensitivity of approximately 0 dB sound pressure level (SPL) in the PL task but much lower hearing sensitivity (nearly 40 dB worse) in the echo task. In the AL task, all bats gradually increased call frequency by 0.8–1.1 kHz, which improved their hearing sensitivity by 25–29 dB. This study underscores the need for studying the sensory capability of subjects engaged in active behaviors.

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Vocal production strongly affects the perceptual hearing sensitivity of bats

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Forward masking explains the reduced hearing sensitivity during vocalization

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Long-term vocal plasticity enables bats to overcome self-generated auditory masking

Doppler shift compensation performance in Hipposideros pratti across experimental paradigms

A central aim of neuroethological research is to discover the mechanisms of natural behaviors in controlled laboratory studies. This goal, however, comes with challenges, namely the selection of experimental paradigms that allow full expression of natural behaviors. Here, we explore this problem in echolocating bats that evolved Doppler shift compensation (DSC) of sonar vocalizations to yield close matching between echo frequency and hearing sensitivity. We ask if behavioral tasks influence the precision of DSC in Pratt's roundleaf bat, Hipposideros pratti, in three classic laboratory paradigms evoking audio-vocal adjustments: Stationary bats listening to echo playbacks, bats transported on a moving pendulum, and bats flying freely. We found that experimental conditions had a strong influence on the expression of the audiovocal frequency adjustments in bats. H. pratti exhibited robust DSC in both free-flying and moving-pendulum experiments but did not exhibit consistent audiovocal adjustments in echo playback experiments. H. pratti featured a maximum compensation magnitude of 87% and a compensation precision of 0.27% in the free flight experiment. Interestingly, in the moving pendulum experiment H. pratti displayed surprisingly high-precision DSC, with an 84% maximum compensation magnitude and a 0.27% compensation precision. Such DSC performance places H. pratti among the bat species exhibiting the most precise audio-vocal control of echo frequency. These data support the emerging view that Hipposiderid bats have a high-precision DSC system and highlight the importance of selecting experimental paradigms that yield the expression of robust natural behaviors.

Sensory error drives fine motor adjustment

Fine audiovocal control is a hallmark of human speech production and depends on precisely coordinated muscle activity guided by sensory feedback. Little is known about shared audiovocal mechanisms between humans and other mammals. We hypothesized that real-time audiovocal control in bat echolocation uses the same computational principles as human speech. To test the prediction of this hypothesis, we applied state feedback control (SFC) theory to the analysis of call frequency adjustments in the echolocating bat, Hipposideros armiger. This model organism exhibits well-developed audiovocal control to sense its surroundings via echolocation. Our experimental paradigm was analogous to one implemented in human subjects. We measured the bats' vocal responses to spectrally altered echolocation calls. Individual bats exhibited highly distinct patterns of vocal compensation to these altered calls. Our findings mirror typical observations of speech control in humans listening to spectrally altered speech. Using mathematical modeling, we determined that the same computational principles of SFC apply to bat echolocation and human speech, confirming the prediction of our hypothesis.

Echolocating Daubenton's bats call louder, but show no spectral jamming avoidance in response to bands of masking noise during a landing task

Echolocating bats listen for weak echoes to navigate and hunt, which makes them prone to masking from background noise and jamming from other bats and prey. Like for electrical fish that display clear spectral jamming avoidance responses (JAR), some studies have reported that bats mitigate the effects of jamming by shifting the spectral contents of their calls, thereby reducing acoustic interference to improve echo-to-noise ratios (ENR). Here we test the hypothesis that FM bats employ a spectral JAR in response to six masking noise-bands ranging from 15-90kHz, by measuring the -3dB endpoints and peak frequency of echolocation calls from five male Daubenton's bats (Myotis daubentonii) during a landing task. The bats were trained to land on a noise generating spherical transducer surrounded by a star-shaped microphone array, allowing for acoustic localization and source parameter quantification of on-axis calls. We show that the bats did not employ spectral JAR as the peak frequency during jamming remained unaltered compared to silent controls (all P>0.05, 60.73±0.96 kHz) (mean±s.e.m.), and -3dB endpoints decreased in noise irrespective of treatment-type. Instead, Daubenton's bats responded to acoustic jamming by increasing call amplitude via a Lombard response that was bandwidth dependent ranging from 0.05 [0.04-0.06 mean±95% CI] dB/dB noise for the most narrowband (15-30 kHz) to 0.17 [0.16-0.18] dB/dB noise for the most broadband noise (30-90 kHz). We conclude that Daubenton's bats, despite the vocal flexibility to do so, do not employ a spectral JAR, but defend ENRs via a bandwidth dependent Lombard response.

Constant Resting Frequency and Auditory Midbrain Neuronal Frequency Analysis of Hipposideros pratti in Background White Noise

Acoustic communication signals are inevitably challenged by ambient noise. In response to noise, many animals adjust their calls to maintain signal detectability. However, the mechanisms by which the auditory system adapts to the adjusted pulses are unclear. Our previous study revealed that the echolocating bat, Hipposideros pratti, increased its pulse intensity in the presence of background white noise. In vivo single-neuron recording demonstrated that the auditory midbrain neurons tuned to the second harmonic (H2 neurons) increased their minimal threshold (MT) to a similar degree as the increment of pulse intensity in the presence of the background noise. Furthermore, the H2 neurons exhibited consistent spike rates at their best amplitudes and sharper intensity tuning with background white noise compared with silent conditions. The previous data indicated that sound intensity analysis by auditory midbrain neurons was adapted to the increased pulse intensity in the same noise condition. This study further examined the echolocation pulse frequency and frequency analysis of auditory midbrain neurons with noise conditions. The data revealed that H. pratti did not shift the resting frequency in the presence of background noise. The auditory midbrain neuronal frequency analysis highly linked to processing the resting frequency with the presence of noise by presenting the constant best frequency (BF), frequency sensitivity, and frequency selectivity. Thus, our results suggested that auditory midbrain neuronal responses in background white noise are adapted to process echolocation pulses in the noise conditions.