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Echolocation signals of wild beluga whales (Delphinapterus leucas) were recorded in 2013 using a vertical, linear 16-hydrophone array at two locations in the pack ice of Baffin Bay, West Greenland. Individual whales were localized for 4:42 minutes of 1:04 hours of recordings. Clicks centered on the recording equipment (i.e. on-axis clicks) were isolated to calculate sonar parameters. We report the first sonar beam estimate of in situ recordings of wild belugas with an average -3 dB asymmetrical vertical beam width of 5.4°, showing a wider ventral beam. This narrow beam width is consistent with estimates from captive belugas; however, our results indicate that beluga sonar beams may not be symmetrical and may differ in wild and captive contexts. The mean apparent source level for on-axis clicks was 212 dB pp re 1 μPa and whales were shown to vertically scan the array from 120 meters distance. Our findings support the hypothesis that highly directional sonar beams and high source levels are an evolutionary adaptation for Arctic odontocetes to reduce unwanted surface echoes from sea ice (i.e., acoustic clutter) and effectively navigate through leads in the pack ice (e.g., find breathing holes). These results provide the first baseline beluga sonar metrics from free-ranging animals using a hydrophone array and are important for acoustic programs throughout the Arctic, particularly for acoustic classification between belugas and narwhals (Monodon monoceros).

Porpoise echolocation parameters may vary depending on their acoustic habitat and predominant behavior. Research was conducted in the Wadden Sea, an acoustically complex, tidally driven habitat with high particle resuspension. Source levels and echolocation parameters of wild harbor porpoises were estimated from time-of-arrival-differences of a six-element hydrophone array. The back-calculated peak-to-peak apparent source level of 169 ± 5 dB re 1 μPa was significantly lower than reported from Inner Danish Waters (−20 dB) and British Columbia (−9 dB) with narrower bandwidth. Porpoises therefore reduce their source level in the Wadden Sea under acoustically complex conditions suggesting an avoidance of cluttering.

A key component in the operation of a biosonar system is the radiation of sound energy from the sound producing head structures of toothed whales and microbats. The current view involves a fixed transmission aperture by which the beam width can only change via changes in the frequency of radiated clicks. To test that for a porpoise, echolocation clicks were recorded with high angular resolution using a 16 hydrophone array. The beam is narrower than previously reported (DI = 24 dB) and slightly dorso-ventrally compressed (horizontal -3 dB beam width: 13°, vertical -3 dB beam width: 11°). The narrow beam indicates that all smaller toothed whales investigated so far have surprisingly similar beam widths across taxa and habitats. Obtaining high directionality may thus be at least in part an evolutionary factor that led to high centroid frequencies in a group of smaller toothed whales emitting narrow band high frequency clicks. Despite the production of stereotyped narrow band high frequency clicks, changes in the directionality by a few degrees were observed, showing that porpoises can obtain changes in sound radiation.

Bats often navigate rapidly through complex environments by using echolocation, a sensory modality that is profoundly different from human vision ( 1 ). Building a sufficient three-dimensional perception of their environment on a lower-dimensional sensory input than human vision, they perform a complex task. They are thus forced to apply a high degree of processing and interpretation to the sensory input, making them prone to sensory deceptions. On page 1045 of this issue, Greif et al. ( 2 ) report that vertical mirrorlike reflecting surfaces, which bats perceive as open flyways, can act as sensory traps.

Acoustic monitoring of bats is increasingly used in biodiversity assessments, population monitoring, and environmental impact assessments. In addition to accurate species identification, additional factors make it challenging to derive population trends or better sizes based on acoustic monitoring. Inter- and intra-species- as well as individual variation of acoustic parameters and acoustic activity result in varying detection probabilities. Changes in environmental conditions result in large changes in the volume monitored by the device. Differences in the devices used for acoustic monitoring make it inherently difficult to compare data collected with different devices. The single call monitoring volume is modelled for bats belonging to different guilds under consideration of the different call parameters such as call intensity, frequency, and directionality. By broadcasting bat echolocation calls from various distances to monitoring devices, the acoustic parameters influencing the successful detection of a call were examined. A microphone array was used to track bats in the vicinity of monitoring devices and the distance between device and bat was measured for each call based on the time of arrival difference. The acoustic detection function, the probability of detecting calls as a function of distance, was then derived for multiple detector types.

We estimated the long-range effects of air gun array noise on marine mammal communication ranges in the Southern Ocean. Air gun impulses are subject to significant distortion during propagation, potentially resulting in a quasi- continuous sound. Propagation modeling to estimate the received waveform was conducted. A leaky integrator was used as a hearing model to assess communication masking in three species due to intermittent/continuous air gun sounds. Air gun noise is most probably changing from impulse to continuous noise between 1,000 and 2,000 km from the source, leading to a reduced communication range for, e.g., blue and fin whales up to 2,000 km from the source. © Springer Science+Business Media New York 2016.

We estimated the long-range effects of air gun array noise on marine mammal communication ranges in the Southern Ocean. Air gun impulses are subject to significant distortion during propagation, potentially resulting in a quasi-continuous sound. Propagation modeling to estimate the received waveform was conducted. A leaky integrator was used as a hearing model to assess communication masking in three species due to intermittent/continuous air gun sounds. Air gun noise is most probably changing from impulse to continuous noise between 1,000 and 2,000 km from the source, leading to a reduced communication range for, e.g., blue and fin whales up to 2,000 km from the source.

Historically, data on toothed whale echolocation parameters and abilities were collected from captive animals. Acoustic parameters under investigation were inter-click-interval, spectral content, source level, directionality, emission direction, including correlation and variation of those parameters. Technological advances over the past decade have allowed collecting data on those parameters from animals at sea using acoustic recording tags or hydrophone arrays. Using a vertical, linear array of 16 hydrophones, echolocation clicks from harbor porpoises, white-beaked dolphins, common dolphins, and bottlenose dolphins were recorded around Iceland and the Azores. The animal’s position at click production was computed for each click based on the time of arrival differences. Intensity and spectral differences at the array allowed measuring source levels, beam width, and spectral variation at different angles relative to on-axis. Advancing knowledge on the use and variation of echolocation signals of toothed w...

Acoustic monitoring of bats is increasingly used in biodiversity assessments and population monitoring. Besides accurate species identification, additional factors make it challenging to derive population trends, yet—sizes based on acoustic monitoring. Inter- and intra-species as well as individual variation of acoustic parameters and acoustic activity result in varying detection probability. Changes in environmental conditions result in a large changes in the volume monitored by the device. Differences in the devices used for acoustic monitoring make it inherently difficult to compare data collected with different devices. By broadcasting bat echolocation calls from various distances to monitoring devices, the acoustic parameters influencing the successful detection of a call were examined. A microphone array was used to track bats in the vicinity of monitoring devices and the distance between device and bat was measured for each call based on the time of arrival difference. The acoustic detection functi...

First studies of the echolocation behaviour of free-ranging white-beaked dolphins (Lagenorhyncus albirostris) were conducted in Faxafloi Bay in the Southwestern part of Iceland in the years 1997 to 1999. However, the sighting rate of white-beaked dolphins has decreased in that area since then and the current studies were conducted in the Northeastern part of Iceland. The aim of this study was to investigate the difference between normal clicks compared to clicks from buzz sequences. The recordings of the Icelandic white-beaked dolphins were conducted using a vertical linear 16-hydrophone array in Skjalfandi Bay, Northeastern Iceland during August 2015 and June 2016. The hydrophones were connected to NI-Boards and to a laptop computer on board using a sample rate of 1 MHz per channel. The group size of the dolphins varied from three individuals up to 30 animals in the area during the recordings. The dolphin echolocation clicks were recorded and it was possible to track individuals and to estimate beam-patt...

Reduction of echolocation call source levels in bats has previously been studied using set-ups with one microphone. By using a 16 microphone array, sound pressure level (SPL) variations, possibly caused by the scanning movements of the bat, can be excluded and the sonar beam aiming can be studied. During the last two meters of approach flights to a landing platform in a large flight room, five big brown bats aimed sonar beams at the landing site and reduced the source level on average by 7 dB per halving of distance. Considerable variation was found among the five individuals in the amount of source level reduction ranging from 4 to 9 dB per halving of distance. These results are discussed with respect to automatic gain control and intensity compensation and the combination of the two effects. It is argued that the two effects together do not lead to a stable echo level at the cochlea. This excludes a tightly coupled closed loop feed back control system as an explanation for the observed reduction of signal SPL in landing big brown bats.

SUMMARYRecordings of the echolocation signals of landing big brown bats with a two-dimensional 16-microphone array revealed that the source level reduction of 7 dB per halving of distance is superimposed by a variation of up to 12 dB within single call groups emitted during the approach. This variation correlates with the wingbeat cycle. The timing of call emission correlates with call group size. First pulses of groups containing many calls are emitted earlier than first calls in groups with fewer calls or single calls. This suggests that the emission of pulse groups follows a fixed motor pattern where the information gained from the preceding pulse group determines how many calls will be emitted in the next group. Single calls and call groups are centred at the middle of the upstroke. Expiration is indicated by call emission. The pause between groups is centred at the middle of the downstroke and indicates inspiration. The hypothesis that the source level variation could be caused by changes in the subglottic pressure due to the contraction of the major flight muscles is discussed.

Echo-locating bats constantly emit ultrasonic pulses and analyze the returning echoes to detect, localize, and classify objects in their surroundings. Echo classification is essential for bats’ everyday life; for instance, it enables bats to use acoustical landmarks for navigation and to recognize food sources from other objects. Most of the research of echo based object classification in echo-locating bats was done in the context of simple artificial objects. These objects might represent prey, flower, or fruit and are characterized by simple echoes with a single up to several reflectors. Bats, however, must also be able to use echoes that return from complex structures such as plants or other types of background. Such echoes are characterized by superpositions of many reflections that can only be described using a stochastic statistical approach. Scientists have only lately started to address the issue of complex echo classification by echo-locating bats. Some behavioral evidence showing that bats can classify complex echoes has been accumulated and several hypotheses have been suggested as to how they do so. Here, we present a first review of this data. We raise some hypotheses regarding possible interpretations of the data and point out necessary future directions that should be pursued.

A critical step on the way to understanding a sensory system is the analysis of the input it receives. In this work we examine the statistics of natural complex echoes, focusing on vegetation echoes. Vegetation echoes constitute a major part of the sensory world of more than 800 species of echolocating bats and play an important role in several of their daily tasks. Our statistical analysis is based on a large collection of plant echoes acquired by a biomimetic sonar system. We explore the relation between the physical world (the structure of the plant) and the characteristics of its echo. Finally, we complete the story by analyzing the effect of the sensory processing of both the echolocation and the auditory systems on the echoes and interpret them in the light of information maximization. The echoes of all different plant species we examined share a surprisingly robust pattern that was also reproduced by a simple Poisson model of the spatial reflector arrangement. The fine differences observed between the echoes of different plant species can be explained by the spatial characteristics of the plants. The bat's emitted signal enhances the most informative spatial frequency range where the species-specific information is large. The auditory system filtering affects the echoes in a similar way, thus enhancing the most informative spatial frequency range even more. These findings suggest how the bat's sensory system could have evolved to deal with complex natural echoes., Author Summary More than 800 species of bats perceive their surroundings through echolocation. They emit ultrasonic pulses and analyze the information conveyed in the echoes returning from objects in their surroundings. This enables bats to orient in space, to acquire food and to perfectly function in complete darkness. In the absence of light, echoes constitute a major part of the sensory world of bats. Understanding their characteristics can thus help to shed light on the echolocation sensory system. The goal of this work is to study the characteristics of natural plant echoes. Plant echoes are very abundant in the world of the bats and are used by them to find food sources and to navigate. We show that some of the features of the echoes can be explained from the physical properties of the plant they were sampled from (e.g., its leaf size and density). We then analyze the effects of the sensory system on the echoes and suggest that they improve the representation of the echoes in a way that enhances the information that is most reliable for the bats.

Classification of plants according to their echoes is an elementary component of bat behavior that plays an important role in spatial orientation and food acquisition. Vegetation echoes are, however, highly complex stochastic signals: from an acoustical point of view, a plant can be thought of as a three-dimensional array of leaves reflecting the emitted bat call. The received echo is therefore a superposition of many reflections. In this work we suggest that the classification of these echoes might not be such a troublesome routine for bats as formerly thought. We present a rather simple approach to classifying signals from a large database of plant echoes that were created by ensonifying plants with a frequency-modulated bat-like ultrasonic pulse. Our algorithm uses the spectrogram of a single echo from which it only uses features that are undoubtedly accessible to bats. We used a standard machine learning algorithm (SVM) to automatically extract suitable linear combinations of time and frequency cues from the spectrograms such that classification with high accuracy is enabled. This demonstrates that ultrasonic echoes are highly informative about the species membership of an ensonified plant, and that this information can be extracted with rather simple, biologically plausible analysis. Thus, our findings provide a new explanatory basis for the poorly understood observed abilities of bats in classifying vegetation and other complex objects., Author Summary Bats are able to classify plants using echolocation. They emit ultrasonic signals and can recognize the plant according to the echo returning from it. This ability assists them in many of their daily activities, like finding food sources associated with certain plants or using landmarks for navigation or homing. The echoes created by plants are highly complex signals, combining together all the reflections from the many leaves that a plant contains. Classifying plants or other complex objects is therefore considered a troublesome task and we are far from understanding how bats do it. In this work, we suggest a simple algorithm for classifying plants according to their echoes. Our algorithm is able to classify with high accuracy plant echoes created by a sonar head that simulates a typical frequency-modulated bat's emitting receiving parameters. Our results suggest that plant classification might be easier than formerly considered. It gives us some hints as to which features might be most suitable for the bats, and it opens possibilities for future behavioral experiments to compare its performance with that of the bats.

SUMMARY We have demonstrated in behavioural experiments that success in capturing prey from surfaces in ‘trawling Myotis’ (Leuconoë-type) depends on the acoustic properties of the surface on which the prey is presented. Two types of surface structure were ensonified with artificial bat signals to probe their acoustic characteristics. We have shown that perception of prey by echolocation is easier if the prey is presented on a smooth surface (such as calm water) than if it is presented on a structured surface (such as vegetation or the ground). This is because the smooth surface reflects a much lower level of clutter echoes than the structured one if ensonified at an angle typical for bats foraging low over water. The ensonification experiments revealed that the sound pressure level of the echo was even higher for mealworms on a smooth surface than for mealworms suspended in air. This might be because waves travelling via the surface also contribute to the echo (e.g. reflection from the surface to the mealworm, back to the surface and then to the receiver). From the behavioural experiments, we conclude that ‘trawling Myotis’ take isolated objects on smooth (water) surfaces for prey. Those objects reflect isolated, stationary acoustic glints back to the echolocating bats. Conversely, ‘trawling Myotis’ will not recognise prey if prey echoes are embedded in numerous clutter echoes. We have demonstrated marked similarities between the three European ‘trawling Myotis’ species M. dasycneme, M. daubentonii and M. capaccinii in echolocation behaviour, search image, foraging strategy and prey perception. We propose that a combination of prey abundance and acoustic advantages could have led to repeated and convergent evolution of ‘trawling’ bats in different parts of the world.

Some echolocation signal parameters can be studied using a single receiver. However, studying parameters such as source level, directionality, and direction of signal emission require the use of multi-receiver arrays. Acoustic localization allows for determination of the position of echolocators at the time of signal emission, and when multiple animals are present, calls can be assigned to individuals based on their location. This combination makes large multi-receiver arrays a powerful tool. Here we present an overview of different array configurations used to study both toothed whales and bats, using a suite of systems ranging from semi-3D-minimum receiver number-number-arrays (3D-MINNAs), linear-2-D-over determined arrays (2D-ODAs), to 3-D-over-determined-arrays (3D-ODAs). We discuss approaches to process and summarize the usually large amounts of data. In some studies, the absolute position of an echolocator and not only relative to the array is crucial. Combining acoustic localizations from a source ...

Click loggers such as C-PODs are an important tool to monitor the spatial distribution and seasonal occurrence of small odontocetes. To determine absolute density, information on the detection function, the detection probability as a function of distance, and derived from this, the effective detection radius (EDR), is needed. In this study a 15 channel hydrophone array, deployed next to 12 C-PODs, was used to localize porpoises and determine their geo-referenced swim paths using the ship’s GPS and motion sensors. The detection function of C-PODs was then computed using the distance between the animals and each C-POD. In addition to this, the acoustic detection function of C-PODs has been measured by playing back porpoise-like clicks using an omni-directional transducer. The EDR for these porpoise-like clicks with a source level of 168 dB re 1 μPa pp varied from 41 to 243 m. This variation seemed to be related to the sensitivity of the devices; however, season and water depth also seemed to have an influence on detectability.

To explore the statistics of complex natural plant echoes, we emitted bat‐like downsweeps (200‐0 kHz) and recorded the echoes of various tree species. A Hilbert transform was used to calculate the envelope of the echoes impulse responses. This corresponds to a one‐dimensional representation of the spatial reflector arrangement of the plant. We then calculated the envelope's power spectrum to asses the amount of periodic structures. In control experiments we compared power spectra of a single leaf, a branch and a few branches, and tested the effect of systematically decreasing the leaf density of a plant. On a bi‐logarithmic plot, the averaged power spectra of all trees have a sigmoid shape with three approximately linear domains that represent different scales of structure, but differ between species. We hypothesize that the first domain is influenced by the gross skeleton of branches, while the others are associated with smaller scale structures. The control experiments showed a similar dependence betwee...

For the first time, the beam structure of single harbour porpoise echolocation clicks was simultaneously measured in both the vertical and horizontal plane. Echolocation clicks of a stationed animal and free‐swimming animals catching fish were recorded with a “plus‐shaped” array consisting of 16 hydrophones. The ‐3 dB beam width in the horizontal and vertical planes is in most cases similar to what has been previously measured by Au et al. (1999). However, individual clicks are sometimes broader or narrower than what has been previously reported. This may be caused by either voluntary control of the animal's beam width, or it may be a secondary effect of variations in intensity and air content within the sound production system.

We reconstruct the sonar beams emitted subsequently by flying bats from recordings of a two‐dimensional microphone array. Based on the bats’ position in space, which can be accessed through a infrared 3D‐video system, and the simultaneous recordings of 16 small microphones, the sonar beams emitted by the bats and the sonar footprints on arbitrary planes can be estimated. For the interpolation of a continuous sonar beam at different frequency bands, directional frequency responses, atmospheric and geometric attenuation, and different lag times are taken into account for each microphone. Mean and maximum call intensities can be analyzed and various analysis parameters can be adjusted, different visualizations are available, and numerical data are exported. The system provides access to estimations of the maximum intensity of each call in front of the bat, beam width, shape, and intensity for arbitrary frequency bands, the aiming of the beam, and the bat’s scanning behavior on its flight path.

The parameters signal duration, pulse interval, and frequency have previously been used to describe the approach behavior of landing bats. An array with 16 microphones was used to investigate additional parameters during the approach flight of landing big brown bats (Eptesicus fuscus). For each signal emitted the position of the landing bat in relation to the microphone array was determined using a 3‐D video system, and the width, maximum intensity, and aiming direction of the sonar beam were calculated. The approach to the landing site begins at a distance of about 1.4 m when the bats pick up the landing site with their sonar beam. This focusing on the landing site is indicated by a clear decrease of the deviation between beam aiming and direction to the landing position. The maximum intensity of the sonar beam is reduced by about 6 dB per half distance during the last 2 m of the approach, which indicates a gain control mechanism. The width of the sonar beam remains constant throughout the whole approach.