Echolocation Beam Research Articles

Stealth echolocation in aerial hawking bats reflects a substrate gleaning ancestry

Predator-prey co-evolution can escalate into an evolutionary arms race.1 Examples of insect countermeasures to bat echolocation are well-known,2 but presumptive direct counter strategies in bats to insect anti-bat tactics are rare. The emission of very low-intensity calls by the hawking Barbastella barbastellus to circumvent high-frequency moth hearing is the most convincing countermeasure known.2,3 However, we demonstrate that stealth echolocation did not evolve through a high-intensity aerial hawking ancestor becoming quiet as previously hypothesized2,3,4 but from a gleaning ancestor transitioning into an obligate aerial hawker. Our ancestral state reconstructions show that the Plecotini ancestor likely gleaned prey using low-intensity calls typical of gleaning bats and that this ability-and associated traits-was subsequently lost in the barbastelle lineage. Barbastelles did not, however, revert to the oral, high-intensity call emission that other hawking bats use but retained the low-intensity nasal emission of closely related gleaning plecotines despite an extremely limited echolocation range. We further show that barbastelles continue to emit low-intensity calls even under adverse noise conditions and do not broaden the echolocation beam during the terminal buzz, unlike other vespertilionids attacking airborne prey.5,6 Together, our results suggest that barbastelles' echolocation is subject to morphological constraints prohibiting higher call amplitudes and beam broadening in the terminal buzz. We suggest that an abundance of eared prey allowed the co-opting and maintenance of low-intensity, nasal echolocation in today's obligate hawking barbastelle and that this unique foraging behavior7 persists because barbastelles remain a rare, acoustically inconspicuous predator to eared moths. VIDEO ABSTRACT.

Does rotation increase the acoustic field of view? Comparative models based on CT data of a live dolphin versus a dead dolphin

Rotational behaviour has been observed when dolphins track or detect targets, however, its role in echolocation is unknown. We used computed tomography data of one live and one recently deceased bottlenose dolphin, together with measurements of the acoustic properties of head tissues, to perform acoustic property reconstruction. The anatomical configuration and acoustic properties of the main forehead structures between the live and deceased dolphins were compared. Finite element analysis (FEA) was applied to simulate the generation and propagation of echolocation clicks, to compute their waveforms and spectra in both near- and far-fields, and to derive echolocation beam patterns. Modelling results from both the live and deceased dolphins were in good agreement with click recordings from other, live, echolocating individuals. FEA was also used to estimate the acoustic scene experienced by a dolphin rotating 180° about its longitudinal axis to detect fish in the far-field at elevation angles of −20° to 20°. The results suggest that the rotational behaviour provides a wider insonification area and a wider receiving area. Thus, it may provide compensation for the dolphin’s relatively narrow biosonar beam, asymmetries in sound reception, and constraints on the pointing direction that are limited by head movement. The results also have implications for examining the accuracy of FEA in acoustic simulations using recently deceased specimens.

Dynamic control of the acoustic scene in echolocating bats

Echolocating bats can detect, localize, classify, pursue, and capture night-flying insects in less than a second. During pursuit, aerial hawking bats dynamically change their emitted calls as they approach prey, increasing call rate, reducing call duration, increasing bandwidth, and reducing source level. Additionally, many bat species dynamically control the shape of the emitted echolocation beam by broadening the beam in close proximity to prey. However, the acoustic scene, as experienced by the bats, depends as much on the characteristics of the emitted call as on the shape and orientation of the outer ears. We show that aerial-hawking bats also dynamically modify the shape and orientation of the outer ear in synchrony with changes to the emitted beam during prey pursuit. We believe that it is the combination of all these dynamic changes, both to the emitted call and to the receiving morphology, that underlie the effectiveness with which bats navigate and forage in the night sky.

WAU! Identifying complex objects with an acoustic flashlight?

A Hawaiian bunker, a bearded and barefoot entrepreneur, some dolphins, and an electrical engineer: A starter recipe for one of the most productive dolphin biosonar researchers in world history? Sound unlikely? Overstated? Apocryphal? And yet… Thirty years with Whit inspired us to investigate a dolphin’s use of wholistic shape matching versus “flashlight” object-parts-focused matching. Our stimuli were 3 3-object sets (Sets W, A,U). Objects within sets varied on shape alone made with identical PVC parts, but PVC parts varied between sets. In Condition Shape, experienced dolphin Calvin matched objects based on shape within sets (matching W1Snail to W1Snail) in a 3-alternative echoic matching-to-sample task: mean accuracy across 12 18-trial sessions/set: Set W: 37%, Set A: 44%, Set U: 39%, chance = 33%. In Condition Parts/NoShape, Calvin performed untrained transfer tests with same-parts/different-shape objects as sample-match pairs within alternative arrays of objects from different sets with different parts (sample W1Snail matched to W2StingRay with alternatives A2Trumpet and U2Whale). Mean accuracy in ten transfer sessions/set (WAU1 to WAU2: 47%, WAU2-WAU3: 48%, WAU3-WAU1: 39%) was above-chance and comparable to Condition Shape even though shape information was absent for matching in this parts-focused condition. Calvin’s narrow echolocation beam may have emphasized object parts versus wholistic shape.

Does rotation during echolocation increase the acoustic field of view? Comparative numerical models based on CT data of a live versus deceased dolphin

Spinning is a natural and common dolphin behavior; however, its role in echolocation is unknown. We used computed tomography (CT) data of a live and a recently deceased bottlenose dolphin together with measurements of the acoustic properties of head tissues to perform acoustic property reconstrcution. The anatomical configuration and acoustic properties of the main forehead structures between the live and deceased dolphins were compared. Finite element analysis (FEA) was applied to simulate the generation and propagation of echolocation clicks, to compute their waveforms and spectra in both near- and far-fields, and to derive echolocation beam patterns. Model results from both the live and deceased dolphins were in good agreement with click recordings from live, echolocating individuals. FEA was also used to estimate the acoustic scene experienced by a dolphin rotating 180ã‚œabout its longitudinal axis to detect fish in the far-field at elevation angles of 0ã‚œ–20ã‚œ. The results suggest that the spinning behavior provides a wider insonification area and compensates for the dolphin’s relatively narrow biosonar beam and constraints on the pointing direction that are limited by head movement. The results also have implications for examining the accuracy of FEA in acoustic simulations using freshly deceased specimens.

Bio-acoustic tracking and localization using heterogeneous, scalable microphone arrays

Microphone arrays are an essential tool in the field of bioacoustics as they provide a non-intrusive way to study animal vocalizations and monitor their movement and behavior. Microphone arrays can be used for passive localization and tracking of sound sources while analyzing beamforming or spatial filtering of the emitted sound. Studying free roaming animals usually requires setting up equipment over large areas and attaching a tracking device to the animal which may alter their behavior. However, monitoring vocalizing animals through arrays of microphones, spatially distributed over their habitat has the advantage that unrestricted/unmanipulated animals can be observed. Important insights have been achieved through the use of microphone arrays, such as the convergent acoustic field of view in echolocating bats or context-dependent functions of avian duets. Here we show the development and application of large flexible microphone arrays that can be used to localize and track any vocalizing animal and study their bio-acoustic behavior. In a first experiment with hunting pallid bats the acoustic data acquired from a dense array with 64 microphones revealed details of the bats’ echolocation beam in previously unseen resolution. We also demonstrate the flexibility of the proposed microphone array system in a second experiment, where we used a different array architecture allowing to simultaneously localize several species of vocalizing songbirds in a radius of 75 m. Our technology makes it possible to do longer measurement campaigns over larger areas studying changing habitats and providing new insights for habitat conservation. The flexible nature of the technology also makes it possible to create dense microphone arrays that can enhance our understanding in various fields of bioacoustics and can help to tackle the analytics of complex behaviors of vocalizing animals.

High resolution three-dimensional beam radiation pattern of harbour porpoise clicks with implications for passive acoustic monitoring

The source properties and radiation patterns of animal vocalisations define, along with propagation and noise conditions, the active space in which these vocalisations can be detected by conspecifics, predators, prey, and by passive acoustic monitoring (PAM). This study reports the 4π (360° horizontal and vertical) beam profile of a free-swimming, trained harbour porpoise measured using a 27-element hydrophone array. The forward echolocation beam is highly directional, as predicted by a piston model, and is consistent with previous measurements. However, at off-axis angles greater than ±30°, the beam attenuates more rapidly than the piston model and no side lobes are present. A diffuse back beam is also present with levels about -30 dB relative to the source level. In PAM, up to 50% of detections can be from portions of the beam profile with distorted click spectra, although this drops substantially for higher detection thresholds. Simulations of the probability of acoustically detecting a harbour porpoise show that a traditional piston model can underestimate the probability of detection compared to the actual three-dimensional radiation pattern documented here. This highlights the importance of empirical 4π measurements of beam profiles of toothed whales, both to improve understanding of toothed whale biology and to inform PAM.

The sonar beam of Macrophyllum macrophyllum implies ecological adaptation under phylogenetic constraint.

All animals are adapted to their ecology within the bounds of their evolutionary heritage. Echolocating bats clearly show such adaptations and boundaries through their biosonar call design. Adaptations include not only the overall time-frequency structure, but also the shape of the emitted echolocation beam. Macrophyllum macrophyllum is unique within the phyllostomid family, being the only species to predominantly hunt for insects in the open, on or above water, and as such it presents an interesting case for comparing the impact of phylogeny and ecology as it originates from a family of low-intensity, high-directionality gleaning bats, but occupies a niche dominated by very loud and substantially less-directional bats. Here, we examined the sonar beam pattern of M. macrophyllum in the field and in a flight room and compared it to closely related species with very different feeding ecology and to that of the niche-sharing but distantly related Myotis daubentonii Our results show that M. macrophyllum uses higher source levels and emits less-directional calls than other phyllostomids. In the field, its call directionality is comparable to M. daubentonii, but in the flight room, M. macrophyllum is substantially more directional. Hence our results indicate that ecology influences the emitted call, pushing the bats to emit a louder and broader beam than other phyllostomids, but that phylogeny does limit the emitted intensity and flexibility of the overall beam pattern.

Sensory gaze stabilization in echolocating bats.

Sensing from a moving platform is challenging for both man-made machines and animals. Animals' heads jitter during movement, so if the sensors they carry are not stabilized, any spatial estimation might be biased. Flying animals, like bats, seriously suffer from this problem because flapping flight induces rapid changes in acceleration which moves the body up and down. For echolocating bats, the problem is crucial. Because they emit a sound to sense the world, an unstable head means sound energy pointed in the wrong direction. It is unknown how bats mitigate this problem. By tracking the head and body of flying fruit bats, we show that they stabilize their heads, accurately maintaining a fixed acoustic-gaze relative to a target. Bats can solve the stabilization task even in complete darkness using only echo-based information. Moreover, the bats point their echolocation beam below the target and not towards it, a strategy that should result in better estimations of target elevation.

Detailed analysis of two detected overlaying transient components within the echolocation beam of a bottlenose dolphin (Tursiops truncatus).

Dolphin echolocation clicks measured far off-axis contain two time-separated components. Whether these components overlap and appear as a single signal on axis has received little attention. Here, the scaled reassigned spectrogram analysis was used to examine if bottlenose dolphin (Tursiops truncatus) clicks measured near- or on-axis of the echolocation beam contained overlapping components. Across click trains, the number of overlapping components spatially varied within the echolocation beam. Two overlapping components were found to predominantly occur in the upper portion of the beam, whereas the lower portion of the beam predominantly contained a single component. When components overlapped, the trailing component generally had a higher center frequency and arrived less than 5 μs after the leading component. The spatial relationship of components was consistent with previous findings of two vertically distinct beam lobes with separated frequency content. The two components in the upper portion of the beam possibly result from a single transient click propagating through a geometrically dispersive media; specifically, the slower sound speed of the dolphin melon's core slightly delays the more directional, high frequency energy of the click, whereas the less directional, lower frequency energy propagates through more peripheral but higher sound speed portions of the melon.

Sound Generating Structures of the Humpback Dolphin Sousa plumbea (Cuvier, 1829) and the Directionality in Dolphin Sounds.

The macroscopic morphology of structures involved in sound generation in the Indian Ocean humpback dolphin (Sousa plumbea) were described for the first time using computed tomography imaging and standard gross dissection techniques. The Indian Ocean humpback dolphin may represent a useful comparative model to the bottlenose dolphin (Tursiops sp.) to provide insights into the functional anatomy of the sound production in dolphins, since these coastal dolphins exhibit similar body size and share similarities on acoustic behavior. The general arrangement of sound generating structures, that is, air sacs and muscles, was similar in both the bottlenose dolphin and the Indian Ocean humpback dolphin. The main difference between the two species existed in a small left posterior branch of the melon in the Indian Ocean humpback dolphin, which was not found in the bottlenose dolphin and might reflect an adaptation of directionality for high frequency communication sounds as seen in some other delphinids (e.g., Lagenorhynchus sp., Grampus griseus). Thus, this may be the main reason for the asymmetry of the sound production structures in dolphins. Additionally, the longer rostrum in Indian Ocean humpback dolphins might suggest a more directional echolocation beam compared to the Lahille's bottlenose dolphin. Anat Rec, 302:849-860, 2019. © 2018 Wiley Periodicals, Inc.

Arrayvolution: using microphone arrays to study bats in the field

Some parameters of echolocation signals can be studied using a single receiver. However, studying parameters such as source level, echolocation beam shape, and direction of signal emission require the use of multireceiver arrays. Acoustic localization allows for determination of the position of bats at the time of signal emission. When multiple animals are present, calls can be assigned to individuals based on their location. This combination makes large multireceiver arrays a powerful tool in bioacoustics research. Here, an overview of different array configurations used to record bats in the field is presented. In some studies, the absolute position of bats and not only relative to the array is crucial. Combining acoustic localizations from a source with geo-referenced receivers allows for determining geo-referenced movements of bats. Current applications of arrays aim to improve acoustic monitoring of bats and study anthropogenic impact.

Objective detection and time-frequency localization of components within transient signals.

An automatic component detection method for overlapping transient pulses in multi-component signals is presented and evaluated. The recently proposed scaled reassignment technique is shown to have the best achievable resolution for closely located Gaussian shaped transient pulses, even in heavy disruptive noise. As a result, the method automatically detects and counts the number of transients, giving the center times and center frequencies of all components with considerable accuracy. The presented method shows great potential for applications in several acoustic research fields, where coinciding Gaussian shaped transients are analyzed. The performance is tested on measured data from a laboratory pulse-echo setup and from a dolphin echolocation signal measured simultaneously at two different locations in the echolocation beam. Since the method requires little user input, it should be easily employed in a variety of research projects.

Spear-nosed bats train echolocation beam

![Figure][1] A pale spear-nosed bat ( Phyllostomus discolour ). Photo credit: Kathrin Kugler. Visual information streaming through our peripheral vision is essential for keeping us on the straight and narrow. ‘When you move past an object, you expect the object to move sideways and if it

Directionality of nose-emitted echolocation calls from bats without a nose leaf (Plecotus auritus).

All echolocating bats and whales measured to date emit a directional bio-sonar beam that affords them a number of advantages over an omni-directional beam, i.e. reduced clutter, increased source level and inherent directional information. In this study, we investigated the importance of directional sound emission for navigation through echolocation by measuring the sonar beam of brown long-eared bats, Plecotus auritusPlecotus auritus emits sound through the nostrils but has no external appendages to readily facilitate a directional sound emission as found in most nose emitters. The study shows that P. auritus, despite lacking an external focusing apparatus, emits a directional echolocation beam (directivity index=13 dB) and that the beam is more directional vertically (-6 dB angle at 22deg) than horizontally (-6 dB angle at 35deg). Using a simple numerical model, we found that the recorded emission pattern is achievable if P. auritus emits sound through the nostrils as well as the mouth. The study thus supports the hypothesis that a directional echolocation beam is important for perception through echolocation and we propose that animals with similarly non-directional emitter characteristics may facilitate a directional sound emission by emitting sound through both the nostrils and the mouth.

Beam patterns of the dolphin demonstrate task-dependent dynamics

The echolocation beam of the bottlenose dolphin was first carefully described by Au and colleagues (1978)[“Propagation of Atlantic bottlenose dolphin echolocation signals,” J. Acoust. Soc. Am. 64, 411-422] using various hydrophone array configurations and targets located in front of the dolphin and along its longitudinal axis. Measured beams were described as vertically elevated with mean vertical and horizontal 3-dB beamwidths of ~10°. The experimental paradigm was later augmented with denser hydrophone arrays, greater spatial coverage of the acoustic field, and the training of target detection with targets presented to the left or right of the dolphin’s longitudinal axis. Utilizing two dolphins, beam steering capabilities and beamwidth control were demonstrated. The two dolphins steered the axis of the echolocation beam up to 18° and 28° in the horizontal plane. Horizontal beamwidths were bimodally distributed in one dolphin, with peaks at 16° and 26°. The other dolphin had a unimodal distribution of beamwidths (peak at ~25°), likely reflecting a learned strategy for the off-axis detection task. The use of dense hydrophone arrays and challenging echolocation tasks continue to build on the work of Au and colleagues, who provided a paradigm that has greatly contributed to our understanding of dolphin biosonar.

The echolocation beam of bottlenose dolphins (Tursiops truncatus): High-resolution measurements of horizontal beam patterns and nearfield/farfield transitions

The work of Whitlow Au and colleagues has demonstrated that dolphin biosonar forms a highly directional, forward-facing beam. In our recent studies, we have expanded upon previous work by making biosonar beam measurements using high-resolution hydrophone arrays with up to 48 hydrophones. Bottlenose dolphins were trained to echolocate on both physical targets and phantom echo generators, with clicks simultaneously recorded on all hydrophones at a sampling rate of 2 MHz. Target ranges (and simulated target ranges for phantom echoes) were varied in order to examine the resulting effects on the spatial characteristics of the acoustic field. The directivity index of the echolocation beam increased with increasing click level and center frequency, and recordings from extreme off-axis azimuths displayed a two-pulse pattern best explained by internal reflections off of the premaxillary bones. High-density hydrophone arrays placed near echolocating dolphins’ heads demonstrated that a transition from the geometric ...

Transmission beam characteristics of a spinner dolphin (Stenella longirostris)

Transmission beam characteristics have been described in a small number of odontocete species, providing insight into the biological and ecological factors that have influenced the design of the outgoing echolocation beam. The current study measured the on-axis spectral characteristics and transmission beam pattern of echolocation clicks from a small oceanic delphinid, the spinner dolphin (Stenella longirostis). A formerly stranded individual was rehabilitated in captivity and trained to station underwater in front of a 16 element hydrophone array. Preliminary analysis of a subset of recorded clicks showed on-axis spectral characteristics with a mean center frequency of 68 kHz and a mean peak frequency of 52 kHz. The dolphin exhibited both a circular beam shape and also varying degrees of a dorso-ventrally compressed transmission beam. The mean angular beamwidth for all clicks was 16.6 and 14.3 degrees in the horizontal and vertical planes, respectively, though some clicks exhibited horizontal beamwidths that were almost twice as wide as the vertical beamwidth. The overall beam was generally broader than the beams of previously described species, with a mean directivity index for all clicks of 21.5 dB. These results are the first reports of transmission beam characteristics from a spinner dolphin.

Intra-click time-frequency patterns across the echolocation beam of a beluga whale

The echolocation beam of toothed whales has been studied ever since it was first discovered in 1960. Recent studies have focused on the frequency distributions across the cross sections of the beams. Other studies have focussed on describing the entire acoustic field around the animal. However, no one has yet described the timing of each frequency component in the main lobe beam in relation to the other frequency components. Even though the echolocation clicks of broadband click species like the beluga whales (Delphinapterus leucas) are short in time (around 70 μs), previous results have shown indications on a frequency dependence with time, within each click. Little is known about the details of how the signal is generated and transmitted into the water. Investigations of when in time the frequency components occur within each click would give us further knowledge to how the signals are generated. This study takes a closer look at these intra-click time-frequency patterns across the echolocation beam of ...

Nearfield and farfield measurements of dolphin echolocation beam patterns: No evidence of focusing.

The potential for bottlenose dolphins to actively focus their biosonar transmissions was examined by measuring emitted clicks in four dolphins using horizontal, planar hydrophone arrays. Two hydrophone configurations were used: a rectangular array with hydrophones 0.2 to 2 m from the dolphins and a polar array with hydrophones 0.5 to 5 m from the dolphins. The biosonar task was a target change detection utilizing physical targets at ranges from 1.3 to 6.3 m with all subjects and "phantom" targets at simulated ranges from 2.5 to 20 m with two subjects. To provide a basis for evaluating the experimental data, sound fields radiated from flat and focused circular pistons were mathematically simulated using transient excitation functions similar to dolphin clicks. The array measurements showed no evidence that the dolphins adaptively focused their click emissions; axial amplitudes and iso-amplitude contours matched the pattern of the simulation results for flat transducers and showed a single region of maximum amplitude, beyond which spherical spreading loss was approximated.