Noseleaf furrows in a horseshoe bat act as resonance cavities shaping the biosonar beam

The evolutionary history and ancestral biogeographic range estimation of old-world Rhinolophidae and Hipposideridae (Chiroptera)

BACKGROUND

Family Rhinolophidae (horseshoe bats), Hipposideridae (leaf-nosed bats) and Rhinonycteridae (trident bats) are exclusively distributed in the Old-World, and their biogeography reflects the complex historic geological events throughout the Cenozoic. Here we investigated the origin of these families and unravel the conflicting family origin theories using a high resolution tree covering taxa from each zoogeographic realm from Africa to Australia. Ancestral range estimations were performed using a probabilistic approach implemented in BioGeoBEARS with subset analysis per biogeographic range [Old-World as whole, Australia-Oriental-Oceania (AOO) and Afrotropical-Madagascar-Palearctic (AMP)].

RESULT

Our result supports an Oriental origin for Rhinolophidae, whereas Hipposideridae originated from the Oriental and African regions in concordance with fossil evidence of both families. The fossil evidence indicates that Hipposideridae has diversified across Eurasia and the Afro-Arabian region since the Middle Eocene. Meanwhile, Rhinonycteridae (the sister family of Hipposideridae) appears to have originated from the Africa region splitting from the common ancestor with Hipposideridae in Africa. Indomalaya is the center of origin of Rhinolophidae AOO lineages, and Indomalayan + Philippines appears to be center of origin of Hipposideridae AOO lineage indicating allopatric speciation and may have involved jump-dispersal (founder-event) speciation within AOO lineage. Wallacea and the Philippines may have been used as stepping stones for dispersal towards Oceania and Australia from the Oriental region. Multiple colonization events via different routes may have occurred in the Philippines (i.e., Palawan and Wallacea) since the Late Miocene. The colonization of Rhinolophidae towards Africa from Asia coincided with the estimated time of Tethys Ocean closure around the Oligocene to Miocene (around 27 Ma), allowing species to disperse via the Arabian Peninsula. Additionally, the number of potential cryptic species in Rhinolophidae in Southeast Asia may have increased since Plio-Pleistocene and late Miocene.

CONCLUSION

Overall, we conclude an Oriental origin for Rhinolophidae, and Oriental + African for Hipposideridae. The result demonstrates that complex historical events, in addition to species specific ecomorphology and specialization of ecological niches may shape current distributions.

How to Accurately Delineate Morphologically Conserved Taxa and Diagnose Their Phenotypic Disparities: Species Delimitation in Cryptic Rhinolophidae (Chiroptera)

Systematics and taxonomy are the backbone of all components of biology and ecology, yet cryptic species present a major challenge for accurate species identification. This is especially problematic as they represent a substantial portion of undiscovered biodiversity, and have implications for not only species conservation, but even assaying potential risk of zoonotic spillover. Here, we use integrative approaches to delineate potential cryptic species in horseshoe bats (Rhinolophidae), evaluate the phenotypic disparities between cryptic species, and identify key traits for their identification. We tested the use of multispecies coalescent models (MSC) using Bayesian Phylogenetic and Phylogeography (BPP) and found that BPP was useful in delineating potential cryptic species, and consistent with acoustic traits. Our results show that around 40% of Asian rhinolophid species are potentially cryptic and have not been formally described. In order to avoid potential misidentification and allow species to be accurately identified, we identified quantitative noseleaf sella and acoustic characters as the most informative traits in delineating between potential cryptic species in Rhinolophidae. This highlights the physical differences between cryptic species that are apparent in noseleaf traits which often only qualitatively described but rarely measured. Each part of the noseleaf including the sella, lateral lappets, and lancet furrows, play roles in focusing acoustic beams and thus, provide useful characteristics to identify cryptic Rhinolophus species. Finally, species delimitation for cryptic species cannot rely on genetic data alone, but such data should be complemented by other evidence, including phenotypic, acoustic data, and geographic distributions to ensure accurate species identification and delineation.

How signaling geometry shapes the efficacy and evolution of animal communication systems

Animal communication is inherently spatial. Both signal transmission and signal reception have spatial biases-involving direction, distance and position-that interact to determine signaling efficacy. Signals, be they visual, acoustic, or chemical, are often highly directional. Likewise, receivers may only be able to detect signals if they arrive from certain directions. Alignment between these directional biases is therefore critical for effective communication, with even slight misalignments disrupting perception of signaled information. In addition, signals often degrade as they travel from signaler to receiver, and environmental conditions that impact transmission can vary over even small spatiotemporal scales. Thus, how animals position themselves during communication is likely to be under strong selection. Despite this, our knowledge regarding the spatial arrangements of signalers and receivers during communication remains surprisingly coarse for most systems. We know even less about how signaler and receiver behaviors contribute to effective signaling alignment over time, or how signals themselves may have evolved to influence and/or respond to these aspects of animal communication. Here, we first describe why researchers should adopt a more explicitly geometric view of animal signaling, including issues of location, direction, and distance. We then describe how environmental and social influences introduce further complexities to the geometry of signaling. We discuss how multimodality offers new challenges and opportunities for signalers and receivers. We conclude with recommendations and future directions made visible by attention to the geometry of signaling.

An experimental link between fast noseleaf deformations and biosonar pulse dynamics in hipposiderid bats

Old-World leaf-nosed bats (Hipposideridae) are echolocating bats with peculiar emission-side dynamics where beamforming baffles ("noseleaves") that surround the points of ultrasound emission (nostrils) change shape while diffracting the outgoing biosonar pulses. While prior work with numerical and robotic models has suggested that these noseleaf deformations could have an impact on the output characteristics of the bat's biosonar system, testing the hypothesis that this is the case in bats remains a critical step to be taken. The work presented here has tested the hypothesis that the noseleaf dynamics in a species of hipposiderid bat (Pratt's roundleaf bat, H. pratti) leads to time-variant acoustical properties on the output side of the bats' biosonar emission system. The time-variant effects of the noseleaf motion could be detected even in the presence of other sources of variability by comparing the distribution of pulse energy over the angle at different points in time. Furthermore, a convolutional neural network was able to classify the noseleaf motion state based on microphone array recordings with 85.3% accuracy. These results hence demonstrate that these nose-emitting bats have access to a substrate for behavioral flexibility on the emission-side of their biosonar systems.

Differential Entropy Analysis of the Acoustic Characteristics of a Biomimetic Dynamic Sonar Emitter

Active noseleaf deformations during pulse emission observed in hipposiderid and rhinolophid bats have been shown to add a time dimension to the bats’ acoustic emission characteristics beyond the established dependencies on frequency and direction. In this study, a dense three-dimensional acoustic characteristics were obtained by the time series of smoothed signal amplitudes at different directions and frequencies collected by a biomimetic dynamic sonar emitter. These data have been analyzed using differential entropy which was used as a measure to compare the encoding capacity for sensory information between the three different dimensions. The capacity for sensory information encoding measured in this way along time dimension was found to be similar to that along the frequency dimension. But both of them provided less information than provided by the direction dimension.

Variability in the rigid pinna motions of hipposiderid bats and their impact on sensory information encoding

Many bat species, e.g., in the rhinolophid and hipposiderid families, have dynamic biosonar systems with highly mobile pinnae. Pinna motion patterns have been shown to fall into two distinct categories: rigid rotations and non-rigid motions (i.e., deformations). In the present work, two questions regarding the rigid rotations have been investigated: (i) what is the nature of the variability (e.g., discrete subgroups or continuous variation) within the rigid motions, (ii) what is its acoustic impact? To investigate the first question, rigid pinna motions in Pratt's leaf-nosed bats (Hipposideros pratti) have been tracked with stereo vision and a dense set of landmark points on the pinna surface. Axis-angle representations of the recorded rigid motions have shown a continuous variation in the rotation axes that covered a range of almost 180° in azimuth and elevation. To investigate the second question, the observed range of rigid pinna motions has been reproduced with a biomimetic pinna. Normalized mutual information between acoustic inputs associated with every pair of the rigid pinna motions showed that even small changes in the rotation axis resulted in more than 50% new sensory information encoding capacity (i.e., normalized mutual information less than 50%). This demonstrates a potential sensory benefit to the observed variability in the rigid pinna rotations.

Maxilloturbinal Aids in Nasophonation in Horseshoe Bats (Chiroptera: Rhinolophidae)

Horseshoe bats (Family Rhinolophidae) show an impressive array of morphological traits associated with use of high duty cycle echolocation calls that they emit via their nostrils (nasophonation). Delicate maxilloturbinal bones inside the nasal fossa of horseshoe bats have a unique elongated strand-like shape unknown in other mammals. Maxilloturbinal strands also vary considerably in length and cross-sectional shape. In other mammals, maxilloturbinals help direct respired air and prevent respiratory heat and water loss. We investigated whether strand-shaped maxilloturbinals in horseshoe bats perform a similar function to those of other mammals, or whether they were shaped for a role in nasophonation. Using histology, we studied the mucosa of the nasal fossa in Rhinolophus lepidus, which we compared with Hipposideros lankadiva (Hipposideridae) and Megaderma lyra (Megadermatidae). Using micro-CT scans of 30 horseshoe bat species, we quantified maxilloturbinal surface area and skull shape within a phylogenetic context. Histological results showed horseshoe bat maxilloturbinals are covered in a thin, poorly vascularized, sparsely ciliated mucosa poorly suited for preventing respiratory heat and water loss. Maxilloturbinal surface area was correlated with basicranial width, but exceptionally long and dorsoventrally flat maxilloturbinals did not show enhanced surface area for heat and moisture exchange. Skull shape variation appears to be driven by structures linked to nasophonation, including maxilloturbinals. Resting echolocation call frequency better predicted skull shape than did skull size, and was specifically correlated with dimensions of the rostral inflations, palate, and maxilloturbinals. These traits appear to form a morphological complex, indicating a nasophonatory role for the strand-shaped rhinolophid maxilloturbinals. Anat Rec, 2018. © 2018 American Association for Anatomy.

Investigation on acoustic reception pathways in finless porpoise (Neophocaena asiaorientalis sunameri) with insight into an alternative pathway

Sound transmission and reception are both vital components to odontocete echolocation and daily life. Here, we combine computed tomography (CT) scanning and finite element modeling to investigate the acoustic propagation of finless porpoise (Neophocaena asiaorientalis sunameri) echolocation pulses. The CT scanning and finite element method wave propagation model results support the well-accepted jaw-hearing pathway hypothesis and suggest an additional alternative auditory pathway composed of structures, mandible (lower jaw) and internal mandibular fat, with different acoustic impedances, which may also conduct sounds to the ear complexes. The internal mandibular fat is attached to the ear complex and encased by the mandibles laterally and anteriorly. The simulations show signals in this pathway initially propagate along the solid mandibles and are transmitted to the acoustically coupled soft tissue of the internal mandibular fat which conducts the stimuli posteriorly as it eventually arrives at ear complexes. While supporting traditional theories, this new bone-tissue conduction pathway might be meaningful to understand the hearing and sound reception processes in a wide variety of odontocetes species.

Design of a dynamic sonar emitter inspired by hipposiderid bats

The ultrasonic emission of the biosonar systems of bats, such as Old World leaf-nosed bats (family Hipposideridae) and the related horseshoe bats (family Rhinolophidae), is characterized by a unique dynamics where baffle shapes ('noseleaves') deform while diffracting the outgoing wave packets. As of now, nothing comparable to this dynamics has been used in any related engineering application (e.g. sonar or radar). Prior work with simple concave baffle shapes has demonstrated the impact of the dynamics on the emission characteristics, but it has remained unclear whether this was simply due to the change in aperture size or also influenced by the geometrical shape detail. Hence, it has also remained unclear whether it would be possible to further enhance the time-variant effects reported so far through different static and dynamic geometries. To address this issue, we have created a dynamic emission baffle with biomimetic shape detail modeled after Pratt's roundleaf bats (Hipposideros pratti). The impact of the dynamic deformation of the shape on the time-variant emission characteristics was evaluated by virtue of the gradient magnitude and the entropy in the gradient orientation. The results have shown that the dynamics results in much larger gradients in signal representation, which change jointly over direction and time.

Quantitative approaches to sensory information encoding by bat noseleaves and pinnae

The biosonar systems of horseshoe bats (Rhinolophidae) and Old World round leaf-nosed bats (Hipposideridae) incorporate a pervasive dynamic at the interfaces for ultrasound emission (noseleaves) and reception (pinnae). Changes in the shapes of these structures alter the acoustic characteristics of the biosonar system and could hence influence the encoding of sensory information. The focus of the present work is on approaches that can be used to investigate the hypothesis that the interface dynamic effects sensory information encoding. Mutual information can be used as a metric to quantify the extent to which the different ultrasonic emission and reception characteristics (beampatterns) provide independent views of the environment. Two different quantitative approaches have been taken to evaluate the relationship between dynamically encoded additional sensory information and sensing performance in finding the direction of a biosonar target. The first approach is to determine an upper bound on the number of different directions that can be distinguished by virtue of distinct spectral signatures. The second approach is based on a lower bound (Cramér–Rao) on the variance of direction estimates. All these different metrics demonstrate that the peripheral dynamics seen in bats result in the encoding of additional sensory information that is suitable for enhancing biosonar performance.

A comparison of the role of beamwidth in biological and engineered sonar

Sonar is an important sensory modality for engineers as well as in nature. In engineering, sonar is the dominating modality for underwater sensing. In nature, biosonar is likely to have been a central factor behind the unprecedented evolutionary success of bats, a highly diverse group that accounts for over 20% of all mammal species. However, it remains unclear to what extent engineered and biosonar follow similar design and operational principles. In the current work, the key sonar design characteristic of beamwidth is examined in technical and biosonar. To this end, beamwidth data has been obtained for 23 engineered sonar systems and from numerical beampattern predictions for 151 emission and reception elements (noseleaves and ears) representing bat biosonar. Beamwidth data from these sources is compared to the beamwidth of a planar ellipsoidal transducer as a reference. The results show that engineered and biological both obey the basic physical limit on beamwidth as a function of the ratio of aperture size and wavelength. However, beyond that, the beamwidth data revealed very different behaviors between the engineered and the biological sonar systems. Whereas the beamwidths of the technical sonar systems were very close to the planar transducer limit, the biological samples showed a very wide scatter away from this limit. This scatter was as large, if not wider, than what was seen in a small reference data set obtained with random aluminum cones. A possible interpretation of these differences in the variability could be that whereas sonar engineers try to minimize beamwidth subject to constraints on device size, the evolutionary optimization of bat biosonar beampatterns has been directed at other factors that have left beamwidth as a byproduct. Alternatively, the biosonar systems may require beamwidth values that are larger than the physical limit and differ between species and their sensory ecological niches.

Unique Turbinal Morphology in Horseshoe Bats (Chiroptera: Rhinolophidae)

The mammalian nasal fossa contains a set of delicate and often structurally complex bones called turbinals. Turbinals and associated mucosae function in regulating respiratory heat and water loss, increasing surface area for olfactory tissue, and directing airflow within the nasal fossa. We used high-resolution micro-CT scanning to investigate a unique maxilloturbinal morphology in 37 species from the bat family Rhinolophidae, which we compared with those of families Hipposideridae, Megadermatidae, and Pteropodidae. Rhinolophids exhibit numerous structural modifications along the nasopharyngeal tract associated with emission of high duty cycle echolocation calls via the nostrils. In rhinolophids, we found that the maxilloturbinals and a portion of ethmoturbinal I form a pair of strand-like bony structures on each side of the nasal chamber. These structures project anteriorly from the transverse lamina and complete a hairpin turn to project posteriorly down the nasopharyngeal duct, and vary in length among species. The strand-like maxilloturbinals in Rhinolophidae were not observed in our outgroups and represent a synapomorphy for this family, and are unique in form among mammals. Within Rhinolophidae, maxilloturbinal size and cross-sectional shape were correlated with phylogeny. We hypothesize that strand-shaped maxilloturbinals may function to reduce respiratory heat and water loss without greatly impacting echolocation call transmission since they provide increased mucosal surface area for heat and moisture exchange but occupy minimal space. Alternatively, they may play a role in transmission of echolocation calls since they are located directly along the path sound travels between the larynx and nostrils during call emission. Anat Rec, 300:309-325, 2017. © 2016 Wiley Periodicals, Inc.

Species-specific control of acoustic gaze by echolocating bats, Rhinolophus ferrumequinum nippon and Pipistrellus abramus, during flight

Based on the characteristics of the ultrasounds they produce, echolocating bats can be categorized into two main types: broadband FM (frequency modulated) and narrowband CF (constant frequency) echolocators. In this study, we recorded the echolocation behavior of a broadband FM (Pipistrellus abramus) and a narrowband CF echolocator species (Rhinolophus ferrumequinum nippon) while they explored an unfamiliar space in a laboratory chamber. During flight, P. abramus smoothly shifted its acoustic gaze in relation to its flight direction, whereas R. ferrumequinum nippon frequently shifted its acoustic gaze from side to side. The distribution of the acoustic gazes of R. ferrumequinum nippon was twice as wide as that of P. abramus. Furthermore, R. ferrumequinum nippon produced double pulses twice as often as P. abramus. Because R. ferrumequinum nippon has a horizontal beam width (−6 dB off-axis angle) half as wide (±20.8 ± 6.0°) as that of P. abramus (±38.3 ± 6.0°), it appears to double the width of its acoustical field of view by shifting its acoustic gaze further off-axis and emitting direction-shifted double pulses. These results suggest that broadband FM and narrowband CF bats actively control their acoustic gazes in a species-specific manner based on the acoustic features of their echolocation signals.

A dynamic ultrasonic emitter inspired by horseshoe bat noseleaves

The emission of biosonar pulses in horseshoe bats (family Rhinolophidae) differs from technical sonar in that it has dynamic features at the interface to the free field. When the horseshoe bats emit their biosonar pulses through the nostrils, the walls of a horn-shaped baffle (anterior leaf) are in motion while diffracting the outgoing ultrasonic wave packets. Here, biomimetic reproductions of the dynamic emission shapes of horseshoe bats have been studied for their ability to impose time-variant signatures onto the outgoing pulses. It was found that an elliptical sound outlet with rotating baffles that were attached along the direction of the major axis can be well suited for this purpose. Most importantly, concave baffle shapes were found to produce strongly time-dependent devices characteristics that could reach a root-mean-square-difference between beampatterns of almost 6 dB within a rotation angle of 10°. In contrast to this, a straight baffle shape needs to be rotated over 60° for a similar result. When continuously rotated in synchrony with the emitted pulses, the concave biomimetic baffles produced time-variant device characteristics that depended jointly on direction, frequency, and time. Since such device properties are so easily produced, it appears well worthwhile to explore their use in engineering.

The acoustical role of vocal tract in the horseshoe bat, Rhinolophus pusillus

The sound field distribution in the vocal tract with a single sound source in the glottis and the transfer function of the supraglottal vocal tract of the horseshoe bat, Rhinolophus pusillus, have been obtained using the finite-element method (FEM) technique. The models of vocal tracts used for FEM calculation are constructed by tomography scanning. These models are used to set up a finite-element model for calculating the sound field distribution by loading the unit sound source in the glottis. By changing the frequency of the unit sound source, the frequency response was figured out and the acoustic role of vocal tract chambers was examined by obtaining the transfer function and sound pressure distribution before and after filling the chambers using voxels. Sound pressures in the trachea and nostrils are recorded and some analysis of the acoustics of the subglottal and vocal tract was made to find the function of the construction in the vocal tract and subglottal parts. The results show nasal chambers can effectively improve the Q (quality factor) value near the second harmonic, and alternate the sound distribution in the supraglottal part. Whereas the tracheal chambers can reduce the amplitude second harmonic in the subglottal part, its function is like a notch filter which can block the second harmonic component of the back propagation sound under the glottis.

An assessment of the direction-finding accuracy of bat biosonar beampatterns

In the biosonar systems of bats, emitted acoustic energy and receiver sensitivity are distributed over direction and frequency through beampattern functions that have diverse and often complicated geometries. This complexity could be used by the animals to determine the direction of incoming sounds based on spectral signatures. The present study has investigated how well bat biosonar beampatterns are suited for direction finding using a measure of the smallest estimator variance that is possible for a given direction [Cramér-Rao lower bound (CRLB)]. CRLB values were estimated for numerical beampattern estimates derived from 330 individual shape samples, 157 noseleaves (used for emission), and 173 outer ears (pinnae). At an assumed 60 dB signal-to-noise ratio, the average value of the CRLB was 3.9°, which is similar to previous behavioral findings. Distribution for the CRLBs in individual beampatterns had a positive skew indicating the existence of regions where a given beampattern does not support a high accuracy. The highest supported accuracies were for direction finding in elevation (with the exception of phyllostomid emission patterns). No large, obvious differences in the CRLB (greater 2° in the mean) were found between the investigated major taxonomic groups, suggesting that different bat species have access to similar direction-finding information.

Inducing rostrum interfacial waves by fluid-solid coupling in a Chinese river dolphin (Lipotesvexillifer)

Through numerically solving the appropriate wave equations, propagation of biosonar signals in a Chinese river dolphin (baiji) was studied. The interfacial waves along the rostrum-tissue interfaces, including both compressional (longitudinal) and shear (transverse) waves in the solid rostrum through fluid-solid coupling were examined. The baiji's rostrum was found to effect acoustic beam formation not only as an interfacial wave generator but also as a sound reflector. The wave propagation patterns in the solid rostrum were found to significantly change the wave movement through the bone. Vibrations in the rostrum, expressed in solid displacement, initially increased but eventually decreased from posterior to anterior sides, indicating a complex physical process. Furthermore, the comparisons among seven cases, including the combination of (1) the rostrum, melon, and air sacs; (2) rostrum-air sacs; (3) rostrum-melon; (4) only rostrum; (5) air sacs-melon; (6) only air sacs; and (7) only melon revealed that the cases including the rostrum were better able to approach the complete system by inducing rostrum-tissue interfacial waves and reducing the differences in main beam angle and -3 dB beam width. The interfacial waves in the rostrum were considered complementary with reflection to determine the obbligato role of the rostrum in the baiji's biosonar emission. The far-field beams formed from complete fluid-solid models and non-fluid-solid models were compared to reveal the effects brought by the consideration of shear waves of the solid structures of the baiji. The results may provide useful information for further understanding the role of the rostrum in this odontocete species.

Interplay of lancet furrows and shape change in the horseshoe bat noseleaf

Horseshoe bats emit biosonar pulses through the nostrils and diffract the outgoing ultrasonic pulses with baffles, so-called "noseleaves," that surround the nostrils. The noseleaves have complex static geometries and can furthermore undergo dynamic shape changes during emission of the biosonar pulses. The posterior noseleaf part, the lancet, has been shown to carry out anterior-posterior flicking motions during biosonar emissions with average lancet tip displacements of about 1 mm. Here, the acoustic effects of the interplay between the lancet furrows and shape change (lancet rotation) on the emission beam were investigated using the animated digital models obtained from the noseleaves of greater horseshoe bats (Rhinolophus ferrumequinum). It was found that forward lancet rotations increase the amount of sound energy allocated to secondary amplitude maxima (sidelobes) in the beampattern, but only in the presence of the furrows. The interaction between static and dynamic features can be readily quantified by roughness (standard deviation about local mean) of the amplitude distribution of the beampatterns. This effect goes beyond the static impact of the furrows on the width of the mainlobe. It could allow the bats to send out their pulses through a sequence of qualitatively different beampatterns.

Lancet dynamics in greater horseshoe bats, Rhinolophus ferrumequinum

Echolocating greater horseshoe bats (Rhinolophus ferrumequinum) emit their biosonar pulses nasally, through nostrils surrounded by fleshy appendages (‘noseleaves’) that diffract the outgoing ultrasonic waves. Movements of one noseleaf part, the lancet, were measured in live bats using two synchronized high speed video cameras with 3D stereo reconstruction, and synchronized with pulse emissions recorded by an ultrasonic microphone. During individual broadcasts, the lancet briefly flicks forward (flexion) and is then restored to its original position. This forward motion lasts tens of milliseconds and increases the curvature of the affected noseleaf surfaces. Approximately 90% of the maximum displacements occurred within the duration of individual pulses, with 70% occurring towards the end. Similar lancet motions were not observed between individual pulses in a sequence of broadcasts. Velocities of the lancet motion were too small to induce Doppler shifts of a biologically-meaningful magnitude, but the maximum displacements were significant in comparison with the overall size of the lancet and the ultrasonic wavelengths. Three finite element models were made from micro-CT scans of the noseleaf post mortem to investigate the acoustic effects of lancet displacement. The broadcast beam shapes were found to be altered substantially by the observed small lancet movements. These findings demonstrate that—in addition to the previously described motions of the anterior leaf and the pinna—horseshoe bat biosonar has a third degree of freedom for fast changes that can happen on the time scale of the emitted pulses or the returning echoes and could provide a dynamic mechanism for the encoding of sensory information.

The aerodynamic cost of head morphology in bats: maybe not as bad as it seems

At first sight, echolocating bats face a difficult trade-off. As flying animals, they would benefit from a streamlined geometric shape to reduce aerodynamic drag and increase flight efficiency. However, as echolocating animals, their pinnae generate the acoustic cues necessary for navigation and foraging. Moreover, species emitting sound through their nostrils often feature elaborate noseleaves that help in focussing the emitted echolocation pulses. Both pinnae and noseleaves reduce the streamlined character of a bat's morphology. It is generally assumed that by compromising the streamlined charactered of the geometry, the head morphology generates substantial drag, thereby reducing flight efficiency. In contrast, it has also been suggested that the pinnae of bats generate lift forces counteracting the detrimental effect of the increased drag. However, very little data exist on the aerodynamic properties of bat pinnae and noseleaves. In this work, the aerodynamic forces generated by the heads of seven species of bats, including noseleaved bats, are measured by testing detailed 3D models in a wind tunnel. Models of Myotis daubentonii, Macrophyllum macrophyllum, Micronycteris microtis, Eptesicus fuscus, Rhinolophus formosae, Rhinolophus rouxi and Phyllostomus discolor are tested. The results confirm that non-streamlined facial morphologies yield considerable drag forces but also generate substantial lift. The net effect is a slight increase in the lift-to-drag ratio. Therefore, there is no evidence of high aerodynamic costs associated with the morphology of bat heads.

Characterization of the diversity in bat biosonar beampatterns with spherical harmonics power spectra

The biosonar beampatterns found across different bat species are highly diverse in terms of global and local shape properties such as overall beamwidth or the presence, location, and shape of multiple lobes. It may be hypothesized that some of this variability reflects evolutionary adaptation. To investigate this hypothesis, the present work has searched for patterns in the variability across a set of 283 numerical predictions of emission and reception beampatterns from 88 bat species belonging to four major families (Rhinolophidae, Hipposideridae, Phyllostomidae, Vespertilionidae). This was done using a lossy compression of the beampatterns that utilized real spherical harmonics as basis functions. The resulting vector representations showed differences between the families as well as between emission and reception. These differences existed in the means of the power spectra as well as in their distribution. The distributions were characterized in a low dimensional space found through principal component analysis. The distinctiveness of the beampatterns across the groups was corroborated by pairwise classification experiments that yielded correct classification rates between ~85 and ~98%. Beamwidth was a major factor but not the sole distinguishing feature in these classification experiments. These differences could be seen as an indication of adaptive trends at the beampattern level.

Intensity and directionality of bat echolocation signals

The paper reviews current knowledge of intensity and directionality of bat echolocation signals. Recent studies have revealed that echolocating bats can be much louder than previously believed. Bats previously dubbed "whispering" can emit calls with source levels up to 110 dB SPL at 10 cm and the louder open space hunting bats have been recorded at above 135 dB SPL. This implies that maximum emitted intensities are generally 30 dB or more above initial estimates. Bats' dynamic control of acoustic features also includes the intensity and directionality of their sonar calls. Aerial hawking bats will increase signal directionality in the field along with intensity thus increasing sonar range. During the last phase of prey pursuit, vespertilionid bats broaden their echolocation beam considerably, probably to counter evasive maneuvers of eared prey. We highlight how multiple call parameters (frequency, duration, intensity, and directionality of echolocation signals) in unison define the search volume probed by bats and in turn how bats perceive their surroundings. Small changes to individual parameters can, in combination, drastically change the bat's perception, facilitating successful navigation and food acquisition across a vast range of ecological niches. To better understand the function of echolocation in the natural habitat it is critical to determine multiple acoustic features of the echolocation calls. The combined (interactive) effects, not only of frequency and time parameters, but also of intensity and directionality, define the bat's view of its acoustic scene.

Adaptive beam-width control of echolocation sounds by CF–FM bats, Rhinolophus ferrumequinum nippon, during prey-capture flight

The echolocation sounds of Japanese CF–FM bats (Rhinolophus ferrumequinum nippon) were measured while the bats pursued a moth (Goniocraspidum pryeri) in a flight chamber. Using a 31-channel microphone array system, we investigated how CF–FM bats adjust pulse direction and beam width according to prey position. During the search and approach phases, the horizontal and vertical beam widths were ±22±5 and ±13±5 deg, respectively. When bats entered the terminal phase approximately 1 m from a moth, distinctive evasive flight by G. pryeri was sometimes observed. Simultaneously, the bats broadened the beam widths of some emissions in both the horizontal (44% of emitted echolocation pulses) and vertical planes (71%). The expanded beam widths were ±36±7 deg (horizontal) and ±30±9 deg (vertical). When moths began evasive flight, the tracking accuracy decreased compared with that during the approach phase. However, in 97% of emissions during the terminal phase, the beam width was wider than the misalignment (the angular difference between the pulse and target directions). These findings indicate that bats actively adjust their beam width to retain the moving target within a spatial echolocation window during the final capture stages.

Noseleaf dynamics during pulse emission in horseshoe bats

Horseshoe bats emit their biosonar pulses nasally and diffract the outgoing ultrasonic waves by conspicuous structures that surrounded the nostrils. Here, we report quantitative experimental data on the motion of a prominent component of these structures, the anterior leaf, using synchronized laser Doppler vibrometry and acoustic recordings in the greater horseshoe bat (Rhinolophus ferrumequinum). The vibrometry data has demonstrated non-random motion patterns in the anterior leaf. In these patterns, the outer rim of the walls of the anterior leaf twitches forward and inwards to decrease the aperture of the noseleaf and increase the curvature of its surfaces. Noseleaf displacements were correlated with the emitted ultrasonic pulses. After their onset, the inward displacements increased monotonically towards their maximum value which was always reached within the duration of the biosonar pulse, typically towards its end. In other words, the anterior leaf's surfaces were moving inwards during most of the pulse. Non-random motions were not present in all recorded pulse trains, but could apparently be switched on or off. Such switches happened between sequences of consecutive pulses but were never observed between individual pulses within a sequence. The amplitudes of the emitted biosonar pulse and accompanying noseleaf movement were not correlated in the analyzed data set. The measured velocities of the noseleaf surface were too small to induce Doppler shifts of a magnitude with a likely significance. However, the displacement amplitudes were significant in comparison with the overall size of the anterior leaf and the sound wavelengths. These results indicate the possibility that horseshoe bats use dynamic sensing principles on the emission side of their biosonar system. Given the already available evidence that such mechanisms exist for biosonar reception, it may be hypothesized that time-variant mechanisms play a pervasive role in the biosonar sensing of horseshoe bats.

Asymmetry and dynamics of a narrow sonar beam in an echolocating harbor porpoise

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.

The furrows of Rhinolophidae revisited

Rhinolophidae, a family of echolocating bats, feature very baroque noseleaves that are assumed to shape their emission beam. Zhuang & Muller (Zhuang & Muller 2006 Phys. Rev. Lett. 97, 218701 (doi:10.1103/PhysRevLett.97.218701); Zhuang & Muller 2007 Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 76(Pt. 1), 051902 (doi:10.1103/PhysRevE.76.051902)) have proposed, based on finite element simulations, that the furrows present in the noseleaves of these bats act as resonance cavities. Using Rhinolophus rouxi as a model species, they reported that a resonance phenomenon causes the main beam to be elongated at a particular narrow frequency range. Virtually filling the furrows reduced the extent of the main lobe. However, the results of Zhuang & Muller are difficult to reconcile with the ecological background of R. rouxi. In this report, we replicate the study of Zhuang & Muller, and extend it in important ways: (i) we take the filtering of the moving pinnae into account, (ii) we use a model of the echolocation task faced by Rhinolophidae to estimate the effect of any alterations to the emission beam on the echolocation performance of the bat, and (iii) we validate our simulations using a physical mock-up of the morphology of R. rouxi. In contrast to Zhuang & Muller, we find the furrows to focus the emitted energy across the whole range of frequencies contained in the calls of R. rouxi (both in simulations and in measurements). Depending on the frequency, the focusing effect of the furrows has different consequences for the estimated echolocation performance. We argue that the furrows act to focus the beam in order to reduce the influence of clutter echoes.

Ear deformations give bats a physical mechanism for fast adaptation of ultrasonic beam patterns

A large number of mammals, including humans, have intricate outer ear shapes that diffract incoming sound in a direction- and frequency-specific manner. Through this physical process, the outer ear shapes encode sound-source information into the sensory signals from each ear. Our results show that horseshoe bats could dynamically control these diffraction processes through fast nonrigid ear deformations. The bats' ear shapes can alter between extreme configurations in about 100 ms and thereby change their acoustic properties in ways that would suit different acoustic sensing tasks.

Information generated by the moving pinnae of Rhinolophus rouxi: tuning of the morphology at different harmonics

Bats typically emit multi harmonic calls. Their head morphology shapes the emission and hearing sound fields as a function of frequency. Therefore, the sound fields are markedly different for the various harmonics. As the sound field provides bats with all necessary cues to locate objects in space, different harmonics might provide them with variable amounts of information about the location of objects. Also, the ability to locate objects in different parts of the frontal hemisphere might vary across harmonics. This paper evaluates this hypothesis in R. rouxi, using an information theoretic framework. We estimate the reflector position information transfer in the echolocation system of R. rouxi as a function of frequency. This analysis shows that localization performance reaches a global minimum and a global maximum at the two most energetic frequency components of R. rouxi call indicating tuning of morphology and harmonic structure. Using the fundamental the bat is able to locate objects in a large portion of the frontal hemisphere. In contrast, using the 1 overtone, it can only locate objects, albeit with a slightly higher accuracy, in a small portion of the frontal hemisphere by reducing sensitivity to echoes from outside this region of interest. Hence, different harmonic components provide the bat either with a wide view or a focused view of its environment. We propose these findings can be interpreted in the context of the foraging behaviour of R. rouxi, i.e., hunting in cluttered environments. Indeed, the focused view provided by the 1 overtone suggests that at this frequency its morphology is tuned for clutter rejection and accurate localization in a small region of interest while the finding that overall localization performance is best at the fundamental indicates that the morphology is simultaneously tuned to optimize overall localization performance at this frequency.

Numerical analysis of biosonar beamforming mechanisms and strategies in bats

Beamforming is critical to the function of most sonar systems. The conspicuous noseleaf and pinna shapes in bats suggest that beamforming mechanisms based on diffraction of the outgoing and incoming ultrasonic waves play a major role in bat biosonar. Numerical methods can be used to investigate the relationships between baffle geometry, acoustic mechanisms, and resulting beampatterns. Key advantages of numerical approaches are: efficient, high-resolution estimation of beampatterns, spatially dense predictions of near-field amplitudes, and the malleability of the underlying shape representations. A numerical approach that combines near-field predictions based on a finite-element formulation for harmonic solutions to the Helmholtz equation with a free-field projection based on the Kirchhoff integral to obtain estimates of the far-field beampattern is reviewed. This method has been used to predict physical beamforming mechanisms such as frequency-dependent beamforming with half-open resonance cavities in the noseleaf of horseshoe bats and beam narrowing through extension of the pinna aperture with skin folds in false vampire bats. The fine structure of biosonar beampatterns is discussed for the case of the Chinese noctule and methods for assessing the spatial information conveyed by beampatterns are demonstrated for the brown long-eared bat.

What noseleaves do for FM bats depends on their degree of sensorial specialization

Background

Many bats vocalizing through their nose carry a prominent noseleaf that is involved in shaping the emission beam of these animals. To our knowledge, the exact role of these appendages has not been thoroughly investigated as for no single species both the hearing and the emission spatial sensitivities have been obtained. In this paper, we set out to evaluate the complete spatial sensitivity of two species of New World leaf-nosed bats: Micronycteris microtis and Phyllostomus discolor. From an ecological point of view, these species are interesting as they belong to the same family (Phyllostomidae) and their noseleaves are morphologically similar. They differ vastly in the niche they occupy. Comparing these species allows us to relate differences in function of the noseleaf to the ecological background of bat species.

Methodology/Principal Findings

We simulate the spatial sensitivity of both the hearing and the emission subsystems of two species, M. microtis and P. discolor. This technique allows us to evaluate the respective roles played by the noseleaf in the echolocation system of these species. We find that the noseleaf of M. microtis focuses the radiated energy better and yields better control over the emission beam.

Conclusions

From the evidence presented we conclude that the noseleaves serve quantitatively different functions for different bats. The main function of the noseleaf is to serve as an energy focusing mechanism that increases the difference between the reflected energy from objects in the focal area and objects in the periphery. However, despite the gross morphological similarities between the noseleaves of the two Phyllostomid species they focus the energy to a different extent, a capability that can be linked to the different ecological niches occupied by the two species.

Acoustic effects accurately predict an extreme case of biological morphology

The biosonar system of bats utilizes physical baffle shapes around the sites of ultrasound emission for diffraction-based beam forming. Among these shapes, some extreme cases have evolved that include a long noseleaf protrusion (sella) in a species of horseshoe bat. We have evaluated the acoustic cost function associated with sella length with a computational physics approach and found that the extreme length can be predicted accurately from a fiducial point on this function. This suggests that some extreme cases of biological morphology can be explained from their physical function alone.

Model-based automated detection of echolocation calls using the link detector

The link detector combines a model-based spectral peak tracker with an echo filter to detect echolocation calls of bats. By processing calls in the spectrogram domain, the links detector separates calls that overlap in time, including call harmonics and echoes. The links detector was validated by using an artificial recording environment, including synthetic calls, atmospheric absorption, and echoes, which provided control of signal-to-noise ratio and an absolute ground truth. Maximum hit rate (2% false positive rate) for the links detector was 87% compared to 1.5% for a spectral peak detector. The difference in performance was due to the ability of the links detector to filter out echoes. Detection range varied across species from 13 to more than 20 m due to call bandwidth and frequency range. Global features of calls detected by the links detector were compared to those of synthetic calls. The error in all estimates increased as the range increased, and estimates of minimum frequency and frequency of most energy were more accurate compared to maximum frequency. The links detector combines local and global features to automatically detect calls within the machine learning paradigm and detects overlapping calls and call harmonics in a unified framework.

Sound-diffracting flap in the ear of a bat generates spatial information

Sound diffraction by the mammalian ear generates source-direction information. We have obtained an immediate quantification of this information from numerical predictions. We demonstrate the power of our approach by showing that a small flap in a bat's pinna generates useful information over a large set of directions in a central band of frequencies: presence of the flap more than doubled the solid angle with direction information above a given threshold. From the workings of the employed information measure, the Cramér-Rao lower bound, we can explain how physical shape is linked to sensory information via a strong sidelobe with frequency-dependent orientation in the directivity pattern. This method could be applied to any other mammal species with pinnae to quantify the relative importance of pinna structures' contributions to directional information and to facilitate interspecific comparisons of pinna directivity patterns.

Numerical study of the effect of the noseleaf on biosonar beamforming in a horseshoe bat

Around 300 bat species are known to emit their ultrasonic biosonar pulses through the nostrils. This nasal emission coincides with the presence of intricately shaped baffle structures surrounding the nostrils. Some prior experimental evidence indicates that these "noseleaves" have an effect on the shape of the animals' radiation patterns. Here, we present a numerical acoustical analysis of the noseleaf of a horseshoe bat species. We show that all three distinctive parts of its noseleaf ("lancet," "sella," "anterior leaf") have an effect on the acoustic near field as well as on the directivity pattern. Furthermore, we show that furrows in one of the parts (the lancet) also exert such an influence. The underlying physical mechanisms suggested by the properties of the estimated near field are cavity resonance, as well as reflection and shadowing of the sound waves emitted by the nostrils. In their effects on the near field, the noseleaf parts showed a tendency toward spatial partitioning with the effects due to each part dominating a certain region. However, interactions between the acoustic effects of the parts were also evident, most notably, a synergism between two frequency-dependent effects (cavity resonance and shadowing) to produce an even stronger frequency selectivity.