Acoustic effects accurately predict an extreme case of biological morphology

Echolocating bats rapidly adjust their mouth gape to control spatial acquisition when scanning a target

BACKGROUND

As well known to any photographer, controlling the "field of view" offers an extremely powerful mechanism by which to adjust target acquisition. Only a few natural sensory systems can actively control their field of view (e.g., dolphins, whales, and bats). Bats are known for their active sensing abilities and modify their echolocation signals by actively controlling their spectral and temporal characteristics. Less is known about bats' ability to actively modify their bio-sonar field of view.

RESULTS

We show that Pipistrellus kuhlii bats rapidly narrow their sensory field of view (i.e., their bio-sonar beam) when scanning a target. On-target vertical sonar beams were twofold narrower than off-target beams. Continuous measurements of the mouth gape of free-flying bats revealed that they control their bio-sonar beam by a ~3.6 mm widening of their mouth gape: namely, bats open their mouth to narrow the beam and vice versa.

CONCLUSIONS

Bats actively and rapidly control their echolocation vertical beam width by modifying their mouth gape. We hypothesize that narrowing their vertical beam narrows the zone of ensonification when estimating the elevation of a target. In other words, bats open their mouth to improve sensory localization.

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.

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.

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.

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.

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.

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.