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© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ketten, D. R., Simmons, J. A., Riquimaroux, H., & Simmons, A. M. Functional analyses of peripheral auditory system adaptations for echolocation in air vs. water. Frontiers in Ecology and Evolution, 09, (2021): 661216, https://doi.org/10.3389/fevo.2021.661216., The similarity of acoustic tasks performed by odontocete (toothed whale) and microchiropteran (insectivorous bat) biosonar suggests they may have common ultrasonic signal reception and processing mechanisms. However, there are also significant media and prey dependent differences, notably speed of sound and wavelengths in air vs. water, that may be reflected in adaptations in their auditory systems and peak spectra of out-going signals for similarly sized prey. We examined the anatomy of the peripheral auditory system of two species of FM bat (big brown bat Eptesicus fuscus; Japanese house bat Pipistrellus abramus) and two toothed whales (harbor porpoise Phocoena phocoena; bottlenose dolphin Tursiops truncatus) using ultra high resolution (11–100 micron) isotropic voxel computed tomography (helical and microCT). Significant differences were found for oval and round window location, cochlear length, basilar membrane gradients, neural distributions, cochlear spiral morphometry and curvature, and basilar membrane suspension distributions. Length correlates with body mass, not hearing ranges. High and low frequency hearing range cut-offs correlate with basilar membrane thickness/width ratios and the cochlear radius of curvature. These features are predictive of high and low frequency hearing limits in all ears examined. The ears of the harbor porpoise, the highest frequency echolocator in the study, had significantly greater stiffness, higher basal basilar membrane ratios, and bilateral bony support for 60% of the basilar membrane length. The porpoise’s basilar membrane includes a “foveal” region with “stretched” frequency representation and relatively constant membrane thickness/width ratio values similar to those reported for some bat species. Both species of bats and the harbor porpoise displayed unusual stapedial input locations and low ratios of cochlear radii, specializations that may enhance higher ultrasonic frequency signal resolution and deter low frequency co, MicroCT scanning, data analyses, and manuscript preparation were assisted by funding to DK from the Joint Industry Program (contract JIP22 III-16-08 – 55205300) and fellowships from the Hanse-Wissenschaftskolleg ICBM Fellowship and the Helmholtz International Fellow research programs. Big brown bat data collection and analysis were supported by an Office of Naval Research grant N00014-14-1-05880 to JS and an Office of Naval Research MURI grant N00014-17-1-2736 to JS and AS. Specimen collection, histology processing, and helical scanning related to the data reported in this study were supported through multiple grants and contracts since 2010 to DK from NIH, N45/LMRS-United States Navy Environmental Division (EnvDiv), Office of Naval Research, and ONR Global.

Echolocation signals emitted by odontocetes can be roughly classified into three broad categories: broadband echolocation signals, narrowband high-frequency echolocation signals, and frequency modulated clicks. Previous measurements of broadband echolocation signal propagation in the bottlenose dolphin (Tursiops truncatus) did not find any evidence of focusing as the signals travel from the near-field to far-field. Finite element analysis (FEA) of high-resolution computed tomography scan data was used to examine signal propagation of broadband echolocation signals of dolphins and narrowband echolocation signals of porpoises. The FEA results were used to simulate the propagation of clicks from phonic lips, traveling through the forehead, and finally transmission into the water. Biosonar beam formation in the near-field and far-field, including the amplitude contours for the two species, was determined. The finite element model result for the simulated amplitude contour in the horizontal plane was consistent with prior direct measurement results for Tursiops, validating the model. Furthermore, the simulated far-field transmission beam patterns in both the vertical and horizontal planes were also qualitatively consistent with results measured from live animals. This study indicates that there is no evidence of convergence for either Tursiops or Phocoena as the sound propagates from the near-field to the far-field.

Author Posting. © BioOne Complete, 2019. This article is posted here by permission of BioOne Complete for personal use, not for redistribution. The definitive version was published in Ziegler, A., Bock, C., Ketten, D. R., Mair, R. W., Mueller, S., Nagelmann, N., Pracht, E. D., & Schroeder, L. Digital three-dimensional imaging techniques provide new analytical pathways for malacological research. American Malacological Bulletin, 36(2), (2018):248-273, doi:10.4003/006.036.0205., Research on molluscan specimens is increasingly being carried out using high-throughput molecular techniques. Due to their efficiency, these technologies have effectively resulted in a strong bias towards genotypic analyses. Therefore, the future large-scale correlation of such data with the phenotype will require a significant increase in the output of morphological studies. Three-dimensional (3D) scanning techniques such as magnetic resonance imaging (MRI) or computed tomography (CT) can achieve this goal as they permit rapidly obtaining digital data non-destructively or even entirely non-invasively from living, fixed, and fossil samples. With a large number of species and a relatively complex morphology, the Mollusca would profit from a more widespread application of digital 3D imaging techniques. In order to provide an overview of the capacity of various MRI and CT techniques to visualize internal and external structures of molluscs, more than twenty specimens ranging in size from a few millimeters to well over one meter were scanned in vivo as well as ex vivo. The results show that all major molluscan organ systems can be successfully visualized using both MRI and CT. The choice of a suitable imaging technique depends primarily on the specimen's life condition, its size, the required resolution, and possible invasiveness of the approach. Apart from visual examples derived from more than two dozen scans, the present article provides guidelines and best practices for digital 3D imaging of a broad range of molluscan taxa. Furthermore, a comprehensive overview of studies that previously have employed MRI or CT techniques in malacological research is given., We would like to express our gratitude to Adam J. Baldinger, Thomas Bartolomaeus, Patrick Beckers, Rüdiger Bieler, Roger T. Hanlon, Carsten Lüter, Iliana Ruiz-Cooley, Tom Schiøtte, Andreas Schmidt-Rhaesa, and Sid Staubach for help with specimen collection or for providing access to museum material. Cornelius Faber, Julia Koch, Tony Stöcker, and W. Caroline West kindly facilitated use of scanning systems. We would also like to thank Julie Arruda, Scott Cramer, Jörg Döpfert, Charlotte Eymann, Bastian Maus, Malte Ogurreck, Christina L. Sagorny, Gillian Trombke, and Christopher Witte for support with data acquisition and analysis. We are particularly grateful to Elizabeth K. Shea for inviting the present contribution and for her extensive commentary on the manuscript. We also thank two anonymous reviewers for their helpful criticisms. Funding for this study was provided by the Ocean Life Institute, the Office of Naval Research, the Seaver Institute, and the Deutsche Forschungsgemeinschaft (INST 217/849-1 FUGG).

The echolocation signals emitted by odontocetes can be roughly classified into three broad categories: broadband high-frequency echolocation signals, narrowband high-frequency echolocation signals, and frequency modulated clicks. Previous measurements of broadband echolocation signals propagation in the bottlenose dolphin (Tursiops truncatus) did not find any evidence of focusing as the signals travel from the near to far-field. Finite element analysis (FEA) of high-resolution CT scan data was used to examine signal propagation of broadband echolocation signals of dolphins and narrowband echolocation signals of porpoises. The FEA results were used to simulate the propagation of clicks from phonic lips, traveling through the forehead, and finally transmission into the seawater. Biosonar beam formation in the near-field and far-field including the amplitude contours for the two species was determined. The finite element model was validated by finding the simulated amplitude contour in the horizontal plane was consistent with prior direct measurement results for Tursiops. Furthermore, the simulated far-field transmission beam patterns in both vertical and horizontal planes were also qualitatively consistent with results measured from live animals. This study shows that there is no evidence of convergence for either Tursiops or Phocoena as the sound propagates from near-field to far-field.