Acoustic communication is widespread in animals. According to the sensory drive hypothesis [Endler JA (1993) Philos Trans R Soc Lond B Biol Sci 340(1292): [215][216][217][218][219][220][221][222][223][224][225], communication signals and perceptual systems have coevolved. A clear illustration of this is the evolution of the tetrapod middle ear, adapted to life on land. Here we report the discovery of a bone conduction-mediated stimulation of the ear by wave propagation in Sechellophryne gardineri, one of the world's smallest terrestrial tetrapods, which lacks a middle ear yet produces acoustic signals. Based on X-ray synchrotron holotomography, we measured the biomechanical properties of the otic tissues and modeled the acoustic propagation. Our models show how bone conduction enhanced by the resonating role of the mouth allows these seemingly deaf frogs to communicate effectively without a middle ear.earless frog | audition | extra-tympanic pathways | X-ray imaging T he middle ear evolved multiple times independently during the evolution of terrestrial life (1). Indeed, a tympanic middle ear is an adaptation to life on land (2, 3) and compensates for the mismatch in acoustic impedance between air and tissue. Without it, 99.9% of sound energy is reflected by the body wall (4, 5). Sechellophryne gardineri is one of the world's smallest terrestrial tetrapods (6). These sooglossid frogs have a Gondwanan origin and evolved in isolation on the Seychelles Archipelago over the last [47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65]8). They lack a middle ear yet produce acoustic signals (9, 10). In fact, similar to many species of frogs lacking the tympanic middle ear, they are still capable of hearing in air. Nevertheless, the mechanisms for sound transfer to the inner ear are far less clear. Some extratympanic pathways (11) such as the lungs (12), the opercular system (13), and bone conduction (14) have been proposed but remain to be tested experimentally (11). X-ray synchrotron holotomography of a female S. gardineri reveals that the pulmonary system is poorly developed and cannot contribute significantly to a lung-based sound transmission pathway. We used finite-difference simulations to investigate possible pathways through the head itself. These simulations highlighted the role of the mouth. Finite-element simulations further showed that the oral cavity of the animals resonates at the dominant frequency of the advertisement call of the species. In addition, synchrotron holotomography performed on seven different species showed that earless frogs are specialized for sound transmission between the oral cavity and the ears in two ways: (i) by minimizing the tissue thickness between the mouth and the inner ear and (ii) by minimizing the number of layers of tissue. The combination of these extratympanic pathways allows the frog to perceive sound efficiently.
This work deals with the determination of the shape of a generally-non-circular impenetrable cylinder from the way it scatters incident sound. A complete family (of generally non-orthogonal functions) representation of the scattered field is employed to match the total measured field. The resolution of the direct problem during the inversion is bypassed by assuming a priori that the coefficients in the field representation are locally those of an impenetrable circular cylinder. These coefficients are known explicitly to within a single parameter which is determined by resolution of a nonlinear equation. This parameter is none other than the length of the position vector joining the origin to the given point on the boundary of the cylinder, so that by varying the locations of the field measurement point and boundary point one generates a discrete form of the polar coordinate parametric equation of the boundary. Numerical examples of the results of the inversion scheme are given for cylinders with both convex (circular, elliptical) and non-convex boundaries.
This work addresses the inverse problem of the identification of a passive three-dimensional impenetrable object in a shallow-water environment. The latter is assumed to have flat perfectly reflecting ͑sound-soft top and sound-hard bottom͒ boundaries and therefore acts as a guide for acoustic waves. These waves are employed to interrogate the object and the scattered acoustic wavefield is measured on the surface of a ͑virtual͒ vertically oriented cylinder ͑of finite or infinite radius, corresponding to near-or far-field measurements͒ fully enclosing the object. The direct scattering problem is resolved in approximate manner by employing, in a local manner, the known separated-variable solution for a scattering by a vertically oriented cylinder in a perfect waveguide. The inverse problem is resolved in the same manner ͑i.e., with the same approximate field ansatz͒ by least-squares matching of theoretical fields ͑for trial objects͒ to the measured field. Examples are given of successful shape reconstructions for two types of immersed objects. This manner of solving approximately both the forward and inverse problems is generalized to the case of a body of shallow water with an elastic seabed.
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