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      Bone conduction and seismic sensitivity in golden moles (Chrysochloridae)

      Journal of Zoology
      Cambridge University Press (CUP)

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          Auditory systems of Heteromyidae: Functional morphology and evolution of the middle ear

          Middle ears (515) from 26 species of the rodent family Heteromyidae - genera Dipodomys, Microdipodops, Perognathus, and Liomys - were studied both grossly and histologically, for qualitative and quantitative comparisons. Middle ear modifications characteristic of each genus are qualitatively described. Quantitative comparisons are made among the 26 species in the study. Some correlations between middle ear size and other measurements are discussed. The middle ear is an acoustical transformer that for best efficiency must match the impedance of the cochlea to the impedance of the air in the external auditory meatus. It accomplishes this by a pressure increase and a velocity decrease through the combined effects of the lever and areal ratios; however, because the important consideration is a matching of two impedances rather than an absolute pressure increase, the pressure transformer ratio is a less informative measure of the middle ear's efficiency than is the impedance transform ratio. The impedance transformer mechanism is explained (from a morphological point of view), and equations are presented. Dipodomys, Microdipodops, and Perognathus have a theoretical transmission (at the resonant frequency) of 94-100% of the incident acoustical energy; Liomys, 78-80%. The areal ratio of stapes footplate to 2/3 tympanic membrane is remarkably constant among the species, varying only from 0.04 to 0.07: in Dipodomys and Microdipodops this small ratio is due to the very large tympanic membrane; in Perognathus and Liomys it is due to the extremely small stapes footplate. The lever ratio of incus to malleus varies from 0.28 to 0.33 in Dipodpmys and Microdipodops, from 0.37 to 0.46 in Perognathus, and from 0.55 to 0.60 in Liomys. In addition, the middle ear volumes and the morphology of tympanic membrane, ossicles, ligaments, and muscles, all combine to minimize both mass and stiffness. All these data suggest middle ear mechanisms which are very efficient over a broad frequency range. The middle ear modifications found in heteromyids are adaptive in predator avoidance, especially in areas of little natural cover; nevertheless, contrary to expectations, there is no firm relationship between habitat and the extent of these modifications in the 26 species. However, environment did apparently plan an important role in the evolution of the family, and this is discussed.
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            Seismic properties of Asian elephant (Elephas maximus) vocalizations and locomotion.

            Seismic and acoustic data were recorded simultaneously from Asian elephants (Elephas maximus) during periods of vocalizations and locomotion. Acoustic and seismic signals from rumbles were highly correlated at near and far distances and were in phase near the elephant and were out of phase at an increased distance from the elephant. Data analyses indicated that elephant generated signals associated with rumbles and "foot stomps" propagated at different velocities in the two media, the acoustic signals traveling at 309 m/s and the seismic signals at 248-264 m/s. Both types of signals had predominant frequencies in the range of 20 Hz. Seismic signal amplitudes considerably above background noise were recorded at 40 m from the generating elephants for both the rumble and the stomp. Seismic propagation models suggest that seismic waveforms from vocalizations are potentially detectable by instruments at distances of up to 16 km, and up to 32 km for locomotion generated signals. Thus, if detectable by elephants, these seismic signals could be useful for long distance communication.
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              Scaling of the mammalian middle ear.

              This study considers the general question how animal size limits the size and information receiving capacity of sense organs. To clarify this in the case of the mammalian middle ear, I studied 63 mammalian species, ranging from a small bat to the Indian elephant. I determined the skull mass and the masses of the ossicles malleus, incus and stapes (M, I and S), and measured the tympanic membrane area, A1. The ossicular mass (in mg) is generally negatively allometric to skull mass (in g), the regression equation for the whole material (excluding true seals) being y = 1.373 x(0.513). However, for very small mammals the allometry approaches isometry. Within a group of large mammals no distinct allometry can be discerned. The true seals (Phocidae) are exceptional by having massive ossicles. The size relations within the middle ear are generally rather constant. However, the I/M relation is slightly positively allometric, y = 0.554 x(1.162). Two particularly isometric relations were found; the S/(M + I) relation for the ossicles characterized by the regression equation y = 0.054 x(0.993), and the relation between a two-dimensional measure of the ossicles and the tympanic membrane ares, (M + I)2/3 /A1. As in isometric ears the sound energy collected by the tympanic membrane is linearly related to its area, the latter isometry suggests that, regardless of animal size, a given ossicular cross-sectional area is exposed to a similar sound-induced stress. Possible morphological middle ear adaptations to particular acoustic environments are discussed.
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                Author and article information

                Journal
                Journal of Zoology
                J. Zoology
                Cambridge University Press (CUP)
                09528369
                14697998
                April 2003
                : 260
                : 4
                : 405-413
                Article
                10.1017/S0952836903003868
                765c5700-1064-43ae-80ba-2bbfe19cae01
                © 2003

                http://doi.wiley.com/10.1002/tdm_license_1.1

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