The anatomy, physiology, functional significance and evolution of specialized hearing organs of gerbilline rodents

Abstract

Middle and inner ear anatomy correlates with neurophysiological responses to a wide range of sound frequencies for species of the Gerbillinae representing generalized, intermediate, and specialized anatomical conditions. Neurophysiological data were recorded from 81 specimens of 13 species representing six genera. Anatomical parameters involved in the process of hearing were correlated with the neurophysiological data to assess the effects of different degrees of anatomical specialization on hearing. The 13 species tested in this manner have graphic curves of auditory sensitivity of remarkably similar disposition over the frequencies tested and to those published for Kangaroo Rats. Ears with anatomical specializations show greater auditory sensitivity.The natural history of the Gerbillinae, particularly the kinds of predators, degree of predation, and habitat is reviewed and utilized to interpret the significance of the degree of auditory specialization in the forms studied and to evaluate the prevailing hypothesis that these specializations enhance the ability of these rodents to survive in open desert situations by detecting and evading predators.The middle ear anatomy of five additional genera and species was also studied. Thus, data on the entire spectrum of gerbilline middle ear morphology provide a n evolutionary sequence. Certain anatomical parameters of the organ of Corti show a degree of specialization parallel to that of features of the middle ear. The morphological changes and possible functional roles of these features are considered.A very high correlation exists for degree of specialization and aridity of habitat, thus specialization increases with increasing aridity. This increased specialization may result from more effective predation in open xeric environments. Auditory acuity for a wide range of low frequency sounds augmented by auditory specialization is hence more advantageous here. There does not appear to be selection for hearing at particular frequencies in this range. The peaks of greatest auditory sensitivity appear to correspond to the resonant frequencies of the different components of the middle ear transformer and cavity.

Primate auditory diversity and its influence on hearing performance

“…It is also possible the mechanical impedance of the middle ear, determined by the mass, stiffness, and frictional resistance of the middle ear components, differs between the two groups. Until these discrepancies are more clearly understood, this finding should serve as a cautionary note to researchers who use ITRs as direct measures of hearing function in comparative studies (Lay, 1972;Hunt and Korth, 1980;Masali et al, 1992).…”

“…The ITR is often considered an ideal transformer ratio because it ignores the intrinsic impedance of the components of the auditory system itself (Dallos, 1973), and several researchers have found a lack of association between the ITR or its component ratios (e.g., areal convergence ratio) and measures of auditory sensitivity (Lay, 1972;Rosowski, 1994). However, it provides a useful starting point for analyzing middle ear function and continues to be used by investigators as a proxy for estimating middle ear performance (Webster and Webster, 1975;Hunt and Korth, 1980;Masali et al, 1992).…”

Structures that contribute to middle-ear admittance in chinchilla

“…Is this combination of large Y TOC and large air space of any adaptive value in the arid open slopes of the high Andes that are the native habitat of these animals? There is a long history of the association of large middle-ear air spaces and open-arid environments (Heim de Balsac 1936;Legouix and Wisner 1955;Lay 1972;Webster and Webster 1975; Webster and Plassman 1992;Huang et al 2002). The basic tenet of this association is that the atmospheric absorption of sounds of frequencies above 2000 Hz is greatly increased in low-humidity (see e.g., Huang et al 2002) with the result that only sounds of lower frequencies carry for any distance.…”

“…A common misuse of the ideal transformer model is the attempt to extrapolate auditory thresholds from measurements of ossicular lengths and tympanic-membrane areas in groups of species (e.g. Lay 1972;Webster and Webster 1975). To our knowledge no significant correlations between anatomical transformer measurements and auditory thresholds have been reported, though the lack of such correlations has been documented (Rosowski and Graybeal 1991).…”

“…Heim de Balsac 1936;Legouix and Wisner 1955). The families and species most investigated in this regard are the Heteromyidae (kangaroo rats and their relatives (Webster 1961(Webster , 1962 Webster 1975, 1984; Webster and Plassman 1992)), Gerbillinae (Lay 1972(Lay , 1974Ravicz, Rosowski and Voigt 1992) and the sand cat, Felis margarita (Huang et al 2002). These studies found a consistent association between anatomical specializations, i.e.…”