Lizard Ear Research Articles

The middle ear of gekkonoid lizards: interspecific variation of structure in relation to body size and to auditory sensitivity

Wishing to assess the effects of the dimensions of the middle ear on the auditory sensitivity of gekkonoid lizards, we measured middle ear components in preserved geckos, which in life had yielded ‘cochlear microphonics’ audiograms. We examined two to seven specimens of 14 species. The measures of middle ear elements varied relative to head or body length similarly within species and among species. The areas of the external ear opening, tympanic membrane, and columellar footplate, and the ratio between the last two (‘hydraulic lever’), were correlated with animal length. The hydraulic and mechanical (extracolumellar) lever ratios appeared to complement each other, the former being emphasized in large animals, the latter in small animals. The apparent auditory sensitivity correlated with the sizes of the animal, head and external ear opening, and negatively (insignificantly) correlated with the mechanical lever ratio. The correlation of sensitivity with the hydraulic lever was insignificant, perhaps due to a ‘tympanic membrane lever’ (catenary effect). The most sensitive frequency negatively correlated with the area of the external ear opening, the area of the tympanic membrane, and with the level of greatest sensitivity. It was positively correlated with the relative length of the cartilaginous portion of the ossicular chain. However, the number of hair cells in the basilar papilla, too, is known to correlate with animal size. Moreover, the least sensitive species were not only the smallest species, they were also the species known to lack a zone of unidirectional hair cells in the basilar papilla. Hence the apparent sensitivity hypothetically depends on both middle ear dimensions and summation of inner ear output. This hypothesis requires verification by other methods.

Correlated amplitude fluctuations of spontaneous otoacoustic emissions in six lizard species

Spontaneous otoacoustic emissions were recorded from 17 lizard ears (six species: Gerrhosaurus major, Iguana iguana, Basiliscus vittatus, Tupinambis teguixin, Varanus exanthematicus, and Cordylus tropidosternum). The spectrum of each recording contained multiple spectral peaks. For each peak, the envelope cross-correlation R(τ) with all other peaks from the same ear was computed. Of the total of 346 emission peak pairs, 58 (17%) showed a significant correlation. Thus like in humans, multiple emission peaks in lizards frequently interact. In the lizards, the cross-correlation was positive in 30 cases, and negative in 28 cases. The cross-correlation function peaked at either a positive (τpeak>0) or a negative (τpeak<0) delay time. Peak delays ranged from −3.7 ms to 6.2 ms with an average 0.2 ms (s.d. 1.8 ms). The range of observed peak delay values differs from that in human [average τpeak=11.0 ms (s.d. 22.1 ms)].

Statistical properties of spontaneous otoacoustic emissions in one bird and three lizard species

Spontaneous otoacoustic emissions were recorded in the barn owl Tyto alba guttata (five ears), and in three lizard species (Callopistes maculatus, one ear; Varanus exanthematicus, seven ears; Gerrhonotus leiocephalus, one ear). The barn owl ears emitted one or two emission frequencies; the lizard ears had one, two, or three emissions. For each peak in the emission spectrum, an amplitude distribution was obtained. By comparing the spectrum and the histogram, it is concluded that the emission signals obeyed partly sinusoidal and partly noise statistics. The latter is presumably caused by random amplitude fluctuations of the emissions. Four out of 7 owl emissions and 7 out of 16 lizard emissions were dominated by sinusoidal statistics. For these emissions, the relative rms amplitude fluctuation ranged from 0.13 to 0.41 for the barn owl, and from 0.04 to 0.29 for the lizards. These emissions must have been generated by an active oscillator in the ear. In one barn owl ear, mutual suppression of two neighboring emissions was observed. This caused one of the two emissions to deviate substantially from sinusoidal statistics. The 11 remaining emissions were dominated by noise statistics, presumably due to large amplitude fluctuations. For these emissions, our data are not conclusive on whether an active oscillator is involved in the emission generation.

Filtering of distortion-product otoacoustic emissions in the inner ear of birds and lizards

When the output magnitude of more than one order of distortion-product otoacoustic emission (DPOAE) is measured, they reach their maximum at the same DPOAE frequency. This fact led several authors to the assumption that, subsequent to their generation in the cochlea, the DPOAE are band-pass filtered. It was suggested that the tectorial membrane is the structure responsible for this filtering. In this report, we show that the same kind of ‘DPOAE tuning’ is shown by animals which have hearing organs with tectorial structures of very different morphology, or even with no tectorial membrane at all. We therefore conclude that it is unlikely that the filter is the tectorial membrane.

Acoustic input-admittance of the alligator-lizard ear: nonlinear features.

The acoustic input-admittance at the alligator lizard's tympanic membrane varies with stimulus level; the magnitude of the variation can be as much as a factor of three. At 1.6 kHz, the frequency of maximum admittance magnitude, the admittance varies when the stimulus level exceeds 65 dB SPL. At frequencies above or below 1.6 kHz, larger SPLs are needed to produce admittance changes. With stimulus frequencies below 0.3 kHz or above 4.0 kHz the admittance is virtually constant for stimulus levels up to at least 100 dB SPL. The nonlinear behavior (a) is greatly reduced when the cochlear partition is destroyed, (b) does not return when the mechanical load of the partition is replaced, (c) is decreased by the introduction of proteolytic enzymes into the inner ear, and (d) is not affected by some manipulations that greatly reduce cochlear potentials. The results suggest that the mechanical properties of the cochlear partition are the source of the nonlinear admittance. Parallels between this phenomenon and two-tone distortion products in the ear canal (Rosowski et al. (1984): Hearing Res. 13, 141-158) suggest that the same nonlinear mechanical source that generates the level-dependent admittance also produces two-tone distortion products in the lizard ear canal. Published demonstrations of level-dependent admittance in mammalian ears, although rather different from these results, do not rule out the presence of a similar mechanism in the mammalian cochlea.

Micromechanics of the Reptilian Ear

This paper reviews selected physiology and anatomy of the lizard peripheral auditory system and presents the implications of these data for understanding cochlear micromechanics. The reasons for studying the micromechanics of the lizard ear are presented along with the difficulties in conducting such studies in the mammal. Data from the Anguid and Iguanid families of lizards demonstrate the influence of the hair cell system on neural tuning. In addition, evidence from the Iguanid suggests that cilium length may help determine neural tonotopic organization. In general, the reptilian research presented shows the importance of the hair cell system for determining the tuning of associated nerve fibers and in understanding the transduction process in general.

The lizard ear: Gekkonidae.

AbstractThe gecko ear was studied in 36 species belonging to 24 genera. This receptor has attained an advanced level of structure and performance in this group of lizards, but there are many variations among species. To a large extent these variations follow subfamily lines as represented in Kluge's system of classification.Brief consideration is given to features of the outer and middle ear, but chief concern is with inner ear structures and their relations to auditory sensitivity as represented by the cochlear potentials.The auditory papilla is segmented, with a dorsal portion whose hair cells have their ciliary tufts attached to a tectorial membrane, and a ventral portion in which these cells form tow assemblages, one with tectorial connections and the other with connections to a line of sallets.The dorsal segment varies greatly in length and in the form of ciliary orientation. In Eublepharinae and most Gekkoninae the ciliary orientation is unidirectional, and the degree of sensitivity relates to the length of this segment. In Diplodactylinae and Sphaerodactylinae the orientation is bidirectional, and this segment functionally hardly differs from the ventral segment.Auditory sensitivity as measured in terms of the cochlear potentials shows close relations with subfamily groupings, except for the Gekkoninae in which considerable diversity is found.The evidence from structural differentiation, along with that derived from the forms of the cochlear potential functions, leads to the suggestion that these ears possess a high degree of pitch discrimination and capability for the analysis of complex sounds.

The lizard ear: Scincidae

AbstractObservations on inner ear structure were made in five species of Scincidae, together with measurements of auditory sensitivity in terms of cochlear potentials. The basilar membrane and auditory papilla show a characteristic form, with considerable uniformity in dimensions except for a moderate expansion in the dorsal region and a more prominent one at the ventral end.A characteristic feature is the presence of a tectorial membrane that covers a large part of the surface of the medial limbus, but never leaves this surface and thus fails to make any contact with the auditory papilla. Hair‐cell stimulation is achieved entirely through operation of the inertia principle (or equivalent principles) by means of a chain of sallets extending along most of the cochlea but giving way in the region of the ventral expansion to a single large body, the culmen papillae.The sensitivity varies in the five skinks studied from better than average to some‐what below average in comparison with other lizard species. Thus an inertial (or inertia‐like) system of hair‐cell stimulation compares favorably with the tectorial membrane (restraint) system exhibited in the ears of most other lizards and all the higher animals.

Types of Mechanisms for Hair-Cell Stimulation

The final step in the mechanical action of sounds on the ear is the production of relative motion between the ciliary tufts of the hair cells and the bodies of these cells. This relative motion is achieved in three principal ways, depending upon three types of accessory structures in contact with the ciliary tufts. In Type I (tectorial or restraint system) each ciliary tuft is connected with a membrane that is firmly anchored to an immobile part of the cochlear wall, and thus is restrained from moving when the cell body moves. In Type II (sallet or inertial system), each tuft is attached to the undersurface of a body that provides inertia and thereby limits the ciliary motion. In Type III (combined system), each tuft is attached to a massive body that provides inertia and also is anchored to the cochlear wall. All three types of stimulating systems are found in lizard ears in great variety. Other reptiles, and also birds and mammals, employ one or another of these systems. Illustrations will be given of these types of stimulating arrangements, with discussion of their special characteristics and the relative performances of the different ears in sound reception.

The lizard ear: Cordylus, Platysaurus and Gerrhosaurus.

AbstractIn further consideration of the lizard ear, the fine structure of the cochlea has been investigated and related to auditory sensitivity in members of the family Cordylidae. The ear of this group of lizards is unusual in that a tectorial membrane is present only in a modified and seemingly vestigial form, and this membrane makes no connections with the auditory hair cells. These cells are provided instead with a series of sallets, small bodies extending in a single row through the dorsal and middle regions of the cochlea, where they rest upon the tips of the ciliary tufts and evidently bring about a stimulation of the hair cells because of their inertia. At the ventral end of the cochlea this line of sallets ends, and here is a single, relatively enormous structure, the culmen papillae, that serves a similar purpose for a large group of hair cells. Consideration is given to the manner of stimulation of the auditory sense cells in these species in relation to others with the usual arrangements involving connections between the ciliary tufts and a tectorial membrane.Included also is a study of a species of Gerrhosaurus, which some have included in the cordylid family and others have placed in a family of its own. The cochlear structure in this species is similar to that of the cordylids in many respects but differs in the ventral region, where instead of the culmen there is a heavy tectorial plate, similarly covering a large number of hair cells but connected to a tectorial membrane.The functioning of these ears is assessed in terms of the cochlear potentials, and is found to vary with species from better than average to excellent in comparison with other lizards investigated. The structural differentiation also is of fairly high degree, and hence it appears that ears without tectorial connections, or with such connections only in a limited region of the cochlea, can perform in a highly serviceable manner.

The tectorial membrane of the lizard ear: species variations.

AbstractThe forms of the tectorial membrane and its connections to the ciliary tufts of the hair cells have been studied in detail in 18 species of lizards and, less thoroughly, in three others. This group, which represents eight lizard families, exhibits three forms of tectorial membrane: a complete form that connects to all parts of the auditory papilla, an abbreviated form that makes this connection only in one limited region of the papilla, and a dendritic form in which the distal portion of the membrane subdivides into strands reaching all the hair cells.Sometimes the tectorial membrane connects directly with the tufts of the hair cells, but more often it makes this connection through intermediary structures. Seven types of such intermediary structures have been identified (if we include the sallet, which is not “intermediate” in a strict sense).Detailed descriptions are given of the various forms of tectorial membrane structures and their variations along the auditory papilla in 12 lizard species. The description for Iguana iguana is offered as representative of the iguanid pattern found in ten members of this family.Consideration is given to the functioning of the tectorial membrane, and also of the sallet, in the process of hair‐cell stimulation.

Tonal differentiation in the lizard ear.

Tonal differentiation in the lizard ear.

The tectorial membrane of the lizard ear: types of structure.

AbstractThis study is concerned with the forms of the tectorial membrane in the lizard ear and its manner of attachment to the ciliary tufts of the hair cells. These structures and their variations were observed in 20 species representing eight families of lizards.Three forms of tectorial membrane were found, a continuous form that extends throughout the length of the auditory papilla, an abbreviated form that reaches the papilla only in one region, and a dendritic form that is particularly narrow at first and then branches extensively to supply all the hair cells.Occasionally the lower edge of the tectorial membrane makes direct connections with the hair tufts. More often there are special connecting structures between the membrane and the hair tufts. Seven types of these structures were identified, as follows: (1) simple fibers, (2) open network, (3) heavy network, (4) fiber plate, (5) finger processes, (6) sallets, and (7) remote connections. These types of tectorial connections are described and illustrated.

Electrical output of lizard ear: relation to hair-cell population.

Cochlear potentials measured in several species of lizard show a close correlation between maximum electrical output and number of hair cells, whereas there is no uniform relation to sensitivity. These results are interpreted as indicating structural differentiation and frequency discrimination in spatial terms in the more advanced lizard ears.

THE INNER EAR OF LIZARDS. I. GROSS STRUCTURE.

THE INNER EAR OF LIZARDS. I. GROSS STRUCTURE.

THE SOUND-CONDUCTING SYSTEMS OF LIZARDS WITHOUT TYMPANIC MEMBRANES

The possible reconstruction of the captorinomorph and Pelycosaur middle ear can be approached from a comparative anatomical point of view. If there were no tympanic membrane in these forms, as had been supposed (Watson, 1953), its absence must have had an effect on the structure of the sound-conducting parts of the internal ear. In another group of reptiles, the lizards, a tympanic membrane is frequently absent and the detailed structure of the internal ears of these forms can be studied with relative ease. The purpose of the present investigation is therefore to study the structure of the internal ear of lizards and to note particularly the modifications that can be associated with the absence of a tympanic membrane or middle ear cavity.

22. Evolutionary Changes in the Middle Ear of certain Agamid and Iguanid Lizards

Proceedings of the Zoological Society of LondonVolume B108, Issue 3 p. 543-549 22. Evolutionary Changes in the Middle Ear of certain Agamid and Iguanid Lizards Malcolm A. Smith M.R.C.S., F.Z S., Malcolm A. Smith M.R.C.S., F.Z S. Department of Zoology, British Museum (Natural History)Search for more papers by this author Malcolm A. Smith M.R.C.S., F.Z S., Malcolm A. Smith M.R.C.S., F.Z S. Department of Zoology, British Museum (Natural History)Search for more papers by this author First published: October 1938 https://doi.org/10.1111/j.1469-7998.1938.tb08530.xCitations: 2AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature VolumeB108, Issue3October 1938Pages 543-549 RelatedInformation