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

Investigation of Age and Gender Effects on the Middle Ear With Wideband Tympanometry in Adults

OBJECTIVES

Several studies have reported the effects of age and gender on the middle ear of adults using wideband acoustic immittance (WAI) that measures middle ear function over a range of frequencies rather than the traditional measures with a single probe tone. Although these results are often based on WAI measurements under ambient pressure, using WAI under varying ear-canal pressures (wideband tympanometry [WBT]) may be able to provide more information about age and gender effects on the middle ear. The aim of this study is to examine the effects of age and gender on the middle ear with WBT in three different age groups consisting of young, middle-aged, and older adults.

DESIGN

A total of 95 adults with normal middle ear function were assessed, including 32 young adults (16 men, 16 women, aged 20 to 39 years), 31 middle-aged adults (15 men, 16 women, aged 41 to 60 years), and 32 older adults (16 men, 16 women, aged 65 to 82 years). WBT measurements were performed from 226 to 8000 Hz using Interacoustics Titan. Energy absorbance data at tympanometric peak pressure (EA TPP ) and ambient pressure (EA AP ) at 1/3 octave frequencies, and resonance frequency (RF) data were analyzed according to age and gender variables.

RESULTS

Analysis results showed that the mean EA TPP was significantly higher from 500 to 794 Hz and at 5040 and 6350 Hz, and significantly lower from 1587 to 3175 Hz in older adults compared with young adults. The mean EA AP was significantly lower from 1587 to 3175 Hz, and significantly higher at 5040 and 6350 Hz in older adults compared with young adults. There was no significant difference in the mean EA TPP and mean EA AP at any frequency between young and middle-aged adults, and middle-aged and older adults. RF was significantly lower in older adults compared with young adults. In all age groups, men had higher mean EA TPP and mean EA AP at lower frequencies and lower mean EA TPP and mean EA AP at higher frequencies than women. Men had slightly lower RF than women in young and older adults, while men had significantly lower RF than women in middle-aged adults.

CONCLUSIONS

This study demonstrated that there are possible age and gender effects on the middle ear that may affect the mechanical transmission of sound. It may be useful to consider this finding in clinical evaluation in adults of different ages and genders, and to establish age- and gender-specific WBT norms in the adult population.

Scaling of ear morphology across 127 bird species and its implications for hearing performance

The dimensions of auditory structures among animals of varying body size can have implications for hearing performance. Larger animals often have a hearing range focused on lower frequencies than smaller animals, which may be explained by several anatomical mechanisms in the ear and their scaling relationships. While the effect of size on ear morphology and hearing performance has been explored in some mammals, anurans and lizards, much less is known about the scaling relationships for the single-ossicle, internally-coupled ears of birds. Using micro- and nano-CT scans of the tympanic middle and inner ears of 127 ecologically and phylogenetically diverse bird species, spanning more than 400-fold in head mass (2.3 to 950 g), we undertook phylogenetically-informed scaling analyses to test whether 12 morphological traits, of functional importance to hearing, maintain their relative proportions with increasing head mass. We then extended our analysis by regressing these morphological traits with measures of hearing sensitivity and range to better understand morphological underpinnings of hearing performance. We find that most auditory structures scale together in equal proportions, whereas columella length increases disproportionately. We also find that the size of several auditory structures is associated with increased hearing sensitivity and frequency hearing limits, while head mass did not explain these measures. Although both birds and mammals demonstrate proportional scaling between auditory structures, the consequences for hearing in each group may diverge due to unique morphological predictors of auditory performance.

Evaluating the drivers and engines of morphological diversification in the invasive gecko Hemidactylus mabouia (Moreau de Jonnès, 1818) (Squamata: Gekkonidae)

Development determines the range of possible phenotypes that can be produced and exposed to selection and has a major role in the evolutionary trajectories of species. Nevertheless, development is itself subject to evolutionary forces. Here, we describe differences at the ontogenetic and population levels in head and limb proportions of the invasive gecko Hemidactylus mabouia, to assess the developmental mechanisms and extrinsic forces associated with morphological diversification during colonization of novel habitats. We have found that allometric trajectories of most skeletal traits remain constant throughout postnatal development. Linear morphometric analysis did not find multivariate differences between ontogenetic stages or sexes. When comparing populations, our results showed that the divergence of the corresponding external measures was explained by shifts in the intercept of static allometry curves, indicating that differences arose early in development. Populations aggregated into two morphological groups that did not correspond to the groups formed on the basis of genetic structure. Using two different approaches, we found support for an adaptive hypothesis when comparing observed patterns of morphological variation with that expected under neutral evolutionary models.

Geographic variation in the matching between call characteristics and tympanic sensitivity in the Weeping lizard

Effective communication requires a match among signal characteristics, environmental conditions, and receptor tuning and decoding. The degree of matching, however, can vary, among others due to different selective pressures affecting the communication components. For evolutionary novelties, strong selective pressures are likely to act upon the signal and receptor to promote a tight match among them. We test this prediction by exploring the coupling between the acoustic signals and auditory sensitivity in Liolaemus chiliensis, the Weeping lizard, the only one of more than 285 Liolaemus species that vocalizes. Individuals emit distress calls that convey information of predation risk to conspecifics, which may respond with antipredator behaviors upon hearing calls. Specifically, we explored the match between spectral characteristics of the distress calls and the tympanic sensitivities of two populations separated by more than 700 km, for which previous data suggested variation in their distress calls. We found that populations differed in signal and receptor characteristics and that this signal variation was explained by population differences in body size. No precise match occurred between the communication components studied, and populations differed in the degree of such correspondence. We suggest that this difference in matching between populations relates to evolutionary processes affecting the Weeping lizard distress calls.

Comparative anatomy of the middle ear in some lizard species with comments on the evolutionary changes within Squamata

The skeleton of the middle ear of lizards is composed of three anatomical elements: columella, extracolumella, and tympanic membrane, with some exceptions that show modifications of this anatomy. The main function of the middle ear is transforming sound waves into vibrations and transmitting these to the inner ear. Most middle ear studies mainly focus on its functional aspects, while few describe the anatomy in detail. In lizards, the morphology of the columella is highly conservative, while the extracolumella shows variation in its presence/absence, size, and the number of processes present on the structure. In this work, we used diaphanized and double-stained specimens of 38 species of lizards belonging to 24 genera to study the middle ear’s morphology in a comparative framework. Results presented here indicate more variation in the morphology of the extracolumella than previously known. This variation in the extracolumella is found mainly in the pars superior and anterior processes, while the pars inferior and the posterior process are more constant in morphology. We also provide new information about the shape of gekkotan extracolumella, including traits that are diagnostic for the iguanid and gekkonid middle ear types. The data collected in this study were combined with information from published descriptive works. The new data included here refers to the length of the columella relative to the extracolumella central axis length, the general structure of the extracolumella, and the presence of the internal process. These characters were included in ancestral reconstruction analysis using Bayesian and parsimony approaches. The results indicate high levels of homoplasy in the variation of the columella-extracolumella ratio, providing a better understanding of the ratio variation among lizards. Additionally, the presence of four processes in the extracolumella is the ancestral state for Gekkota, Pleurodonta, and Xantusiidae, and the absence of the internal processes is the ancestral state for Gekkota, Gymnophthalmidae, and Scincidae; despite the fact that these groups convergently develop these character states, they could be used in combination with other characters to diagnose these clades. The posterior extension in the pars superior and an anterior process with some small and sharp projections is also a diagnostic trait for Gekkota. A more accurate description of each process of the extracolumella and its variation needs to be evaluated in a comprehensive analysis, including a greater number of species. Although the number of taxa sampled in this study is small considering the vast diversity of lizards, the results provide an overall idea of the amount of variation of the middle ear while helping to infer the evolutionary history of the lizard middle ear.

Sound localization by the internally coupled ears of lizards: From biophysics to biorobotics

As they are generally small and only hear low frequencies, lizards have few cues for localizing sound. However, their ears show extreme directionality (up to 30 dB direction-dependent difference in eardrum vibrations) created by strong acoustical coupling of the eardrums, with almost perfect internal transmission from the contralateral ear over a broad frequency range. The activity of auditory nerve fibers reflects the eardrum directionality, so all auditory neurons are directional by default. This suggests that the ensuing neural processing of sound direction is simple in lizards. Even the simplest configuration of electrical analog models-two tympanic impedances connected via a central capacitor-produces directional patterns that are qualitatively similar to the experimental data on lizard ears. Several models, both analytical and (very recently) finite-element models, have been published. Robotic implementations using simplified models of the ear and of binaural comparison show that robust phonotaxic behavior can be generated with little additional processing and be performed by simple (and thus small and cheap) units. The authors review lizard directional processing and attempts at modeling and robotics with a twofold aim: to clarify the authors' understanding of central processing of sound localization in lizards, and to lead to technological developments of bioinspired robotics.

Anatomical influences on internally coupled ears in reptiles

Many reptiles, and other vertebrates, have internally coupled ears in which a patent anatomical connection allows pressure waves generated by the displacement of one tympanic membrane to propagate (internally) through the head and, ultimately, influence the displacement of the contralateral tympanic membrane. The pattern of tympanic displacement caused by this internal coupling can give rise to novel sensory cues. The auditory mechanics of reptiles exhibit more anatomical variation than in any other vertebrate group. This variation includes structural features such as diverticula and septa, as well as coverings of the tympanic membrane. Many of these anatomical features would likely influence the functional significance of the internal coupling between the tympanic membranes. Several of the anatomical components of the reptilian internally coupled ear are under active motor control, suggesting that in some reptiles the auditory system may be more dynamic than previously recognized.

The Acoustic Properties of Low Intensity Vocalizations Match Hearing Sensitivity in the Webbed-Toed Gecko, Gekko subpalmatus

The design of acoustic signals and hearing sensitivity in socially communicating species would normally be expected to closely match in order to minimize signal degradation and attenuation during signal propagation. Nevertheless, other factors such as sensory biases as well as morphological and physiological constraints may affect strict correspondence between signal features and hearing sensitivity. Thus study of the relationships between sender and receiver characteristics in species utilizing acoustic communication can provide information about how acoustic communication systems evolve. The genus Gekko includes species emitting high-amplitude vocalizations for long-range communication (loud callers) as well as species producing only low-amplitude vocalizations when in close contact with conspecifics (quiet callers) which have rarely been investigated. In order to investigate relationships between auditory physiology and the frequency characteristics of acoustic signals in a quiet caller, Gekko subpalmatus we measured the subjects’ vocal signal characteristics as well as auditory brainstem responses (ABRs) to assess auditory sensitivity. The results show that G. subpalmatus males emit low amplitude calls when encountering females, ranging in dominant frequency from 2.47 to 4.17 kHz with an average at 3.35 kHz. The auditory range with highest sensitivity closely matches the dominant frequency of the vocalizations. This correspondence is consistent with the notion that quiet and loud calling species are under similar selection pressures for matching auditory sensitivity with spectral characteristics of vocalizations.

Environmental influences in the evolution of tetrapod hearing sensitivity and middle ear tuning

Vertebrates inhabit and communicate acoustically in most natural environments. We review the influence of environmental factors on the hearing sensitivity of terrestrial vertebrates, and on the anatomy and mechanics of the middle ears. Evidence suggests that both biotic and abiotic environmental factors affect the evolution of bandwidth and frequency of peak sensitivity of the hearing spectrum. Relevant abiotic factors include medium type, temperature, and noise produced by nonliving sources. Biotic factors include heterospecific, conspecific, or self-produced sounds that animals are selected to recognize, and acoustic interference by sounds that other animals generate. Within each class of tetrapods, the size of the middle ear structures correlates directly to body size and inversely to frequency of peak sensitivity. Adaptation to the underwater medium in cetaceans involved reorganization of the middle ear for novel acoustic pathways, whereas adaptation to subterranean life in several mammals resulted in hypertrophy of the middle ear ossicles to enhance their inertial mass for detection of seismic vibrations. The comparative approach has revealed a number of generalities about the effect of environmental factors on hearing performance and middle ear structure across species. The current taxonomic sampling of the major tetrapod groups is still highly unbalanced and incomplete. Future expansion of the comparative evidence should continue to reveal general patterns and novel mechanisms.

Transient evoked otoacoustic emissions (TEOAEs) in Caucasian and Chinese young adults

The goal of this study was to examine the effect of race and gender on transient evoked otoacoustic emission (TEOAE) characteristics. TEOAE amplitude, noise levels, and hearing thresholds were compared in 81 Caucasian (mean age: 27.8 years) and 81 Chinese (mean age: 24.7 years) young adults with normal hearing. TEOAE amplitude was significantly higher in females than males and in the Chinese group than the Caucasian group. Females had better hearing sensitivity than males consistent with TEOAE results. Hearing sensitivity was not statistically different between the two racial groups; however, the interaction between race and hearing thresholds was significant. As the noise levels between the two racial groups were not statistically different, the observed differences are most likely related to differences in middle-ear transmission properties or to differences in cochlear mechanisms. Documentation of gender and racial differences and understanding the underlying mechanism of these differences will not only assist us in understanding how TEOAE will be affected by middle-ear transmission properties but also will help us in establishing normative data in clinical settings.

Effects of age and size in the ears of gekkotan lizards: auditory sensitivity, its determinants, and new insights into tetrapod middle-ear function

Do related, differently sized species differ in size-related structural or functional traits merely because they mature at different points of a uniform allometric ontogenetic growth curve, or do they evolutionarily diverge? We tested ears of gekkotan lizards through experiments distinguishing the two. Auditory sensitivity was assessed by compound action potential (CAP) thresholds in eight species. The best thresholds characterizing species ranged 22-72 dB sound pressure level at 0.5-1.0 kHz. Direct acoustic stimulation of the columella footplate elevated thresholds by 25-50 dB. Intraspecific CAP sensitivity was primarily affected by body length in Eublepharis macularius, but by tympanic-membrane velocity in Oedura marmorata. The chief factor determining middle-ear function (difference in CAP sensitivity before and after middle-ear ablation) was body length in both species. A secondary factor was the middle-ear hydraulic lever ratio in E. macularius, but the mechanical lever in O. marmorata. When intra- and interspecific data were compared, the relation of CAP thresholds to body size in E. macularius resembled the interspecific regression. The intraspecific regression of auditory sensitivity over tympanic membrane velocity in O. marmorata differed from that calculated interspecifically. Hence, the evolutionary contribution to size effects on CAP sensitivity exceeds the ontogenetic contribution. Putatively, body length affects CAP sensitivity through absolute sizes of tympanic membrane and columella footplate. These newly discovered effects join those of the hydraulic lever and (interspecifically) hair-cell number to improve the hearing of larger species that vocally communicate across wider spaces, apparently throughout the Tetrapoda.

Wideband Reflectance Norms for Caucasian and Chinese Young Adults

OBJECTIVE

This study examined differences between the middle ears of two ethnic groups, Caucasian and Chinese young adults with normal hearing, using a new middle-ear measurement technique, wideband energy reflectance. The goal of this study was to determine whether the Chinese group had different middle-ear transmission properties than the Caucasian group.

DESIGN

There were 126 subjects (237 ears) between the ages of 18 and 32 yr, with 62 subjects in the Caucasian group and 64 subjects in the Chinese group. Wideband energy reflectance data were gathered using Mimosa Acoustics (RMS system version 4.0.4.4) wideband reflectance (WBR) equipment.

RESULTS

The Chinese group had significantly lower wideband energy reflectance than their Caucasian counterparts at higher frequencies; however, the Caucasian group had significantly lower energy reflectance at lower frequencies than the Chinese group. The Chinese group also had significantly lower admittance magnitude than the Caucasian group at lower frequencies. Because body size indices are more comparable between Caucasian females and Chinese males, the effect of body size could be potentially adjusted for by comparing the Caucasian female subjects with the Chinese male subjects. The differences observed between the Caucasian and the Chinese groups were no longer significant when the Caucasian female subjects were compared with the Chinese male subjects. Applying the Caucasian norms to four Caucasian adults with surgically confirmed otosclerosis resulted in an improved hit rate compared with the combined Caucasian and Chinese norms and the Chinese-only norms.

CONCLUSION

Body size may play a role in the observed differences between the Caucasian and Chinese groups. The findings of this study suggest that further research is needed to investigate the effects of body size on wideband energy reflectance. It should be noted that factors other than body size may contribute to the observed differences. Chinese individuals may simply have different middle-ear characteristics than Caucasian individuals, which could affect WBR. In the meantime, overall test performance may be improved by using a more homogeneous norm when evaluating the middle ear of Caucasian or Chinese individuals with WBR.

Eye size in geckos: Asymmetry, allometry, sexual dimorphism, and behavioral correlates

The function of the vertebrate eye depends on its absolute size, and the size is presumably adapted to specific needs. We studied the variation of eye size at all levels, from intra-individual to inter-specific, in lid- less, spectacled, gecko lizards (Gekkonomorpha). We mea sured 1,408 museum specimens of 62 species, representing subfamilies Diplodactylinae, Gekkoninae, and Sphaerodactylinae. Intra-individually, eye size showed significant directional asymmetry in Stenodactylus sthenodactylus. A latitudinal study of six species confirmed that during postnatal ontogeny eye size undergoes conventional negative allometry; the slope is steeper among adults than among juveniles, expressing the need of juveniles for relatively larger eyes. Within species with sexual size dimorphism, commonly the larger sex possessed larger eyes in absolute terms but not relative to head-and-body length. Interspecifically, eye size showed negative allometry, with slope significantly steeper than those of intraspecific ontogenetic allometry, again expressing the need of juveniles for relatively larger eyes and showing that eye-size differences among species do not merely result from body-size differences. Finally, adult eye size varied interspecifically in correlation with parameters of behavioral ecology: eyes were significantly larger in nocturnal than in diurnal species, and significantly larger in cursorial than in scansorial species.

Effects of age and size in the ears of gekkonomorph lizards: middle-ear morphology with evolutionary implications

The function of the ear depends in part on its absolute size and internal proportions. Thus, in both young individuals and small species, the middle ear is expected to be allometrically enlarged despite its smaller absolute size. Here we aim to compare the ontogenetic allometry of relevant middle-ear structures as observed within gecko (gekkonomorph lizards) species, with the evolutionary allometry observed interspecifically. These observations also provide middle-ear data for future evaluation of variation in auditory sensitivity. The material comprised 84 museum specimens of geckos, representing nine species of three gekkonomorph subfamilies. The results of dissections and measurements show that different reports notwithstanding, the middle-ear ossicular chain is indeed structured as described for geckos by Werner and Wever. Some sexual dimorphism is indicated, but this requires further study. During postnatal ontogeny, the allometric growth in the ratio of the columellar footplate area to body length differed between the intraspecific and interspecific levels, hence species differences in the middle ear do not merely result from animal size. The ratio of the tympanic membrane area to the columellar footplate area increased during ontogeny. In this, geckos resemble birds and probably also mammals. Similarly, when the comparison was among adults representing different species, the ratio of the tympanic membrane area to the columellar footplate area increased with body size. In this, however, the geckos differed from birds and mammals, in which this ratio varied taxonomically, irrespective of body size. It would thus seem that middle-ear proportions have evolved among geckos to produce small interspecific differences, but among amniote tetrapods they have evolved according to different principles in the classes reptiles, birds, and mammals.

Age effects and size effects in the ears of gekkonomorph lizards: inner ear

Audiograms have indicated greater auditory sensitivity in larger than in smaller geckos; part of this difference, interspecifically and intraspecifically, is explained by middle-ear proportions. To investigate the contribution of the inner ear to the variation in sensitivity, we examined it in museum specimens representing 11 species and three subfamilies. We measured papilla basilaris length, and, when intact, the saccular otoconial mass. Papilla length approximated 1% of rostrum-anus length in large geckos but 2% in small geckos; in some species some inter-aural difference was indicated. Over the lumped material, relative papilla length varied as a function of body length, with highly significant correlation. Similar relations prevailed within each subfamily. However, intraspecifically the correlation of papilla basilaris length with animal size was usually nonsignificant. Hair cell populations assessed from SEM photographs were larger in the larger species but intraspecifically did not relate to an individual's size. Hence interspecifically, the dependence of auditory sensitivity on animal size seems supported by inner-ear differences but intraspecifically this relation derives only from the middle ear. Otoconial mass, as measured by its volume, was correlated with animal length both interspecifically and intraspecifically.

Effects of age and size in the ears of gekkonomorph lizards: middle-ear sensitivity

Previous studies of electrophysiological audiograms in gekkonomorph lizards revealed greater sensitivity in adults than in juveniles. We investigated whether this difference, as far as it is affected by the middle ear, is due to animal age or size. The velocity transfer function of the tympanic membrane(TM) was examined using laser interferometry in nine samples: adults of three large gekkonomorph species, adults of three small species (each related to one of the former), and juveniles of the large species, their sizes matching those of the small-species adults. Each transfer function exhibited an inverted `V'or `U' shape, with the velocity of TM motion peaking in the mid-frequency range and becoming poorer at lower and higher frequencies. Among samples,maximum TM velocity correlated with animal length, perhaps because of a damping change in the larger TM. The frequency at maximum velocity negatively correlated with measurements of TM area. Presumably, with a larger TM area,the best frequency shifted downward because of increased middle-ear mass or decreased stiffness. The bandwidth of the transfer function negatively correlated with animal length, being broader in smaller animals and sharper in larger animals. This effect can be attributed to increased mass, decreased damping, or both. Among the middle-ear morphological measurements, the one most closely correlated with body length was the length of the extracolumellar anchorage at the TM. Among the physiological variables investigated, maximum velocity was negatively correlated with the frequency at which it occurred. The dependence of these transfer function variables on animal and ear size was similar, regardless of whether the comparison was among adults of species of different sizes, or among age classes within a species, so that age differences appear to be largely the result of size differences.