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

Vocalization by extant nonavian reptiles: A synthetic overview of phonation and the vocal apparatus

Among amniote vertebrates, nonavian reptiles (chelonians, crocodilians, and lepidosaurs) are regarded as using vocal signals rarely (compared to birds and mammals). In all three reptilian clades, however, certain taxa emit distress calls and advertisement calls using modifications of regions of the upper respiratory tract. There is no central tendency in either acoustic mechanisms or the structure of the vocal apparatus, and many taxa that vocalize emit only relatively simple sounds. Available evidence indicates multiple origins of true vocal abilities within these lineages. Reptiles thus provide opportunities for studying the early evolutionary stages of vocalization. The early literature on the diversity of form of the laryngotracheal apparatus of reptiles boded well for the study of form-function relationships, but this potential was not extensively explored. Emphasis shifted away from anatomy, however, and centered instead on acoustic analysis of the sounds that are produced. New investigative techniques have provided novel ways of studying the form-function aspects of the structures involved in phonation and have brought anatomical investigation to the forefront again. In this review we summarize what is known about hearing in reptiles in order to contextualize the vocal signals they generate and the sound-producing mechanisms responsible for them. The diversity of form of the sound producing apparatus and the increasing evidence that reptiles are more dependent upon vocalization as a communication medium than previously thought indicates that they have a significant role to play in the understanding of the evolution of vocalization in amniotes.

Comparison of otoacoustic emissions within gecko subfamilies: morphological implications for auditory function in lizards

Otoacoustic emissions (OAEs) are sounds emitted by the ear and provide a non-invasive probe into mechanisms underlying peripheral auditory transduction. This study focuses upon a comparison of emission properties in two phylogenetically similar pairs of gecko: Gekko gecko and Hemidactylus turcicus and Eublepharis macularius and Coleonyx variegatus. Each pair consists of two closely related species within the same subfamily, with quantitatively known morphological properties at the level of the auditory sensory organ (basilar papilla) in the inner ear. Essentially, the comparison boils down to an issue of size: how does overall body size, as well as the inner-ear dimensions (e.g., papilla length and number of hair cells), affect peripheral auditory function as inferred from OAEs? Estimates of frequency selectivity derived from stimulus-frequency emissions (emissions evoked by a single low-level tone) indicate that tuning is broader in the species with fewer hair cells/shorter papilla. Furthermore, emissions extend outwards to higher frequencies (for similar body temperatures) in the species with the smaller body size/narrower interaural spacing. This observation suggests the smaller species have relatively improved high-frequency sensitivity, possibly related to vocalizations and/or aiding azimuthal sound localization. For one species (Eublepharis), emissions were also examined in both juveniles and adults. Qualitatively similar emission properties in both suggests that inner-ear function is adult like soon after hatching and that external body size (e.g., middle-ear dimensions and interaural spacing) has a relatively small impact upon emission properties within a species.

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.

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.