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

Abstract

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.

FIG. 1

Compiled adult SFOAE data for both Gekko and Hemidactylus, evoked using a stimulus level of L_p = 20 dB SPL. Top plots show magnitude and phase (error bars of individual curves excluded for clarity, different individuals are shown via different shading intensities). Average magnitudes (and associated standard error) within octave-wide bins are indicated via the red square symbols (See “Methods”). Approximate noise floor is indicated via the dashed black line. Dashed red line for the phase indicates the N_SFOAEloess trend integrated with respect to frequency, thereby providing the “average” phase response for visual comparison. Some phase curves have been offset vertically for clarity. Bottom plot (N_SFOAE) indicates the phase-gradient delay (in cycles) as computed from the phase response. Only points whose magnitude was at least 10 dB above the noise floor were included. Solid line for N_SFOAE plot is a locally weighted regression (loess) while solid black lines indicate the associated 95% confidence interval (see “Methods”). The total number of unique ears is indicated in the upper left corner of the N_SFOAE plots. All data were obtained with lizards at a steady-state body temperature of ~32–33°C via a regulated heating pad. Other stimulus parameters used: L_s = 35 dB SPL, f_s = f_p + 40 Hz.

FIG. 2

Similar to the previous figure, except data are shown for adult Eublepharis and Coleonyx.

FIG. 3

Similar to the previous figures, except data are shown for juvenile Eublepharis.

FIG. 4

Comparison of SFOAE phase-gradient delays (expressed in # of stimulus cycles, N_SFOAE) across gecko species (see caption to Fig. 1). Same data as shown in previous figures. Species within similar subfamily pairings are grouped by line style: Gekkoninae—dashed lines, Eublepharinae—solid lines. In figure legend, the number in brackets indicates number of unique ears included in the trend. Brackets on left of legend indicate same subfamily pairings as in Table 1. Shaded regions indicate associated 95% confidence interval. Probe level, L_p = 20 dB SPL.