Sound localization in the lizard using internally coupled ears: A finite-element approach

Area of Science:

• Comparative bioacoustics
• Vertebrate auditory systems
• Evolution of hearing

Background:

• Non-mammalian vertebrates typically have a single middle ear ossicle, limiting hearing frequencies and sound localization.
• Mammals evolved three middle ear ossicles, enhancing high-frequency hearing and sound localization.
• Non-mammalian heads often have air-filled cavities that acoustically couple the middle ears.

Purpose of the Study:

• To investigate the acoustic responses and sound-source localization capabilities of the coupled middle ears in the brown anole (Anolis sagrei).
• To utilize finite-element modeling to simulate the auditory system of the brown anole.
• To understand how interaural connections and inner-ear loading affect sound localization.

Main Methods:

• Constructed 3D finite-element models of the middle ear and interaural cavity using high-resolution micro-CT data.
• Validated the middle ear model by matching experimental data and anatomical features.
• Simulated acoustic responses and sound localization capabilities of both isolated and coupled middle ear models.

Main Results:

• The interaural connection amplifies interaural intensity differences, enhancing sound-source localization.
• The interaural canal doubles the phase differences between eardrums, effectively increasing the head's size for localization.
• Inner-ear loading reduced peak velocity ratios but broadened directionality plateaus, impacting both sound perception and localization.

Conclusions:

• The coupled middle ear system in the brown anole significantly enhances sound-source localization capabilities compared to isolated ears.
• Interaural connections and inner-ear loading play crucial roles in auditory perception and spatial hearing in non-mammals.
• This study provides insights into the evolution of hearing and sound localization mechanisms in terrestrial vertebrates.