Abstract
The semi-circular canals in the inner ear sense head rotations. It is widely recognised that the anatomy of the semi-circular canals is often adapted to the species-specific agility, in order to provide the necessary sensitivity. Based on research on mammals, the ellipticity of the semi-circular canal was so far considered as a non-important factor herein. A dataset of 125 squamate species and 156 mammalian species, now shows that the posterior semi-circular canal of squamates is much more elliptical (eccentricities ranging between 0.76 and 0.94) than that of mammals (eccentricities ranging between 0 and 0.71). Fluid-Structure Interaction computer models show that the effect of the ellipticity on sensitivity is strongest in small semi-circular canals. This new insight indicates that the high ellipticity in squamates leads to a severe reduction in sensitivity of up to 45%. In mammals, on the other hand, the reduction in sensitivity is limited to 13%, which is consistent with previous literature that found a limited effect of semi-circular canal ellipticity in mammals. Further, there is a strongly negative correlation between semi-circular canal size and eccentricity in squamates, which is absent in mammals. Hence, the smallest squamates have the most elliptical semi-circular canals. In general, the smaller the semi-circular canal, the less sensitive it is. Therefore, the highly elliptical squamate canals are probably the result of fitting the largest possible canal in small and flat head. Miniaturising the canals while maintaining a circular shape would reduce the sensitivity by another 73% compared to the highly elliptical canals.
Highlights
The semi-circular canals in the inner ear sense head rotations
Is the high ellipticity related to the small size of their head? Is this a property of the Lacertidae family, or is this a more general feature of lizards or even squamates? How does this compare to the ellipticity of the semi-circular canals of mammals? And what is the impact of the highly elliptical semi-circular canals on the sensitivity of the system and how does this relate to the agility of the animals? These questions were investigated by calculating the semi-circular canal ellipticity of a large sample of 125 squamate species and 156 mammalian species
The only mammalian species that we found in the literature with an eccentricity close to, or within, the range of the lacertid lizards, are Tursiops truncates and Spalax microphtalmus
Summary
The semi-circular canals in the inner ear sense head rotations. It is widely recognised that the anatomy of the semi-circular canals is often adapted to the species-specific agility, in order to provide the necessary sensitivity. Fluid-Structure Interaction computer models show that the effect of the ellipticity on sensitivity is strongest in small semi-circular canals. The size[4,5,6,7,8,9], duct diameter[10,11], and anatomical variation[12,13] of the semi-circular canals have been found to be adapted to the sensitivity required by the animal’s agility and locomotion style. This may be due to the fact that the deviations from circularity are small in mammals[21] Such a negative correlation is hypothesized to exist in squamates if they have highly elliptical canals. An in-depth investigation of the influence of ellipticity on sensitivity for a range of semi-circular canal sizes, will be performed using Fluid-Structure Interaction computer simulation models. Since this deformation defines the stimulation of the hair cells in the cupula, the models give a very precise measure of the sensitivity of the system
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