Shake rattle and roll: the bony labyrinth and aerial descent in squamates.

Controlled aerial descent has evolved many times independently in vertebrates. Squamates (lizards and snakes) are unusual in that respect due to the large number of independent origins of the evolution of this behavior. Although some squamates such as flying geckos of the genus Ptychozoon and the flying dragons of the genus Draco show obvious adaptations including skin flaps or enlarged ribs allowing them to increase their surface area and slow down their descent, many others appear unspecialized. Yet, specializations can be expected at the level of the sensory and neural systems allowing animals to maintain stability during controlled aerial descent. The vestibular system is a likely candidate given that it is an acceleration detector and is well-suited to detect changes in pitch, roll and yaw. Here we use conventional and synchrotron μCT scans to quantify the morphology of the vestibular system in squamates able to perform controlled aerial descent compared to species characterized by a terrestrial or climbing life style. Our results show the presence of a strong phylogenetic signal in the data with the vestibular system in species from the same family being morphologically similar. However, both our shape analysis and an analysis of the dimensions of the vestibular system showed clear differences among animals with different life-styles. Species able to perform a controlled aerial descent differed in the position and shape of the inner ear, especially of the posterior ampulla. Given the limited stability of squamates against roll and the fact that the posterior ampulla is tuned to changes in roll this suggests an adaptive evolution of the vestibular system in squamates using controlled aerial descent. Future studies testing for similar differences in other groups of vertebrates known to use controlled aerial descent are needed to test the generality of this observation.