Comparative morphology of the internal structures of the mammalian cochlea

Abstract Therian mammals possess numerous unique morphological features in their auditory system. Many of these have been well studied in attempts to understand their functional importance, and some aspects of cochlear morphology have been shown to correlate with hearing ability across taxa. Among the unique features of the therian inner ear is the presence of bony structures supporting the sensory epithelia. While these have been implicated in the development of extended high-frequency hearing, their functional role remains unclear. In this study, I use micro-computed tomography (μCT) imaging to take quantitative measurements of the bony spiral laminae and cochlear scalae through the length of the basilar membrane. I present an analysis of the variation showing how these change from the cochlear base to the apex, and how they differ between clades. I find considerable variation in the structures between different clades, particularly in the area of the cochlear scalae which show differences between terrestrial and aquatic species. These results provide a broad survey demonstrating a diversity that could be further exploited to explore functional significance in more detail. A cursory comparison shows correlation between various morphological measures and hearing ability, with the dimensions of the basilar membrane appearing the most useful to make predictions.