Cochlear labyrinth volume and hearing abilities in primates

The primate cochlea is a membranous, fluid-filled receptor organ that is specialized for sound detection. Like other parts of the inner ear, the cochlea is contained within the bony labyrinth of the petrous temporal bone. The close anatomical relationship between the bony cochlear labyrinth and the membranous cochlea provides an opportunity to quantify cochlear size using osteological specimens. Although mechanisms of cochlear frequency analysis are well studied, relatively little is known about the functional consequences of interspecific variation in cochlear size. Previous comparative analyses have linked increases in basilar membrane length to decreases in both the high and low frequency limits of hearing in mammals. However, these analyses did not consider the potentially confounding effects of body mass or phylogeny. Here, we present measurements of cochlear labyrinth volume in 33 primate species based on high-resolution computed tomography. These data demonstrate that cochlear labyrinth volume is strongly negatively allometric with respect to body mass. Scaling of cochlear volume in primates is very similar to scaling of basilar membrane length among mammals generally. Furthermore, an analysis of 10 primate taxa with published audiograms reveals that cochlear labyrinth volume is significantly negatively correlated with the high frequency limit of hearing. This result is independent of body mass and phylogeny, suggesting that cochlear size is functionally related to the range of audible frequencies in primates. Although the nature of this functional relationship remains speculative, our findings suggest that some hearing parameters of extinct taxa may be estimated using fossil petrosals.