Cochlear morphology is correlated with endocranial volume in catarrhine primates

The cochlear dimensions relevant to hearing capability generally follow a pattern of allometric scaling with body mass in mammals. Nevertheless, humans exhibit cochlear dimensions larger than expected from the body mass, and it has been proposed that this is related to encephalization. In this study, we investigate whether the link between brain mass and cochlear morphology is unique to humans and whether cranial features other than brain mass impinge on the cochlear morphology in catarrhine primates that exhibit considerable variation in brain mass. Cochlear and cranial dimensions (cochlear length, oval window area, endocranial volume, cranial size, and interaural distance) were measured on three-dimensional surface models for 30 hominoid individuals (humans, chimpanzees, gorillas, orangutans, and gibbons) and 20 cercopithecine individuals (Japanese macaques, mandrills, and baboons). Relations between cochlear and cranial variables were evaluated using generalized linear mixed model (GLMM) and phylogenetic generalized least squares (PGLS) analyses. The endocranial volume better predicts cochlear morphology than the other cranial measurements in catarrhines. Specifically, the average cochlear length per turn exhibits a strong correlation, while the oval window area exhibits a weaker correlation with the endocranial volume in both GLMM and PGLS. Humans do not deviate from the common allometric pattern of cochlear length against endocranial volume in the catarrhine primates studied here. It appears that humans do not necessarily represent an exceptional cochlear morphology in catarrhine primates after accounting for the cranial dimensions and phylogeny. Our data thus indicate that the link between endocranial volume and cochlear morphology could be a more general pattern for catarrhine primates than previously recognized.