Humans, mammals like any other? The case of the bony labyrinth

The morphology of the osseous molding of the inner ear – the bony labyrinth – is used as an indicator of phylogenetic/geographic relatedness, ecological adaptations, or sex estimation in both modern and fossil hominins. Ear shape variation is also observed at larger taxonomic scales, with a huge disparity across mammals. This disparity may be due to an important evolvability of the mammalian ear, resulting from its genetic, developmental and anatomical complexity relative to other vertebrates. In this exploratory study, we tested whether the human ear, while morphologically distinct from other mammals, follows similar patterns at the intraspecific level. We compared our species (n=29) to squirrels (n=26), buzzards (n=29) and sparrows (n=26). We quantified bony labyrinth morphology using geometric morphometrics (14 anatomical landmarks, 91 semilandmarks). We conducted separate principal component analysis for each species to compare intraspecific variation, and separate two-block partial least square analyses for mammals and birds to compare morphological integration between the cochlear and vestibular systems of the labyrinth. While the overall variation for the human labyrinth is larger than any other species, the main component of shape variation is localised in the cochlea, as for squirrels, and unlike birds. The association between cochlear and vestibular systems is stronger and has a higher dimensionality in mammals, compared to birds. Whereas the integration patterns are different between mammals and birds, we found similar patterns for the two mammal species. These similarities in intraspecific variation and integration of the labyrinth between humans and squirrels are clearly related to the coiled shape of the cochlea, a synapomorphy of therian mammals. If confirmed for other hominids, these results may contribute to a better understanding of developmental patterns in fossil hominins and help to resolve controversies about the taxonomic attribution of some fossils.