103 - Novel Inner Ear Visualizations Offer New Insights into the Function of the Primate Otolith System and Relationships with Phylogeny

Poster Board Number: 103
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Abstract Body : The bony labyrinth of the inner ear contains the sensory organs for balance and hearing. In mammals, this neurosensory complex is housed inside the petrous temporal bone and comprises the semicircular canals (SCCs; detecting angular acceleration), otolith organs (utricle and saccule; detecting linear acceleration, vibration, and head tilt relative to gravity), and cochlea (detecting sound). Due to the high density of the petrous temporal bone, the morphology of the bony labyrinth is often well-preserved in fossils. This can be a useful tool to examine evolutionary trends across mammals. While SCC and cochlear morphology have been well-studied, the key central structure of the bony labyrinth (surrounding the otolith organs; bony vestibule) remains largely unexplored. This limits our understanding of neurosensory evolution within our lineage and potential relationships with locomotion and/or posture.
Using virtual 3D models of 126 inner ears derived from micro-CT scans of 13 extant anthropoid primate species we assess relationships between bony vestibule form (size+shape) and: 1) body mass; 2) phylogeny; and 3) head posture (as the otolith organs are sensors of gravity, from Strait and Ross, 1999). We also assess levels of morphological variation among humans, apes, and monkeys. We visualize bony otolith organ structure using a set of 3D landmark and semilandmark patches on the bony labyrinth which approximate internal organ structure. Principal Components Analyses of Procrustes-superimposed landmark coordinates assess overall shape variation while degree of variation among groups is assessed by measuring Procrustes variance. Phylogenetic signal is estimated using Pagel’s lambda. Ordinary least squares and phylogenetic generalized least squares models determine correlations between bony vestibule form and explanatory variables.
Results indicate that while size and shape both correlate with body mass, only size carries a strong phylogenetic signal while only shape carries a functional signal (i.e., head posture). Further, related to shape, while humans and monkeys show restricted shape variation, considerable variation is present amongst the apes. This variation is related to superoinferior displacement of the otolith organs which indicates the location of the maculae and in turn, sensory hair cell orientation. Such extensive variation reflects significant differences in a key neurosensory system linked with posture and locomotion. New approaches to 3D visualization, such as those used here, are allowing a deeper level of understanding and interpretation than previously available.