Cochlear shape reveals that the human organ of hearing is sex-typed from birth

Abstract

Sex differences in behavioral and neural characteristics can be caused by cultural influences but also by sex-based differences in neurophysiological and sensorimotor features. Since signal-response systems influence decision-making, cooperative and collaborative behaviors, the anatomical or physiological bases for any sex-based difference in sensory mechanisms are important to explore. Here, we use uniform scaling and nonparametric representations of the human cochlea, the main organ of hearing that imprints its adult-like morphology within the petrosal bone from birth. We observe a sex-differentiated torsion along the 3D cochlear curve in samples of 94 adults and 22 juvenile skeletons from cross-cultural contexts. The cochlear sexual dimorphism measured in our study allows sex assessment from the human skeleton with a mean accuracy ranging from 0.91 to 0.93 throughout life. We conclude that the human cochlea is sex-typed from an early post-natal age. This, for the first time, allows nondestructive sex determination of juveniles' skeletal remains in which the biomolecules are too degraded for study but in which the petrosal is preserved, one of the most common bone within archaeological assemblages. Our observed sex-typed cochlear shape from birth is likely associated with complex evolutionary processes in modern humans for reasons not yet fully understood.

Figure 1

Unsupervised learning (clustering) (left) and tangent principal component analysis (TPCA) (right) of cochlear curves, demonstrating a clear distinction between males and females along PC1, irrespective of country of sampling (France or South Africa) or geographic origin (Euro: European, Afro: African).

Figure 2

Fréchet means allowing us to visualize important shape differences between the female (left) and male (right) 3D cochlear curves shown here in exactly the same reference system and orientation.

Figure 3

Fréchet means (top row) in females (left) and males (right) shown with a color-coded torsion function. Functions (bottom row) of the female to male index of torsion (left) and the torsion to curvature index (right) along the cochlear curve from basal (first decile) to apical (10^th decile) locations.