New data about semicircular canal morphology and locomotion in modern hominoids

Abstract

The labyrinth has two functional parts: the cochlea for audition and the vestibular system for equilibrioception. In the latter, the semicircular ducts and the otolithic organs are sensitive to rotational and linear accelerations of the head, respectively. The labyrinthine morphology influences perception accuracy, hence the adaptation to a specific locomotor pattern. The aim of this study is to determine the relationship between locomotion and semicircular canal morphology using geometric morphometrics, and to explain these links with existing functional models. The influence of factors other than functional constraints on labyrinthine morphology is discussed. The left bony labyrinth of 65 specimens was extracted virtually. Five extant hominoid species with various locomotion modes were sampled. A set of 13 landmarks was placed on the semicircular canals. After a Procrustes fit, their coordinates were analyzed using a principal component analysis. It was found that labyrinthine morphology is significantly distinct between species. More specifically, the differences involve a posterolateral projection of the lateral semicircular canal and the rotation of this canal relative to the vertical canals. This rotation occurs in the sagittal plane, which is consistent with previous studies based on traditional morphometrics. Among extant hominoids, the shape of the canals potentially discriminates species based on posture. This result could be used to reconstruct the locomotor pattern of fossil hominoids.

Human bony labyrinth is an indicator of population history and dispersal from Africa

“…The morphology of the bony labyrinth has been shown to be a complex indicator of both functional (i.e., locomotor and auditory) adaptations ( 39 – 42 ; but see ref. 43 ) and phyletic relationships among primates and other mammals ( 40 , 44 ).…”

“… 43 ) and phyletic relationships among primates and other mammals ( 40 , 44 ). Recent studies show potential to elucidate morphological differentiation at the level of hominoid species ( 42 ) and subspecies ( 45 ). Morphometric quantification, such as that used in the present study, differentiates labyrinth morphology among great ape species and gibbon ( 42 ), and between subspecies of chimpanzees ( 45 ), although the contributing factors to such shape differences remain to be elucidated.…”

“…Recent studies show potential to elucidate morphological differentiation at the level of hominoid species ( 42 ) and subspecies ( 45 ). Morphometric quantification, such as that used in the present study, differentiates labyrinth morphology among great ape species and gibbon ( 42 ), and between subspecies of chimpanzees ( 45 ), although the contributing factors to such shape differences remain to be elucidated. Patterns of intraspecific variation of the primate labyrinth have only recently been investigated ( 46 ), and the observation that slow-moving species exhibit higher levels of intraspecific variation than fast-moving species has been interpreted as evidence for relaxed stabilizing selection in slow-moving species.…”

Recent inner ear specialization for high-speed hunting in cheetahs

“…This finding contradicts the pattern observed between cheetahs and other felids (i.e., greater angle between ASC and PSC in cheetahs, Supplementary Table 6 ), but it is possible that the greater vertical development of ASC and PSC in cheetahs has a more important influence than their orthogonality on increasing the afferent vestibular sensitivity to roll and pitch head movements (as radius of curvature R, a measure of canal elongation, is still the most common index used to infer an animal’s agility 21 , 37 ). Moreover, the out-of-plane curvature of ASC and PSC could allow the cheetah to sense accelerations of the head in other directions than the main roll and pitch movements 19 , 33 . Further physiological studies on the possible effect of semicircular canal curvature on the canal afferent sensitivity would enhance interpretations of their interrelationships.…”

“…We predict that the extant cheetah species will display a greater volume of its vestibular system, when compared to other felids, given the primary role of the vestibular system in sensing head motions and enabling co-ordination of body movement and gaze direction, essential features for high-speed running (e.g., enlarged utricular volume increasing semicircular canal system afferent sensitivity 29 ). We investigated the variation of the bony vestibular system shape among felids using 3D geometric morphometrics, a powerful method for determining phylogenetic and functional patterns of mammalian inner ears 14 , 18 , 26 , 30 – 33 . Among the shape factors previously identified as related to the sensitivity of the vestibular system afferents are the size of the semicircular canals, and the variations of orthogonality between canals and their degree of out-of-plane curvatures 21 , 26 , 34 – 38 .…”