The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)

Semicircular canal morphology as a predictor of platyrrhine locomotor behavior


1Evolutionary Anthropology, Duke University, 2Department of Radiology, Duke University Medical Center

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Rotations of the head are detected by the semicircular canals, and it is understood that canal morphology is functionally related to animal movement. A study of 210 mammals has shown a significant correlation between “agility rank,” a qualitative surrogate for head rotation, and the paired variables mean canal radius of curvature (R) and body mass. Recent measurement of head rotations in strepsirrhines, however, indicates that mean angular head velocity (AVM) is significantly correlated with canal orthogonality, not with R and body mass. One possible explanation for this discrepancy is that agility ranks do not adequately describe the head rotations that serve as the stimulus to the semicircular canals.

To explore variation in canal orthogonality and its implications for head movements outside of strepsirrhines, we measured canal orthogonality in 11 extant platyrrhine species. Values of canal orthogonality in platyrrhines fall within the observed strepsirrhine range. Using the strepsirrhine equation to estimate platyrrhine AVM, we predict Callicebus and Pithecia experience the fastest average head rotations, similar to that of the bush baby Galago moholi. At the opposite extreme, the wooly monkey Lagothrix is predicted to exhibit the slowest head rotations — most closely resembling Eulemur. These findings suggest a lack of correspondence between the kinematic variable AVM and behaviorist’s perceptions of agility or locomotor categories—wooly monkeys climb and suspend below branches whereas Eulemur is an above-branch arboreal quadruped. Moreover, the apparently slow-moving howler Alouatta has highly orthogonal canals. These inconsistencies highlight the need for in vivo head movement data from platyrrhines.

Support from NSF 0824546 to RFK and MDM.

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