Clavicular curvature and locomotion in anthropoid primates: A 3D geometric morphometric analysis

Objectives: As a component of the primate shoulder, the clavicle is expected to reflect locomotor adaptations. Whereas previous work has generally focused on clavicular length and torsion, the shape of clavicular curvature may better distinguish taxa and provide additional information about upper limb use in locomotion. This study uses three-dimensional geometric morphometrics to analyze shape differences in the curvatures of the clavicle in different locomotor groups of anthropoid primates. Methods: Sliding semi-landmarks were placed on clavicles of 10 Anthropoid primate species (total n-=-85) that display a range of locomotor behaviors. Landmarks (k-=-39) were chosen to capture the overall curvature of the clavicle in three dimensions. Results: The degree of ventral curvature in the clavicle represents a gradient from most-curved in suspensory genera (e.g., Ateles, Hylobates, and Pongo) to least-curved in genera that are rarely suspensory (e.g., Papio and Gorilla). This curvature may allow an increased range of craniodorsal movement without the clavicle impinging on the thoracic outlet. An inferior curvature of the medial clavicle is found in hominoids and brachiators. This curvature could help stabilize the shoulder and prevent superior dislocation of the clavicle in suspension. Finally, a superior curvature in the lateral part of the clavicle, most pronounced in quadrupedal monkeys, may be related to the relative position of the scapula and sternum. Conclusions: Patterns of clavicular curvature in anthropoid primates reflect locomotor behavior and successfully distinguished among taxonomic and locomotor groups. In the future, this method could be used to assess locomotor behavior in fossil primates.