The pectoralis muscle orientation as an indicator of the modes of wing-propelled locomotion in birds.

Given the diversification of wingbeat utilisation for body propulsion among birds, it is essential to understand the relationship between skeletal morphology and flapping ability, depending on the wing depression exerted by the pectoralis muscle. The path between the origin and insertion of the pectoralis, which can be identified on the skeletal surface, indicates the orientation of the force it exerts. In this study, line segments were made within the pectoralis to represent the lines of muscle action (LoA). We then compared their overall orientation and distribution among different modes of wing-propelled locomotion (WPL) using three-dimensional skeletal models constructed from computed tomography and surface scans of 79 neornithine bird specimens. The species were selected to represent a diverse range of taxonomic groups, WPL modes, and body masses. The pectoralis LoAs in flap-flight and wing-propelled diving birds were oriented craniolaterally, whereas those in soaring birds were oriented more laterally. These differences in pectoralis profiles among species suggest a putative adaptation to generate forces more efficiently in the direction necessary for typical flapping styles. Therefore, the overall morphology of the thoracic skeleton, which reflects pectoral muscle orientation, can be an indicator of WPL mode. It can also be utilised as a more robust approach for reconstructing the locomotor capabilities of fossil taxa with avian-like musculoskeletal systems.