Turtle Girdles: Comparing the Relationships Between Environment and Behavior on Forelimb Function in Loggerhead Sea Turtles (Caretta caretta) and River Cooters (Pseudemys concinna)

Locomotion in water and on land impose dramatically different demands, yet many animals successfully move in both environments. Most turtle species perform both aquatic and terrestrial locomotion but vary in how they use their limbs. Freshwater turtles use anteroposterior movements of the limbs during walking and swimming with contralateral fore- and hindlimbs moving in synchrony. In contrast, sea turtles swim primarily with "powerstroke" movements, characterized by synchronous forelimb motions while the hindlimbs act as rudders. High-speed video has been used to study powerstroking, but pectoral girdle movements and long-axis rotation (LAR) of the humerus are likely both key components to turtle locomotor function and cannot be quantified from external video. Here, we used XROMM to measure pectoral girdle and humeral movements in a sea turtle (loggerhead, Caretta caretta) compared to the freshwater river cooter (Pseudemys concinna) during terrestrial and aquatic locomotion. The largest difference among species was in yaw of the pectoral girdle during swimming, with loggerheads showing almost no yaw during powerstroking whereas pectoral girdle yaw in the cooter during rowing was over 30°. The magnitude of humeral LAR was greatest during loggerhead powerstroking and the temporal pattern of supination and pronation was opposite from that of cooters. We hypothesize that these kinematic differences are driven by differences in how the limbs are used to power propulsion. Rotations at the glenoid drive the overall patterns of movement in freshwater turtles, whereas glenohumeral LAR in loggerheads is used to direct the position and orientation of the elbow, which is the joint that determines the orientation of the thrust-generating structure (the flipper) in loggerheads.