Walking and paddling motions were studied in a semiaquatic bird, the ringed teal (Callonetta leucophrys), to investigate the motions associated with movements in two environments with radically divergent physical properties. A three-dimensional (3D) kinematic reconstruction based on nonsynchronous biplanar cineradiographic data was used to quantify the 3D trajectories of the body and hind limb segments. Our study revealed that two subsystems interact to provide propulsion in water and on land. During paddling, the trunk, the femur, and the tibiotarsus are in a stable position and play the role of the hull. The femur and tibiotarsus are positioned laterally and parasagittaly and the intertarsal joint is fixed and positioned caudally allowing large amplitude movements of the "paddle" (tarsometatarsus and palmate foot). During walking, the center of mass is held above the medially oriented foot, providing stability during the single support phase. During stance, the foot is medially oriented because of the lateral and parasagittal positions of the tibiotarsus and tarsometatarsus during both walking and paddling. This position of the foot during walking imposes trunk translation and results in the typical waddling motion of Anatidae. This study provides new insights into how waddling motion relates to semiaquatic birds' ability to move in both terrestrial and aquatic environments.
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