Fig. 4.

Mean kinematic profiles of swimming in four species of turtle: Carettochelys insculpta (red squares), rowing Apalone ferox (inverted blue triangles), rowing Trachemys scripta (green triangles) and flapping Caretta caretta (black circles). Data for T. scripta were provided by Rivera and Blob (Rivera and Blob, 2010). Data for C. caretta were provided by Rivera et al. (Rivera, A. R. V. et al., 2011). Each trial was normalized to the same duration and angle values were interpolated to represent 0–100% of the limb cycle. For C. insculpta, A. ferox and T. scripta, the limb cycle is defined as protraction of the humerus followed by retraction; for C. caretta, the limb cycle is defined as elevation of the humerus followed by depression. Mean ± s.e.m. angle values are plotted for every third increment (every 3% through the cycle) for all individuals. Solid vertical lines demarcate the switch from protraction to retraction in A. ferox and T. scripta at 43% of the limb cycle. Dashed vertical lines indicate the switch from protraction to retraction in C. insculpta and from elevation to depression in C. caretta at 51% of the limb cycle. (A) Humeral protraction and retraction (i.e. angle from the transverse plane). An angle of 0 deg indicates that the humerus is perpendicular to the midline of the turtle, while an angle of 90 deg indicates a fully protracted forelimb with the distal end of the humerus directed anteriorly (an angle of –90 deg would indicate a fully retracted forelimb with the distal tip of the humerus directed posteriorly). (B) Humeral elevation and depression (i.e. angle from the horizontal plane). An angle of 0 deg indicates that the humerus is in the horizontal plane. Angles greater than zero indicate elevation above the horizontal (distal end above proximal end) and negative angles indicate depression of the humerus (distal end lower than proximal end). Peak elevation is coincident with peak protraction for T. scripta and C. caretta, meaning that limb protraction happens at the same time as elevation and retraction is concurrent with depression. (C) Elbow flexion and extension. Extension is indicated by larger angles and flexion is indicated by smaller angles. An angle of 0 deg indicates complete flexion, 180 deg indicates a fully extended elbow, and 90 deg indicates that the humerus is perpendicular to the radius and ulna. (D) Forefoot orientation angle is calculated as the angle between a vector pointing forwards along the anteroposterior midline (also the path of travel) and a vector emerging from the palmar surface of a plane defined by the tips of digits 1 and 5 and the elbow; this angle is transformed by subtracting 90 deg from each value. A high-drag orientation of the forefoot paddle with the palmar surface of the paddle directed opposite the direction of travel (and in the same direction as the flow of water) is indicated by a feathering angle of 90 deg, and a perfect low-drag orientation of the forefoot paddle is indicated by a feathering angle of 0 deg.