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Comparative kinematics of the forelimb during swimming in red-eared slider (Trachemys scripta) and spiny softshell (Apalone spinifera) turtles

Cinnamon M. Pace, Richard W. Blob^* and Mark W. Westneat

Department of Zoology, Division of Fishes, Field Museum of Natural History, 1400 South Lake Shore Drive, Chicago, IL 60605, USA

View larger version (148K): [in a new window]   Fig. 1. Ventral views of the right forelimb of Trachemys scripta, the red-eared slider (A), and Apalone spinifera, the spiny softshell turtle (B). Scale bars, 1 cm.

View larger version (132K): [in a new window]   Fig. 2. Frames from video footage of swimming turtles during forelimb protraction (recovery phase). (A–C) Trachemys scripta, lateral view. (D–F) T. scripta, ventral view. (G–I) Apalone spinifera, ventral view. (J–L) A. spinifera, lateral view. The grid scale is 1 cm in each frame. Movements of the left forelimb were analyzed; thus, in lateral views, the focal forelimb is the one closest to the viewer, but in ventral views (which were filmed using a mirror) the focal forelimb is the one closest to the bottom of the video frame. Lateral and ventral views for each species are from the same experimental trial. Images in each column depict an approximately equivalent instant in the locomotor cycle: left-hand column (A,D,G,J), early recovery phase; center column (B,E,H,K), mid recovery phase; right-hand column (C,F,I,L), late recovery phase. The white arrow in K points to the forelimb in A. spinifera, which is difficult to see in still lateral frames while the forefoot is feathered. Note that video frames illustrated in Fig. 3 depict thrust phase from the same swimming trials, so that sequential viewing of Fig. 2 and Fig. 3 will allow examination of entire kinematic cycles for turtle forelimbs during swimming.

View larger version (134K): [in a new window]   Fig. 3. Frames from video footage of swimming turtles during forelimb retraction (thrust phase). (A–C) Trachemys scripta, lateral view. (D–F) T. scripta, ventral view. (G–I) Apalone spinifera, ventral view. (J–L) A. spinifera, lateral view. The grid scale is 1 cm in each frame. As in Fig. 2, in lateral views the focal forelimb is the one closest to the viewer, but in ventral views the focal forelimb is the one closest to the bottom of the video frame. Lateral and ventral views for each species are from the same experimental trial, and the trials illustrated are the same as those illustrated in Fig. 2. As in Fig. 2, images in each column depict an approximately equivalent instant in the locomotor cycle: left-hand column (A,D,G,J), early thrust phase; center column (B,E,H,K), mid thrust phase; right-hand column (C,F,I,L), late thrust phase.

View larger version (29K): [in a new window]   Fig. 4. Mean kinematic profiles of humerus protraction and retraction (i.e. angle from the transverse plane) during turtle swimming. (A) Trachemys scripta, (B) Apalone spinifera. Each kinematic trial was normalized to the same duration, and values of kinematic angles were interpolated for 100 equally spaced increments through the stroke cycle (Walker, 1998), allowing mean angles and standard errors to be calculated for each 1 % increment through the stroke for each species. Angle values ±1 S.E.M. are plotted for every second increment (every 2 % through the cycle); N=15 trials for each species. Angles of 0° indicate that the humerus is perpendicular to the anteroposterior midline of the turtle, with negative angles indicating that the distal end of the humerus is directed posteriorly and positive angles indicating that the distal end of the humerus is directed anteriorly. An angle of +90°, for example, indicates that the distal end of the humerus is directed straight ahead of the turtle. The bold vertical line in each plot demarcates recovery phase (R) from thrust phase (T).

View larger version (29K): [in a new window]   Fig. 5. Mean kinematic profiles of humerus elevation and depression (i.e. angle from the horizontal plane) during turtle swimming. (A) Trachemys scripta, (B) Apalone spinifera. The format and method of profile calculation are the same as in Fig. 4. Angles of 0° indicate a horizontal humerus, while positive angles indicate that the distal end of the humerus is elevated above the proximal end, and negative angles indicate that the distal end of the humerus is depressed below the proximal end. The bold vertical line in each plot demarcates recovery phase (R) from thrust phase (T).

View larger version (27K): [in a new window]   Fig. 6. Mean kinematic profiles of elbow extension and flexion (true angle in three dimensions) during turtle swimming. (A) Trachemys scripta, (B) Apalone spinifera. The format and method of profile calculation are the same as in Fig. 4. Larger angles indicate greater elbow extension: 180° would indicate a straight elbow joint. The bold vertical line in each plot demarcates recovery phase (R) from thrust phase (T).

View larger version (29K): [in a new window]   Fig. 7. Mean kinematic profiles of wrist dorsiflexion and palmarflexion (true angle in three dimensions) during turtle swimming. (A) Trachemys scripta, (B) Apalone spinifera. The format and method of profile calculation are the same as in Fig. 4. Angles of 0° indicate a straight wrist joint, while positive angles indicate dorsiflexion and negative angles indicate palmarflexion. The bold vertical line in each plot demarcates recovery phase (R) from thrust phase (T).

View larger version (33K): [in a new window]   Fig. 8. Mean kinematic profiles of forefoot (paddle) orientation during turtle swimming. (A) Trachemys scripta, (B) Apalone spinifera. The format and method of profile calculation are the same as in Fig. 4. The angle plotted is the angle between a vector pointing forwards along the anteroposterior midline of the turtle and a vector emerging from the palmar surface of a plane defined by the wrist and the tips of digits 1 and 5 on the turtle manus, transformed by subtracting 90° from each value. Thus, a low-drag orientation of the forefoot (perfect feathering) is indicated by an angle of 0°, whereas a high-drag forefoot orientation, with the palmar surface of the manus facing in the direction opposite to the direction of travel, is indicated by an angle of 90°. The bold vertical line in each plot demarcates recovery phase (R) from thrust phase (T).