Fig. 10. Pelvic fin steady swimming functional hypotheses. The complete pelvic oscillation cycle of left fin is represented in two identical polar plots. Body excursion (black), fin area (green), fin angle with transverse plane (dark blue), and fin angle with sagittal plane (light blue) are represented on the plots. 0 deg. arbitrarily represents the start of the stroke when the fin is held against the body. 180 deg. represents mid-stroke, when the outside fin tip is maximally abducted. Thick bars represent maximum values for each variable and thin bars represent minimum values. Data represent the left pelvic fins of all fish during all swimming trials. Widened areas on bars represent the mean and 95% confidence interval; thin lines represent the angular variance s² (Batschelet, 1965; Batschelet, 1981). (A) Motion relative to the transverse plane. (i) As the pelvic fin supinates towards the transverse plane it actively pushes against the oncoming flow producing a braking force. (ii) The fin passively adducts away from the transverse plane owing to water drag, stabilizing and straightening the body in the flow. (B) Motion relative to the sagittal plane. (i) As the pelvic fin adducts away from the sagittal plane it moves in the same direction as the body, actively pushing against the induced flow and producing a lateral force in the direction the body is oscillating. This force may act to dampen body oscillation, helping to slow and reverse body motion. (ii) As the body changes direction the fin continues supinating away from the sagittal plane, passively moved by body induced water flow. This motion maximally supinates the fin, preparing for the next active cycle. (iii) The fin begins pronating towards the sagittal plane, in the same direction as body oscillation, against body induced flow. This produces a lateral force in the direction of body motion with maximum fin area dampening the body oscillation and helping to reverse the body direction. (iv) Body oscillation changes direction while the fin continues pronation. Induced flow due to body motion passively moves the fin to maximize pronation towards sagittal plane, preparing for the lateral force production of the next stroke.