Fig. 1. Schematic summary of two alternative models illustrating the forces acting on the body of a shark during steady horizontal swimming. (A) Modified version of the classical model (with data on body angle and pectoral fin function incorporated from Wilga and Lauder, 2000, 2001) in which the beating of the tail is proposed to generate an upward lift force (F_tail) that generates a torque around the center of mass (shaded circle). Force on the water is directed posteroventrally (F_water), and an equal and opposite reaction force is directed anterodorsally, dorsal to the center of mass (F_reaction). Torques generated by the tail are countered by equal and opposite torques resulting from lift forces produced by the body (F_body), which has a positive angle of attack during horizontal locomotion. The net upward lift forces are balanced by the weight (F_weight) of the negatively buoyant shark. The pectoral fins do not generate lift during steady horizontal locomotion (Wilga and Lauder, 2000, 2001) and, hence, no forces are shown acting on these fins. (B) Modified version of the model of Thomson (1976) (to include our previously published data on shark body angle and pectoral fin function) in which the tail generates a reaction force that is directed anteriorly through the center of mass.

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