Fig. 3. Role of differential tissue stiffness in biomechanics of the Hydra somersault.
(A) The stiffer shoulder region is depicted as a hypothetical spring. The Hydra and the deformations in this spring are shown as somersaults to reach the upside-down position. (B) The forces acting on the Hydra body column during the somersault. FB represents the force of buoyancy in water acting against the gravitational force on the organism Fg. F D is the drag force acting against the direction of motion. F bend is the representation of the force acting on the head region due to energy stored in the bend. The changes in force as the Hydra goes from stage 1/2 (dotted outline) to stage 4 (dotted outline)/5 can be seen. (C) A schematic illustration of the calculation of energy stored in the bent shoulder (U; U=YIL/2R2, where I is the second moment of inertia). The shoulder region of the Hydra was fitted to a circle to measure the radius of curvature (R) of the bend. The length (L) is 25% of the total length (the stiff region), and Y is the measured Young’s modulus. (D) The progression of energy in the bend (E bend) with time after release (t=0). E bend first increases to a peak and then exponentially decays as the bend straightens to bring the body to an upside-down position. The continuous line represents the fit to a single exponent (τ≈0.4 s, n=1). See also Table S2.