Fig. 6. The effect of motion on visibility. (A) An optimal static camouflage pattern where the pattern matches the background, and the orientation minimises the number of lines obscured. Unfortunately, if the animal moves, this pattern is far from optimal: the relative motion between the stripes on the animals back and the background is highly visible, and there is a strong relative motion signal together with occlusions along the side (marked by the arrows). (B) A pattern without stripes minimises motion signals. Further, orientating the body orthogonally to the substrate, the area of high relative motion and occlusion is minimised. (C) Sand ripples in the natural environment are oriented at 90° to the water flow (Ayrton, 1910). Thus an orientated substrate provides important information about the current. Similar principles apply to shadows cast by waves. Orientating the body in the direction shown by the arrow, as was observed, minimises drag, and maximises the efficiency with which the animals can compensate for involuntary movements.