Fish scale the surface, interacting only with the riblet tips, not causing any high-veloctiy flow in the valleys. Since this high velocity flow now only interacts with the riblet-tip, which is a very small surface area, the momentum transfer which causes drag is now much lower than before, thereby effectively reducing drag. Also, this reduces the cross-stream velocity fluctuations, which aids in momentum transfer too. The rough, sandpaper-like texture of shark and ray skin, coupled with its toughness, has led it to be valued as a source of rawhide leather, called shagreen. One of the many historical applications of shark shagreen was
Kangaroo leather fibres are often at angles as much as 90 degrees to the skin surface. Cattle hide also contains sweat glands, erector pili muscles and a distinct gradation in elastin levels, concentrated in the upper part of the skin. Kangaroo hide on the other hand has been shown to have a highly uniform orientation of fibre bundles in parallel with the skin surface. It does not contain sweat glands or erector pili muscles and elastin is evenly distributed throughout the skin thickness. This structural uniformity explains both the greater tensile strength of the whole leather and the greater retention of strength