Schematics of the most common form of packing of river otter underhairs, showing how hairs interlock. A: Individual hair. B: Two adjacent hairs. C: Two hairs with fins fitting into grooves. D: Top view of two hairs. E: Cross section of underhair packing.

Credit: John W. Weisel, Chandrasekaran Nagaswami, Rolf O. Peterson, University of Pennsylvania School of Medicine; Michigan Technological University; NRC Research Press