River otter hair structure facilitates interlocking to impede penetration of water and allow trapping of air

Abstract

Unlike many other mammals spending a considerable amount of time in water, river otters (Lutra canadensis (Schreber, 1777)) do not have a thick layer of body fat. Instead, they have a very densely packed layer of thin underhairs. The structure of river otter hair was examined by scanning electron microscopy and polarizing light microscopy. Guard hairs were hollow and became thicker distally and then tapered to a point and had different cuticle scales in proximal and distal regions. The cuticle of the thin underhairs had a striking pattern of sharply sculpted fins with deep grooves between them; usually there were four fins at each level, rotated 45° with respect to those at an adjacent level. Underhairs varied in diameter and the scales were sometimes petal-shaped. Polarizing light microscopy images showed interlocking arrangements of the underhairs that help to impede the penetration of water. Also, these images showed that the grooves between fins or petals of underhairs entrap air bubbles. The structure of the hairs allows them to interact loosely with each other, despite variations in size and structure. Furthermore, the nature of the interactions between the fins and depressions allows space between the hairs that can trap air bubbles to increase the thermal insulation of the otter's coat.