The cuticle structure of the wool hairs (secondary hairs) of six otter species was examined by scanning electron microscopy to clarify the specific function of this hair type in the Lutrinae. The species studied were chosen according to the different genera, climatic regions, and degrees of association to water of the Lutrinae. Independent of their preferred habitats, the cuticle of every wool hair examined exhibited in all animals a rather similar shape and arrangement of the scales. This specific adaptive feature allows a flexible interlocking of adjacent wool hairs, which also helps to form thin wool hair bundles that surround small oval shaped spaces. Thus, the trapping of an effective insulating air layer is facilitated and heat loss from the body is reduced.