The transition from a terrestrial to an aquatic lifestyle in mammals required the evolution of specific adaptations for locomotion and stability in water. We postulated that the non-wettable fur of semi-aquatic mammals is an important adjunct to buoyancy control. Fur buoyancy characteristics and hair morphology were examined for two terrestrial mammals (opossum, Didelphis virginiana and Norway rat, Rattus norvegicus), and seven semi-aquatic mammals, (beaver, Castor canadensis; sea otter, Enhydra lutris; Australian water rat, Hydromys chrysogaster; river otter, Lutra canadensis; American mink, Mustela vison; muskrat, Ondatra zibethicus; and platypus, Ornithorhynchus anatinus). We determined buoyancy hydrostatically, measured hair density on histological skin samples, and hair length and diameter. Buoyancy was positively correlated with hair density, but was not with hair length or thickness. As expected, buoyancy was considerably greater in aquatic mammals than in terrestrial mammals. Enhydra displayed the greatest hair density (1188.8 hairs/mm2) with a buoyant force of 0.94 N, whereas Rattus had the lowest hair density (95.2 hairs/mm2) and Didelphis had the lowest fur buoyancy of 0.12 N. High hair density of non-wettable fur traps large amounts of air and thus provides semi-aquatic mammals with positive buoyancy and decreases the effort needed to float.Frank E. Fish, Jennifer Smelstoys, Russell V. Baudinette, and Penny S. Reynold
