FISH, F.E.*; SMELSTOYS, J.; BAUDINETTE, R.V.; REYNOLDS, P.S.: Fur doesn't fly, it floats: buoyancy of hair in semi-aquatic mammals

The evolution from a terrestrial existence to an aquatic lifestyle in mammals required the development of adaptations for locomotion and stability in water. We postulated that the non-wettable fur of semi-aquatic mammals, in addition to providing thermal insulation in water, also aids in buoyancy control. The buoyancy characteristics of the fur and hair morphology were examined for a variety of terrestrial mammals, including Didelphis and Rattus, and semi-aquatic mammals, including Castor, Enhydra, Hydromys, Lutra, Mustela, Ondatra, and Ornithorhynchus. We determined buoyancy by mounting fur samples onto circular glass plates and weighed underwater. Histological sections of the skin were made to determine hair density. Buoyancy was dependent on hair density, but was not correlated with hair length or thickness. Semi-aquatic mammals had greater hair density and buoyancy than terrestrial mammals. Enhydra displayed the greatest hair density (1188.8 hairs/mm2) with a buoyant force of 83.13 N/m2, whereas Rattus had the lowest hair density (95.2 hairs/mm2) and Didelphis had the lowest fur buoyancy of 10.4 N/m2. High hair density of non-wettable fur entraps large amounts of low-density air, and thus provides semi-aquatic mammals with positive buoyancy and decreases the effort needed to float.