Abstract
One of the means for body insulation in mammals during their transition from terrestrial to semi-aquatic and then aquatic life is based on the use of fur in water as an effective “dry” insulator. Fur functional conditions are changed drastically and instantaneously when fur makes contact with water. The physical mechanisms underlying fur waterproofing were analyzed, and their relations to both fur structure and animal body size were established. The results of the analysis allow us to define to what extent the structural features of fur of a mammal in relation to its body size are capable of trapping an insulating air layer when the animal swims in water. On the basis of our analysis, both the body size of the ancestors of fur seals and the type of thermal insulation changing in the postnatal ontogeny of fur seals are hypothesized.