INFLUENCE OF HYDRODYNAMIC-DESIGN AND PROPULSIVE MODE ON MAMMALIAN SWIMMING ENERGETICS

Optimisation of energy by aquatic mammals requires adaptations that reduce drag, and improve thrust production and efficiency. Drag is minimised by streamlining the body and appendages. Highly derived aquatic mammals have body shapes close to the optimal hydrodynamic design for drag reduction. There is no conclusive evidence for specialised drag reduction mechanisms, although decreasing hair density is associated with reduced drag. Improvement in thrust production and efficiency is accomplished by changes in propulsive mode and appendage design. Semiaquatic mammals employ. drag-based propulsion using paddles, whereas fully aquatic mammals use lift-based propulsion with hydrofoils. Because paddling generates thrust through half the stroke cycle, propulsive efficiency is low and energetic cost is high compared with that for mammals using hydrofoils. Lift-based swimming is a rapid and high-powered propulsive mode. Oscillations of the hydrofoil generate thrust throughout the stroke cycle. For cetaceans and pinnipeds, propulsive efficiency is approximately 80%, and transport cost is below that of semiaquatic mammals. Behavioural adaptations help minimise energy expenditure by swimming mammals. Submerged swimming avoids increased drag from energy lost in formation of surface waves. Porpoising and wave riding, characteristic of dolphins, can reduce the transport costs, allowing for longer-duration swimming at high speeds.