Intermittent Swimming by Mammals: A Strategy for Increasing Energetic Efficiency During Diving

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Abstract
The evolutionary history of marine mammals involved marked physiological and morphological modifications to change from terrestrial to aquatic locomotion. A consequence of this ancestry is that swimming is energetically expensive for mammals in comparison to fish. This study examined the use of behavioral strategies by marine mammals to circumvent these elevated locomotor costs during horizontal swimming and vertical diving. Intermittent forms of locomotion, including wave-riding and porpoising when near the water surface, and prolonged gliding and a stroke and glide mode of propulsion when diving, enabled marine mammals to increase the efficiency of aquatic locomotion. Video instrumentation packs (8-mm camera, video recorder and time-depth microprocessor) deployed on deep diving bottlenose dolphins (Tursiops truncatus), northern elephant seals (Mirounga angustirostris), and Weddell seals (Leptonychotes weddellii) revealed exceptionally long periods of gliding during descent to depth. Glide duration depended on depth and represented nearly 80% of the descent for dives exceeding 200 m. Transitions in locomotor mode during diving were attributed to buoyancy changes with compression of the lungs at depth, and were associated with a 9–60% reduction in the energetic cost of dives for the species examined. By changing to intermittent locomotor patterns, marine mammals are able to increase travelling speed for little additional energetic cost when surface swimming, and to extend the duration of submergence despite limitations in oxygen stores when diving.