The diving physiology of bottlenose dolphins (Tursiops truncatus) - II. Biomechanics and changes in buoyancy at depth

During diving, marine mammals must balance the conservation of limited oxyg en reserves with the metabolic costs of swimming exercise. As a result, ene rgetically efficient modes of locomotion provide an advantage during period s of submergence and will presumably increase in importance as the animals perform progressively longer dives. To determine the effect of a limited ox ygen supply on locomotor performance, we compared the kinematics and behavi or of swimming and diving bottlenose dolphins. Adult bottlenose dolphins (T ursiops truncatus) were trained to swim horizontally near the water surface or submerged at 5 m and to dive to depths ranging from 12 to 112 m. Swimmi ng kinematics (preferred swimming mode, stroke frequency and duration of gl ides) were monitored using submersible video cameras (Sony Hi-8) held by SC UBA divers or attached to a pack on the dorsal fin of the animal. Drag and buoyant forces were calculated from patterns of deceleration for horizontal ly swimming and vertically diving animals. The results showed that dolphins used a variety of swimming gaits that correlated with acceleration. The pe rcentage of time spent gliding during the descent phase of dives increased with depth, Glide distances ranged from 7.1+/-1.9 m for 16 m dives to 43.6/-7.0 m (means +/- S.E.M.) for 100 m dives. These Sliding patterns were att ributed to changes in buoyancy associated with lung compression at depth. B y incorporating prolonged glide periods, the bottlenose dolphin realized a theoretical 10-21% energetic savings in the cost of a 100 m dive in compari son with dives based on neutral buoyancy models, Thus, modifying locomotor patterns to account for physical changes with depth appears to be one mecha nism that enables diving mammals with limited oxygen stores to extend the d uration of a dive.