Cornering in the Water: An Investigation of Dolphin Swimming Performance
Marine mammal biomechanics research has focused on straight-line swimming at consistent speeds, resulting in a lack of knowledge about how animals select movement strategies to balance cost vs performance during tasks like cornering. In this work we examine performance, maneuverability and cost tradeoffs for bottlenose dolphins (Tursiops truncatus) during prescribed swimming. During the task, animals completed two straight-line sections of swimming with a cornering event (180-degree turn). Movement kinematics were measured with a biologging tag (speed, orientation, and depth), and used to estimate the path of the animal during cornering events using a dead reckoning approach. A hydrodynamic model was used to estimate thrust power and energetic cost during lap swimming. Three animals performed the same swimming task, but the path, cornering strategy, and speed varied between individuals. From the kinematic analysis, TT02 was the fastest lap swimmer, with the highest average lap speed, along with the largest energetic cost. TT01 selected a strategy that reduced energetic cost by sacrificing task performance; the animal took about 1.5 times longer to finish each lap compared to TT02 (36 s vs 23 s). TT03 swam more slowly than TT02 (28 s vs 23 s), but at a 50% reduction in cost per lap. The improved efficiency seen in TT03's movement strategy was the result of reducing transient costs during the lap. This included selecting cornering trajectories that balanced trade-offs between distance traveled and speed loss during the turn. Results from this work provide new insight into maneuverability and movement strategies that the dolphins adopt to balance performance and cost during movement, and provide new knowledge for the design and control of bio-inspired marine robotic systems.