The cost of transition: investigating swimming biomechanics of bottlenose dolphins

Understanding how often and how effectively animals locomote is essential for accurate estimates of time-energy budgets. Yet, effectively quantifying locomotor costs for marine animals during dynamic swimming remains challenging. Here, we investigated swimming energetics and biomechanics of bottlenose dolphins (Tursiops truncatus) during periods of transient and consistent speed swimming. Measured kinematics (speed, depth, acceleration and orientation) served as inputs for a hydrodynamic model of a bottlenose dolphin that was used to estimate propulsive power and energetic cost of transport. These estimates of propulsive power were verified with respirometry-based estimates of metabolic cost during a prescribed swimming task. Over 1246 active-fluking and consistent-speed periods of swimming were segmented from tag data for the analysis. Swimming speeds ranged from 1.8 to 5.0 m s-1 and the thrust power ranged from 0.1 to 2.0 kW. Prescribed swimming tasks were characterized by extended periods of transient and continuous fluking, with the transient movements requiring approximately 27% more power output than consistent speed swimming. In contrast, the animals tended to self-select locomotor speeds and a transient fluke-and-glide gait that minimized cost of transport (0.47 body lengths s-1, or 1.09 m s-1). Faster speeds require a larger power output to overcome hydrodynamic drag, but the magnitude of acceleration to reach these faster swimming speeds is also costly. To reduce energetic cost, the animals swam at slower speeds using a fluke-and-glide gait characterized by periods of fluking to accelerate the body followed by extended glides.