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Decompression theory has been mainly based on studies on terrestrial mammals, and may not translate well to marine mammals. However, evidence that marine mammals experience gas bubbles during diving is growing, causing concern that these bubbles may cause gas emboli pathology (GEP) under unusual circumstances. Marine mammal management, and usual avoidance, of gas emboli and GEP, or the bends, became a topic of intense scientific interest after sonar-exposed, mass-stranded deep-diving whales were observed with gas bubbles. Theoretical models, based on our current understanding of diving physiology in cetaceans, predict that the tissue and blood N2 levels in the bottlenose dolphin (Tursiops truncatus) are at levels that would result in severe DCS symptoms in similar sized terrestrial mammals. However, the dolphins appear to have physiological or behavioral mechanisms to avoid excessive blood N2 levels, or may be more resistant to circulating bubbles through immunological/biochemical adaptations. Studies on behavior, anatomy and physiology of marine mammals have enhanced our understanding of the mechanisms that are thought to prevent excessive uptake of N2. This has led to the selective gas exchange hypothesis, which provides a mechanism how stress-induced behavioral change may cause failure of the normal physiology, which results in excessive uptake of N2, and in extreme cases may cause formation of symptomatic gas emboli. Studies on cardiorespiratory function have been integral to the development of this hypothesis, with work initially being conducted on excised tissues and cadavers, followed by studies on anesthetized animals or trained animals under human care. These studies enabled research on free-ranging common bottlenose dolphins in Sarasota Bay, FL, and off Bermuda, and have included work on the metabolic and cardiorespiratory physiology of both shallow- and deep-diving dolphins and have been integral to better understand how cetaceans can dive to extreme depths, for long durations.

Instrumenting animals with tags contributes additional resistive forces (weight, buoyancy, lift, and drag) that may result in increased energetic costs; however, additional metabolic expense can be moderated by adjusting behavior to maintain power output. We sought to increase hydrodynamic drag for near-surface swimming bottlenose dolphins, to investigate the metabolic effect of instrumentation. In this experiment, we investigate whether (1) metabolic rate increases systematically with hydrodynamic drag loading from tags of different sizes or (2) whether tagged individuals modulate speed, swimming distance, and/or fluking motions under increased drag loading. We detected no significant difference in oxygen consumption rates when four male dolphins performed a repeated swimming task, but measured swimming speeds that were 34% (>1 m s-1) slower in the highest drag condition. To further investigate this observed response, we incrementally decreased and then increased drag in six loading conditions. When drag was reduced, dolphins increased swimming speed (+1.4 m s-1; +45%) and fluking frequency (+0.28 Hz; +16%). As drag was increased, swimming speed (-0.96 m s-1; -23%) and fluking frequency (-14 Hz; 7%) decreased again. Results from computational fluid dynamics simulations indicate that the experimentally observed changes in swimming speed would have maintained the level of external drag forces experienced by the animals. Together, these results indicate that dolphins may adjust swimming speed to modulate the drag force opposing their motion during swimming, adapting their behavior to maintain a level of energy economy during locomotion.Summary Statement: Biologging and tracking tags add drag to study subjects. When wearing tags of different sizes, dolphins changed their swimming paths, speed, and movements to modulate power output and energy consumption.