Swimming Energy Economy in Bottlenose Dolphins Under Variable Drag Loading

Julie M Van Der Hoop,Victor Petrov,Julie Rocho-Levine,Joaquin Gabaldon,Michael J Moore,K Alex Shorter,Andreas Fahlman

doi:10.3389/fmars.2018.00465

Abstract

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 behaviour 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 animal. Together, these results indicate that dolphins may adjust swimming speed to modulate the drag force opposing their motion during swimming, adapting their behaviour to maintain a level of energy economy during locomotion.

Highlights

Tagging studies strive to collect novel data in an environment where observations are difficult; tags can measure animal movement as well as environmental and physiological variables to help interpret animal behavior or performance (Johnson et al, 2009; Crossin et al, 2014; Hussey et al, 2015)
The tag alone was relatively hydrodynamic with the flow remaining attached to the tag body, but the cup geometry created drag-inducing areas of stagnate recirculating flow
Energy economy plays an important part in behavioral strategies animals employ to locomote (Fish, 1998; Williams et al, 2000); as such, tagged animals may compensate for drag from biologging tags by modifying swimming biomechanics or behavior

Summary

Introduction

Tagging studies strive to collect novel data in an environment where observations are difficult; tags can measure animal movement as well as environmental and physiological variables to help interpret animal behavior or performance (Johnson et al, 2009; Crossin et al, 2014; Hussey et al, 2015). Contribute additional weight and bulk, and more relevant in the marine realm, perturb the hydrodynamics of highly streamlined animals. At a given speed, resistive forces (e.g., drag, weight or buoyancy) created by a tag will require additional work from the animal. Drag forces increase with speed-squared and present a steep trade-off (Bannasch et al, 1994; Jones et al, 2013): animals can maintain speed under higher drag conditions but will need to expend more energy to overcome added drag. Diving vertebrates may have several compensatory mechanisms for this added drag; for example, changes in dive behavior (Webb et al, 1998) that do not result in detectable changes in energy requirements (Fahlman et al, 2008). Animals may compensate for added drag by reducing their speed to modulate power output (van der Hoop et al, 2014). Experimental protocols manipulating drag on swimmers have been effective in quantifying these trade-offs, effects, and compensation strategies (e.g., Webb, 1971a,b)

Methods

Results

Discussion

Conclusion