Sink fast and swim harder! Round-trip cost-of-transport for buoyant divers

Efficient locomotion between prey resources at depth and oxygen at the surface is crucial for breath-hold divers to maximize time spent in the foraging layer, and thereby net energy intake rates. The body density of divers, which changes with body condition, determines the apparent weight (buoyancy)...

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Bibliographic Details
Published in:Journal of Experimental Biology
Main Authors: Miller, Patrick, Biuw, Martin, Watanabe, Yuuki Y., Thompson, David, Fedak, Mike
Format: Article in Journal/Newspaper
Language:English
Published: 2012
Subjects:
WHALES
DIVING BEHAVIOR
STRATEGIES
drag
LOCOMOTION
SPEED
swimming gait
buoyancy
BREATH-HOLD DIVERS
cost-of-transport
BODY DENSITY
STROKING PATTERNS
NORTHERN ELEPHANT SEALS
DEPTH
Elephant Seals
Online Access:https://risweb.st-andrews.ac.uk/portal/en/researchoutput/sink-fast-and-swim-harder-roundtrip-costoftransport-for-buoyant-divers(0a8fa431-ab70-4b3c-843d-f496a8ed0d63).html
https://doi.org/10.1242/jeb.070128
Description
Summary:Efficient locomotion between prey resources at depth and oxygen at the surface is crucial for breath-hold divers to maximize time spent in the foraging layer, and thereby net energy intake rates. The body density of divers, which changes with body condition, determines the apparent weight (buoyancy) of divers, which may affect round-trip cost-of-transport (COT) between the surface and depth. We evaluated alternative predictions from external-work and actuator-disc theory of how non-neutral buoyancy affects round-trip COT to depth, and the minimum COT speed for steady-state vertical transit. Not surprisingly, the models predict that one-way COT decreases (increases) when buoyancy aids (hinders) one-way transit. At extreme deviations from neutral buoyancy, gliding at terminal velocity is the minimum COT strategy in the direction aided by buoyancy. In the transit direction hindered by buoyancy, the external-work model predicted that minimum COT speeds would not change at greater deviations from neutral buoyancy, but minimum COT speeds were predicted to increase under the actuator disc model. As previously documented for grey seals, we found that vertical transit rates of 36 elephant seals increased in both directions as body density deviated from neutral buoyancy, indicating that actuator disc theory may more closely predict the power requirements of divers affected by gravity than an external work model. For both models, minor deviations from neutral buoyancy did not affect minimum COT speed or round-trip COT itself. However, at body-density extremes, both models predict that savings in the aided direction do not fully offset the increased COT imposed by the greater thrusting required in the hindered direction.

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