MEPS 430:171-182 (2011)  -  doi:10.3354/meps08868

Drag reduction by air release promotes fast ascent in jumping emperor penguins—a novel hypothesis

John Davenport1,*, Roger N. Hughes2, Marc Shorten1, Poul S. Larsen3

1Department of Zoology, Ecology and Plant Science, University College Cork, Distillery Fields, North Mall, Cork, Ireland
2School of Biological Sciences, Bangor University, Deiniol Road, Bangor, Gwynedd LL57 2UW, UK
3Department of Mechanical Engineering, Fluid Mechanics Section, Technical University of Denmark, Building 403,
2800 Kgs. Lyngby, Denmark

ABSTRACT: To jump out of water onto sea ice, emperor penguins must achieve sufficient underwater speed to overcome the influence of gravity when they leave the water. The relevant combination of density and kinematic viscosity of air is much lower than for water. Injection of air into boundary layers (‘air lubrication’) has been used by engineers to speed movement of vehicles (ships, torpedoes) through sea water. Analysis of published and unpublished underwater film leads us to present a hypothesis that free-ranging emperor penguins employ air lubrication in achieving high, probably maximal, underwater speeds (mean ± SD: 5.3 ± 1.01 m s–1), prior to jumps. Here we show evidence that penguins dive to 15 to 20 m with air in their plumage and that this compressed air is released as the birds subsequently ascend whilst maintaining depressed feathers. Fine bubbles emerge continuously from the entire plumage, forming a smooth layer over the body and generating bubbly wakes behind the penguins. In several hours of film of hundreds of penguins, none were seen to swim rapidly upwards without bubbly wakes. Penguins descend and swim horizontally at about 2 m s–1; from simple physical models and calculations presented, we hypothesize that a significant proportion of the enhanced ascent speed is due to air lubrication reducing frictional and form drag, that ­buoyancy forces alone cannot explain the observed speeds, and that cavitation plays no part in ­bubble formation.

KEY WORDS: Emperor penguins · Air lubrication · Bubbly wakes · Jumping

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Cite this article as: Davenport J, Hughes RN, Shorten M, Larsen PS (2011) Drag reduction by air release promotes fast ascent in jumping emperor penguins—a novel hypothesis. Mar Ecol Prog Ser 430:171-182

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