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First published online May 13, 2004
Journal of Experimental Biology 207, 2101-2114 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00997

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How do cormorants counter buoyancy during submerged swimming?

Gal Ribak¹, Daniel Weihs² and Zeev Arad^1,*

¹ Faculty of Biology, Technion – Israel Institute of Technology, Israel
² Faculty of Aerospace Engineering, Technion – Israel Institute of Technology, Haifa 32000, Israel

^* Author for correspondence (e-mail: zarad{at}tx.technion.ac.il)

Accepted 23 March 2004

Buoyancy is a de-stabilizing force for diving cormorants that forage at shallow depths. Having to counter this force increases the cost of transport underwater. Cormorants are known to be less buoyant than most water birds but are still highly buoyant (=0.8 kg m^–3) due to their adaptations for aerial flight. Nevertheless, cormorants are known to dive at a wide range of depths, including shallow dives where buoyancy is maximal. We analyzed the kinematics of underwater swimming of the great cormorant (Phalacrocorax carbo sinensis) in a shallow pool to discover and evaluate the mechanisms countering buoyancy while swimming horizontally. The birds maintained a very uniform cyclic paddling pattern. Throughout this cycle, synchronized tilting of the body, controlled by the tail, resulted in only slight vertical drifts of the center of mass around the average swimming path. We suggest that this tilting behavior serves two purposes: (1) the elongated bodies and the long tails of cormorants, tilted at a negative angle of attack relative to the swimming direction, generate downward directed hydrodynamic lift to resist buoyancy and (2) during the propulsive phase, the motion of the feet has a significant vertical component, generating a vertical component of thrust downward, which further helps to offset buoyancy. The added cost of the drag resulting from this tilting behavior may be reduced by the fact that the birds use a burst-and-glide pattern while swimming.

Key words: buoyancy, diving, kinematics, underwater, cormorant, Phalacrocorax

This article has been cited by other articles:

				G. Ribak, D. Weihs, and Z. Arad Consequences of buoyancy to the maneuvering capabilities of a foot-propelled aquatic predator, the great cormorant (Phalcrocorax carbo sinensis) J. Exp. Biol., September 15, 2008; 211(18): 3009 - 3019. [Abstract] [Full Text] [PDF]

				G. Ribak, D. Weihs, and Z. Arad Submerged swimming of the great cormorant Phalacrocorax carbo sinensis is a variant of the burst-and-glide gait J. Exp. Biol., October 15, 2005; 208(20): 3835 - 3849. [Abstract] [Full Text] [PDF]

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