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Adjustment of submerged swimming to changes in buoyancy in cormorants

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Abstract:

Waterbirds are buoyant because of volumes of air in their plumage and respiratory tract. When they are submerged, their buoyancy is reduced, owing to compression of these volumes of air with depth. We tested how the horizontal submerged swimming of cormorants (Phalacrocorax carbo sinensis (Blumelbach, 1798)) changed when their buoyancy was artificially reduced. Birds were filmed swimming under water once with lead weights (density 11 000 kg·m–3) and again with "dummy" weights (density 1100 kg·m–3) attached to their body. The dummy weights had negligible weight under water and served as control for the increased drag in the experiment. Cormorants swimming with weights tilted their bodies at an angle of 3°–7° below the swimming direction, whereas the body of birds in the control groups was tilted at 14°–16°. The tilt of the body affected the orientation and trajectory of the tail and feet during swimming. A hydrodynamic analysis showed that the lesser tilt of the body (while swimming with weights equivalent to 26% of body weight) resulted in a 55%–57% reduction of the vertical hydrodynamic forces (lift, drag, and thrust) generated by the birds to overcome buoyancy. When more weights were added and the birds became negatively buoyant, these vertical forces changed direction to prevent sinking. Thus, by adjusting the tilt of the body, the birds may dynamically control their buoyancy to maintain straight horizontal swimming despite changes in buoyancy.

Les oiseaux aquatiques flottent sur l'eau à cause des volumes d'air présents dans leur plumage et leur système respiratoire. Une fois submergés, leur flottabilité est réduite à cause de la compression de ces volumes d'air en fonction de la profondeur. Nous avons évalué de quelle manière la nage horizontale sous l'eau de cormorans (Phalacrocorax carbo sinensis (Blumelbach, 1798)) change lorsque leur flottabilité est réduite artificiellement. Nous avons filmé des oiseaux qui nagent sous l'eau, une fois avec du lest de plomb (densité de 11 000 kg·m–3) fixé au corps et une seconde fois avec du faux lest (densité de 1100 kg·m–3). Le faux lest a une masse négligeable sous l'eau et sert de témoin de la traînée accrue durant l'expérience. Les cormorans qui nagent avec du lest inclinent leur corps d'un angle de 3°–7° sous la direction de nage, alors que les oiseaux du groupe témoin l'inclinent de 14°–16°. L'inclinaison du corps affecte l'orientation et la trajectoire de la queue et des pattes durant la nage. Une analyse hydrodynamique indique que l'inclinaison réduite du corps (durant la nage avec du lest correspondant à 26 % de la masse corporelle) entraîne une réduction de 55 % – 57 % des forces hydrodynamiques verticales (portance, traînée et poussée) générées par les oiseaux pour contrecarrer la flottabilité. Si on ajoute du lest supplémentaire et que la flottabilité des oiseaux devient négative, ces forces verticales changent de sens afin de combattre l'enfoncement. Ainsi, en ajustant l'inclinaison de leur corps, les oiseaux peuvent contrôler leur flottabilité, afin de maintenir une nage horizontale droite malgré les changements de flottabilité.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: March 1, 2006

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  • Published since 1929, this monthly journal reports on primary research contributed by respected international scientists in the broad field of zoology, including behaviour, biochemistry and physiology, developmental biology, ecology, genetics, morphology and ultrastructure, parasitology and pathology, and systematics and evolution. It also invites experts to submit review articles on topics of current interest.
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