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First published online August 8, 2003

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The Journal of Experimental Biology 206, 3139-3147 (2003)
doi: 10.1242/jeb.00540

Kinematics of hovering hummingbird flight along simulated and natural elevational gradients

Douglas L. Altshuler^1,* and Robert Dudley^1,2,

¹ Section of Integrative Biology, University of Texas at Austin, Austin, Texas 78712, USA
² Smithsonian Tropical Research Institute, PO Box 2072, Balboa, Republic of Panama

^* Author for correspondence at present address: Bioengineering, Mail Code: 138-78, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA (e-mail: doug{at}caltech.edu)

Accepted 12 June 2003

Hovering flight is one of the most energetically demanding forms of animal locomotion. Despite the cost, hummingbirds regularly hover at high elevations, where flight is doubly challenging because of reduced air density and oxygen availability. We performed three laboratory experiments to examine how air density and oxygen partial pressure influence wingbeat kinematics. In the first study, we experimentally lowered air density but maintained constant oxygen partial pressure. Under these hypodense but normoxic conditions, hummingbirds increased stroke amplitude substantially and increased wingbeat frequency slightly. In the second experiment, we maintained constant air density but decreased oxygen partial pressure. Under these normodense but hypoxic conditions, hummingbirds did not alter stroke amplitude but instead reduced wingbeat frequency until they could no longer generate enough vertical force to offset body weight. In a final combined experiment, we decreased air density but increased oxygen availability, and found that the wingbeat kinematics were unaffected by supplemental oxygen.

We also studied hovering and maximally loaded flight performance for 43 hummingbird species distributed along a natural elevational gradient in Peru. During free hovering flight, hummingbirds showed increased stroke amplitude interspecifically at higher elevations, mirroring the intra-individual responses in our first laboratory experiment. During loaded flight, hummingbirds increased both wingbeat frequency and wing stroke amplitude by 19% relative to free-flight values at any given elevation. We conclude that modulation of wing stroke amplitude is a major compensatory mechanism for flight in hypodense or hypobaric environments. By contrast, increases in wingbeat frequency impose substantial metabolic demands, are only elicited transiently and anaerobically, and cannot be used to generate additional sustained lift at high elevations.

Key words: air density, elevation, flight, hummingbird, hyperoxia, hypoxia, kinematics, load lifting, oxygen concentration, wingbeat frequency, stroke amplitude

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