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First published online September 9, 2005
Journal of Experimental Biology 208, 3573-3579 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01775

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Energy metabolism in orchid bee flight muscles: carbohydrate fuels all

Raul K. Suarez¹, Charles-A. Darveau^2,*, Kenneth C. Welch, Jr.¹, Diane M. O'Brien³, David W. Roubik⁴ and Peter W. Hochachka^2,

¹ Department of Ecology, Evolution and Marine Biology, University of California Santa Barbara, Santa Barbara, CA 93106-9610, USA
² Department of Zoology, University of British Columbia, Vancouver, BC, Canada V6T 1Z4
³ Biology and Wildlife Department, University of Alaska, Fairbanks, AK 99775, USA
⁴ Smithsonian Tropical Research Institute, Balboa, Republic of Panama

^* Author for correspondence (e-mail: suarez{at}lifesci.ucsb.edu)

Accepted 6 July 2005

The widely accepted idea that bees fuel flight through the oxidation of carbohydrate is based on studies of only a few species. We tested this hypothesis as part of our research program to investigate the size-dependence of flight energetics in Panamanian orchid bees. We succeeded in measuring rates of O₂ consumption and CO₂ production in vivo during hovering flight, as well as maximal activities (V_max values) in vitro of key enzymes in flight muscle energy metabolism in nine species belonging to four genera. Respiratory quotients (ratios of rates of CO₂ production to O₂ consumption) in all nine species are close to 1.0. This indicates that carbohydrate is the main fuel used for flight. Trehalase, glycogen phosphorylase and hexokinase activities are sufficient to account for the glycolytic flux rates estimated from rates of CO₂ production. High activities of other glycolytic enzymes, as well as high activities of mitochondrial oxidative enzymes, are consistent with the estimated rates of carbohydrate-fueled oxidative metabolism. In contrast, hydroxyacylCoA dehydrogenase, an enzyme involved in fatty acid oxidation, was not detectable in any species. Thoracic homogenates displayed ADP-stimulated oxidition of pyruvate + proline, but did not oxidize palmitoyl L-carnitine + proline as substrates. A metabolic map, based on data reported herein and information from the literature, is presented. The evidence available supports the hypothesis that carbohydrate serves as the main fuel for flight in bees.

Key words: respiratory quotient, hovering flight, Euglossine bee, enzymes, glycolysis, flux, carbohydrate

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