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Hummingbirds can fuel expensive hovering flight completely with either exogenous glucose or fructose
Author(s) -
Chen Chris Chin Wah,
Welch Kenneth Collins
Publication year - 2014
Publication title -
functional ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.272
H-Index - 154
eISSN - 1365-2435
pISSN - 0269-8463
DOI - 10.1111/1365-2435.12202
Subject(s) - fructose , sucrose , biology , sugar , monosaccharide , carbohydrate , hexose , carbohydrate metabolism , biochemistry , enzyme
Summary Hummingbirds have specialized on a diet consisting almost exclusively of a mixture of sucrose, glucose and fructose found in floral nectar. Previous studies have shown that hummingbirds can fuel energetically expensive hovering flight almost exclusively using recently ingested sucrose. However, the relative capacities for the direct utilization of glucose and fructose by hovering hummingbirds remain unknown. 13 C‐enriched solutions of glucose and fructose were fed to ruby‐throated hummingbirds ( A rchilochus colubris ) separately. Along with simultaneous measurements of gas exchange during hovering we collected exhaled breath samples using feeder‐mask respirometry and analysed these to determine the isotopic signatures of exhaled carbon dioxide. We found that hovering hummingbirds transition from exclusively oxidizing endogenous fatty acids when fasted, to oxidizing newly ingested carbohydrates when given access to either glucose or fructose solutions. We then switched hummingbirds to the respective unlabelled solutions of glucose or fructose to estimate carbohydrate turnover kinetics. During the period of availability of enriched solutions, the percentage of metabolism supported by exogenous sugar increased from 0% to near 100% in some individuals. On average, hummingbirds fuelled 81% and 88% of their metabolism during hovering flight with exogenous glucose and fructose, respectively. The amount of energy ingested, fractional turnover of ingested sugars in the pool of actively metabolized substrates, amount oxidized, energy expended and proportion of hovering metabolism supported by each hexose were all similar between glucose and fructose. By foraging frequently and fuelling hovering flight directly with ingested monosaccharides hummingbirds avoid the energetic tax associated with the cost of synthesis of fats from these sugars prior to their oxidation. Remarkably, hovering hummingbirds are able to utilize fructose and glucose equally, a physiological feat which no mammals are thought to match, and one that suggests novel physiological capacities for the oxidation of fructose by active muscle tissues in hummingbirds. The data presented here indicate hummingbirds enhance net energy intake though specialization of diet, behaviour, and, uniquely, metabolic physiology.