Meeting Abstract

113.5  Tuesday, Jan. 7 11:30  Glucose, fructose, and sucrose use in hovering hummingbirds WELCH, KC*; CHEN, CCW; University of Toronto Scarborough; University of Toronto Scarborough kwelch@utsc.utoronto.ca

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 remain unknown. We investigated the use of each monosaccharide as a fuel by feeding separate ¹³C-enriched fructose, glucose, or sucrose solutions to ruby-throated hummingbirds (Archilochus colubris). We collected breath samples from hovering hummingbirds using feeder-mask respirometry to determine the isotopic signatures of exhaled carbon dioxide. We found that hummingbirds transition from exclusively oxidizing endogenous fatty acids when fasted, to oxidizing newly ingested carbohydrates when given access to any of the above solutions. The percentage of hovering metabolism supported by exogenous sugar increased from 0% to near 100% in some individuals, averaging 81% and 88% for exogenous glucose and fructose treatments, respectively. Every measure of carbon turnover kinetics, energy intake and expenditure was similar between glucose and fructose treatments. By foraging frequently and fueling 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. These findings indicate hummingbirds enhance net energy intake though specialization of diet, behaviour, and, uniquely, metabolic physiology.