Metabolic substrate use and the turnover of endogenous energy reserves in broad - tailed hummingbirds ( Selasphorus platycercus )

Introduction The nutrients that animals assimilate can follow several pathways. They can be stored for future use, immediately oxidized to fuel metabolism, or used to synthesize materials for reproduction, growth and tissue maintenance. The sugars assimilated by a non-reproductive nectar-feeding bird, for example, can be stored as glycogen, used to synthesize lipid, or oxidized immediately (Alexander, 1999). Historically, studies investigating metabolic substrate use have relied on the respiratory quotient (RQ=VCO2VO2) to assess whether carbohydrates, lipids or proteins support respiration (Surarez et al., 1990; Powers, 1991). Advances in elemental stable isotope analysis now allow an alternative/complementary method to RQ to determine metabolic substrate use. Recently, stable isotopes have been used to clarify nutritional, physiological and ecological questions that respirometry could not, for instance, to quantify the relative contribution of ingested (‘income’) to stored (‘capital’) nutrients for reproduction in adults of both moths and butterflies (O’Brien et al., 2000; O’Brien et al., 2004). Stable isotopes have also been used in migratory birds to discriminate between nutrients ingested on the wintering grounds and during migration versus those ingested on the breeding grounds (Hobson et al., 2000). These studies relied on animals that either naturally or artificially switched between diets of distinct isotopic composition. We used a similar diet-shifting approach to clarify metabolic substrate use in hummingbirds. Carleton et al. found that the carbon stable isotope composition ( 13C) of respired CO2 from feeding rufous hummingbirds (Selasphorus rufus) closely resembled that of dietary nutrients (Carleton et al., 2004). When they switched birds to a diet with a contrasting isotopic composition, 13C of respired CO2 was intermediate between diets, which indicated that hummingbirds were metabolizing both exogenous nutrients and endogenous reserves. Here, by measuring 13C of exhaled CO2 in animals that were shifted between diets with contrasting carbon isotope compositions, we were able to quantify the fraction of metabolism fueled by income (assimilated sugars) and capital (endogenous reserves) in broad-tailed hummingbirds (Selasphorus platycercus Swainson). Additionally, because stable isotopes allow determining isotopic incorporation of assimilated nutrients into an organism’s tissues (Carleton et al., 2004), we examined both the isotopic incorporation of carbon and the mean residence time of a carbon atom in the endogenous reserves of broadtailed hummingbirds.