Abstract
We sought to characterize the ability of hummingbirds to fuel their energetically expensive hovering flight using dietary sugar by a combination of respirometry and stable carbon isotope techniques. Broadtailed hummingbirds (Selasphorus platycercus) were maintained on a diet containing beet sugar with an isotopic composition characteristic of C3 plants. Hummingbirds were fasted and then offered a solution containing cane sugar with an isotopic composition characteristic of C4 plants. By monitoring the rates of CO2 production and O2 consumption, as well as the stable carbon isotope composition of expired CO2, we were able to estimate the relative contributions of carbohydrate and fat, as well as the absolute rate at which dietary sucrose was oxidized during hovering. The combination of respirometry and carbon isotope analysis revealed that hummingbirds initially oxidized endogenous fat following a fast and then progressively oxidized proportionately more carbohydrates. The contribution from dietary sources increased with each feeding bout, and by 20 min after the first meal, dietary sugar supported approximately 74% of hovering metabolism. The ability of hummingbirds to satisfy the energetic requirements of hovering flight mainly with recently ingested sugar is unique among vertebrates. Our finding provides an example of evolutionary convergence in physiological and biochemical traits among unrelated nectar-feeding animals.
Highlights
The habit of feeding on floral nectar has promoted convergence in morphological, behavioral, and physiological traits among insects, birds, and mammals
Respiratory quotient (RQ), which is equal to the rate of CO2 production (Vco2) divided by the rate of oxygen consumption (Vo2; Ferrannini 1988), indicates that fasted hummingbirds switch from oxidizing lipids (RQ p 0.7) to oxidizing carbohydrates (RQ p 1.0) within minutes of feeding (Suarez et al 1990)
RQ can distinguish between lipid and carbohydrate oxidation (Ferrannini 1988), it makes no distinction between endogenous and exogenous nutrient use
Summary
The habit of feeding on floral nectar has promoted convergence in morphological, behavioral, and physiological traits among insects, birds, and mammals. We examine whether broadtailed hummingbirds (Selasphorus platycercus) share an additional trait with nectarivorous insects: the ability to support metabolism entirely with newly ingested sugar. RQ can distinguish between lipid and carbohydrate oxidation (Ferrannini 1988), it makes no distinction between endogenous and exogenous nutrient use. This analytical limitation, can be overcome by using elemental stable isotope analysis in a diet-shifting experimental design (O’Brien et al 2002). We combine respirometry and stable isotope analysis to determine (1) the substrate supporting metabolism during hovering flight in hummingbirds and (2) whether that substrate is endogenous or recently ingested. Our approach allows us to estimate the rate of exogenous sugar oxidation in hovering hummingbirds (Adopo et al 1994)
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