Oxidation rate and turnover of ingested sugar in hovering Anna's(Calypte anna) and rufous (Selasphorus rufus)hummingbirds

Abstract

SUMMARY Hummingbirds obtain most of their dietary calories from floral nectar ingested during hovering flight. Despite the importance of dietary sugar to hummingbird metabolism, the turnover of newly ingested carbon in the pool of actively metabolized substrates has not been adequately characterized in hovering hummingbirds. By combining respirometry with stable carbon isotope analysis of respired breath, we show that in rufous (Selasphorus rufus) and Anna's (Calypte anna) hummingbirds at high foraging frequencies, utilization of newly ingested sugars increased over a period of 30–45 min until it accounted for virtually 100% of the fuel oxidized. This newly ingested sugar disappears from the actively metabolized pool of substrates over a similar time course. These results demonstrate that turnover of carbon in the pool of actively metabolized substrates is rapid; carbon from ingested sucrose is available for oxidation for 30–45 min before being cleared. By monitoring expired CO2 for the appearance and disappearance of the signature characteristic of newly ingested sugar and then calculating energy budgets using video recordings of hummingbird activity, we estimated the proportion of recently ingested sugar used to fuel ongoing metabolism as well as the proportion devoted to energy storage. Consistent differences between species in the percentage of ingested cane sugar oxidized during the 2 h experimental periods suggest that individuals of each species adopted energy intake patterns appropriate to their needs. This approach provides a means by which to examine the partitioning of dietary carbon intake between energy expenditure and storage using non-invasive, field-compatible techniques.

Tracking the oxidative kinetics of carbohydrates, amino acids and fatty acids in the house sparrow using exhaled 13CO2

“…The results of this study suggest that sparrows may not be capable of oxidizing exogenous carbohydrates as rapidly as nectarivores (Welch et al, 2006;Welch et al, 2008;Welch and Suarez, 2007), although it is difficult to make direct comparisons among these animals because the sparrows were digesting a mixed diet rather than just sugar. A comparison of exogenous glucose oxidation in exercising humans and resting sparrows, which have similar mass specific V CO2 (0.0371mlCO 2 min (Jentjens et al, 2004)] in 2h.…”

“…Although the past few years have witnessed an increase in the use of breath testing in nectarivorous endotherms (Voigt et al, 2008a;Voigt et al, 2003;Voigt et al, 2008b;Voigt and Speakman, 2007;Welch et al, 2006;Welch et al, 2008;Welch and Suarez, 2007), we suggest that breath testing can be used to study a much wider range of animal models. The approaches we outline here offer a robust methodological framework that can be applied to all types of animals and be tailored to test a variety of hypotheses.…”

“…As a result, two experimental diets with identical isotopic signatures could have very different macronutrient composition. These and other limitations to using naturally enrichedIn recent studies on nectarivorous bats and birds, researchers have overcome the obstacle of specificity by feeding the animal a single organic molecule, usually a solution of a single monosaccharide or disaccharide derived from C 3 or C 4 plants (Carleton et al, 2006;Voigt et al, 2008a;Voigt and Speakman, 2007;Welch et al, 2006;Welch et al, 2008;Welch and Suarez, 2007). However, given the comparatively small differences in isotopic signatures of such diets, such an approach may still lack the precision required to identify minor, but important differences in oxidative kinetics.…”

Regulation of nutrient intake in nectar-feeding birds: insights from the geometric framework

“…In addition, nectar contains inorganic ions, amino acids, proteins, lipids and secondary metabolites in smaller quantities (Nicolson and Thornburg 2007). Nectar-feeding birds have high energy requirements due to their small body size and energetic lifestyles, and use recently ingested carbohydrates to fuel up to 95% of their metabolism (Welch and Suarez 2007).…”

Drinking problems on a ‘simple’ diet: physiological convergence in nectar-feeding birds

“…This rapid absorption of sugar in nectarivores has been demonstrated using stable isotope tracing, which takes advantage of the fact that the 13 C: 12 C signatures of cane sugar and beet sugar are completely different, allowing experimental testing using diet switching. Changing the dietary sugar source and measuring the isotopic composition of expired CO 2 clearly shows the rapid oxidation of ingested sugars during foraging flight in hummingbirds (Welch et al, 2006;Welch and Suarez, 2007). Importantly, direct oxidation of dietary sugar is 16% more efficient than using fat to fuel flight (Suarez et al, 1990).…”