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First published online June 27, 2008
Journal of Experimental Biology 211, 2233-2238 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.018523

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Stable carbon isotopes in exhaled breath as tracers for dietary information in birds and mammals

Christian C. Voigt^1,*, Leonie Baier², John R. Speakman³ and Björn M. Siemers²

¹ Evolutionary Ecology Research Group, Leibniz Institute for Zoo and Wildlife Research, Alfred-Kowalke-Straße 17, D-10315 Berlin, Germany
² Max-Planck-Institute for Ornithology, Sensory Ecology Group, Eberhard-Gwinner-Straße, 82319 Seewiesen, Germany
³ Aberdeen Centre for Energy Regulation and Obesity, School of Biological Sciences, University of Aberdeen, Tillydrone Avenue, Aberdeen AB24 2TZ, UK

^* Author for correspondence (e-mail: voigt{at}izw-berlin.de)

Accepted 7 May 2008

The stable carbon isotope ratio of exhaled CO₂ (¹³C_breath) reflects the isotopic signature of the combusted substrate and is, therefore, suitable for the non-invasive collection of dietary information from free-ranging animals. However, ¹³C_breath is sensitive to changes in ingested food items and the mixed combustion of exogenous and endogenous substrates. Therefore, experiments under controlled conditions are pivotal for the correct interpretation of ¹³C_breath of free-ranging animals. We measured ¹³C_breath in fasted and recently fed insectivorous Myotis myotis (Chiroptera) to assess the residence time of carbon isotopes in the pool of metabolized substrate, and whether ¹³C_breath in satiated individuals levels off at values similar to the dietary isotope signature (¹³C_diet) in insect-feeding mammals. Mean ¹³C_breath of fasted individuals was depleted by –5.8 (N=6) in relation to ¹³C_diet. After feeding on insects, bats exchanged 50% of carbon atoms in the pool of metabolized substrates within 21.6±10.5 min, which was slower than bats ingesting simple carbohydrates. After 2 h, ¹³C_breath of satiated bats levelled off at –2.6 below ¹³C_diet, suggesting that bats combusted both exogenous and endogenous substrate at this time. A literature survey revealed that small birds and mammals metabolize complex macronutrients at slower rates than simple macronutrients. On average, ¹³C_breath of fasting birds and mammals was depleted in ¹³C by –3.2±2.0 in relation to ¹³C_diet. ¹³C_breath of satiated animals differed by –0.6±2.3 from ¹³C_diet when endogenous substrates were not in isotopic equilibrium with exogenous substrates and by +0.5±1.8 (N=6 species) after endogenous substrates were in isotopic equilibrium with exogenous substrates.

Key words: bats, dietary preferences, exogenous substrate, fat, metabolism, stable isotopes

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