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

SUMMARY The stable carbon isotope ratio of exhaled CO2 (δ13Cbreath) 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,δ 13Cbreath 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 δ13Cbreath of free-ranging animals. We measured δ13Cbreath 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 δ13Cbreath in satiated individuals levels off at values similar to the dietary isotope signature (δ13Cdiet) in insect-feeding mammals. Meanδ 13Cbreath of fasted individuals was depleted by– 5.8‰ (N=6) in relation toδ 13Cdiet. 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, δ13Cbreath of satiated bats levelled off at –2.6‰ belowδ 13Cdiet, 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, δ13Cbreath of fasting birds and mammals was depleted in 13C by– 3.2±2.0‰ in relation toδ 13Cdiet. δ13Cbreath of satiated animals differed by –0.6±2.3‰ fromδ 13Cdiet 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.

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