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

SUMMARY The stable carbon isotope ratio of exhaled CO 2 (δ 13 C 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,δ 13 C 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 δ 13 C breath of free-ranging animals. We measured δ 13 C 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 δ 13 C breath in satiated individuals levels off at values similar to the dietary isotope signature (δ 13 C diet ) in insect-feeding mammals. Meanδ 13 C breath of fasted individuals was depleted by– 5.8‰ ( N =6) in relation toδ 13 C 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, δ 13 C breath of satiated bats levelled off at –2.6‰ belowδ 13 C 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, δ 13 C breath of fasting birds and mammals was depleted in 13 C by– 3.2±2.0‰ in relation toδ 13 C diet . δ 13 C breath of satiated animals differed by –0.6±2.3‰ fromδ 13 C 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.