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

The stable carbon isotope ratio of exhaled CO2 (delta C-13(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, delta C-13(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 delta C-13(breath) of free- ranging animals. We measured delta C-13(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 delta C-13(breath) in satiated individuals levels off at values similar to the dietary isotope signature (delta C-13(diet)) in insect- feeding mammals. Mean delta C-13(breath) of fasted individuals was depleted by - 5.8% ( N= 6) in relation to delta C-13(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, delta C-13(breath) of satiated bats levelled off at - 2.6% below delta C-13(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, delta C-13(breath) of fasting birds and mammals was depleted in C-13 by - 3.2 +/- 2.0% in relation to delta C-13(diet). delta C-13(breath) of satiated animals differed by - 0.6 +/- 2.3% from delta C-13(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.