Nutrient routing in omnivorous animals tracked by stable carbon isotopes in tissue and exhaled breath
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- Voigt, C.C., Rex, K., Michener, R.H. et al. Oecologia (2008) 157: 31. doi:10.1007/s00442-008-1057-3
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Omnivorous animals feed on several food items that often differ in macronutrient and isotopic composition. Macronutrients can be used for either metabolism or body tissue synthesis and, therefore, stable C isotope ratios of exhaled breath (δ13Cbreath) and tissue may differ. To study nutrient routing in omnivorous animals, we measured δ13Cbreath in 20-g Carollia perspicillata that either ate an isotopically homogeneous carbohydrate diet or an isotopically heterogenous protein-carbohydrate mixture. The δ13Cbreath converged to the δ13C of the ingested carbohydrates irrespective of whether proteins had been added or not. On average, δ13Cbreath was depleted in 13C by only ca. −2‰ in relation to the δ13C of the dietary carbohydrates and was enriched by +8.2‰ in relation to the dietary proteins, suggesting that C. perspicillata may have routed most ingested proteins to body synthesis and not to metabolism. We next compared the δ13Cbreath with that of wing tissue (δ13Ctissue) in 12 free-ranging, mostly omnivorous phyllostomid bat species. We predicted that species with a more insect biased diet—as indicated by the N isotope ratio in wing membrane tissue (δ15Ntissue)—should have higher δ13Ctissue than δ13Cbreath values, since we expected body tissue to stem mostly from insect proteins and exhaled CO2 to stem from the combustion of fruit carbohydrates. Accordingly, δ13Ctissue and δ13Cbreath should be more similar in species that feed predominantly on plant products. The species-specific differences between δ13Ctissue and δ13Cbreath increased with increasing δ15Ntissue, i.e. species with a plant-dominated diet had similar δ13Ctissue and δ13Cbreath values, whereas species feeding at a higher trophic level had higher δ13Ctissue than δ13Cbreath values. Our study shows that δ13Cbreath reflect the isotope ratio of ingested carbohydrates, whereas δ13C of body tissue reflect the isotope ratio of ingested proteins, namely insects, supporting the idea of isotopic routing in omnivorous animals.