Nutrient routing in omnivorous animals tracked by stable carbon isotopes in tissue and exhaled breath

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 (delta C-13(breath)) and tissue may differ. To study nutrient routing in omnivorous animals, we measured delta C-13(breath) in 20-g Carollia perspicillata that either ate an isotopically homogeneous carbohydrate diet or an isotopically heterogenous protein-carbohydrate mixture. The delta C-13(breath) converged to the delta C-13 of the ingested carbohydrates irrespective of whether proteins had been added or not. On average, delta C-13(breath) was depleted in delta C-13 by only ca. - 2 parts per thousand in relation to the delta C-13 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 delta C-13(breath) with that of wing tissue (delta C-13(tissue)) 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 (delta(15)Ntissue)-should have higher delta C-13(tissue) than delta C-13(breath) values, since we expected body tissue to stem mostly from insect proteins and exhaled CO2 to stem from the combustion of fruit carbohydrates. Accordingly, delta C-13(tissue) and delta C-13(breath) should be more similar in species that feed predominantly on plant products. The species-specific differences between delta C-13(tissue) and delta C-13(breath) increased with increasing delta N-13(tissue), i.e. species with a plant-dominated diet had similar delta C-13(tissue) and delta C-13(breath) values, whereas species feeding at a higher trophic level had higher delta C-13(tissue) than delta C-13(breath) values. Our study shows that delta C-13(breath) reflect the isotope ratio of ingested carbohydrates, whereas delta C-13 of body tissue reflect the isotope ratio of ingested proteins, namely insects, supporting the idea of isotopic routing in omnivorous animals.