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.
- exhaled carbon dioxide
- Carollia perspicillata
- stable carbon isotopes
- infra-red stable isotope analyser
- metabolic substrate use