Introduction
Stable isotope analysis is widely used in ecology to reconstruct diets and characterize trophic interactions. The carbon (δ 13C) and nitrogen (δ 15N) isotope composition of animal tissues is a result of the assimilated diet and the isotopic fractionation that results from metabolic processes 1,2. These processes allow for a unique method to analyze an animal’s diet becauseδ 15N values can be an indicator of relative trophic position in the local food web3. Values ofδ 13C remain relatively stable as carbon moves through food webs, allowing inference of primary sources of carbon4,5. An estimate of the magnitude of shift between a consumer and its diet is measured as the trophic discrimination factor (TDF), reported as 𝚫13C for carbon and 𝚫15N for nitrogen, defined as 𝚫13Ctissue-diet =δ 13Ctissueδ 13Cdiet (likewise for nitrogen)2.
Utilizing the appropriate TDF for an organism is critical to the accuracy of isotopic mixing models for diet reconstruction6. Incorrect TDFs can bias estimated proportions of each diet component and have conservation and management implications6. Depending on the TDF that was used, mixing models for critically endangered Balearic Shearwaters varied substantially (between 2%-56%) of reliance on fishery discards7. Many researchers suggest moving away from assumed discrimination factors and determining species-specific TDFs to avoid bias in the estimation of wild animal foraging ecology6,8,9.
Studies using captive animals with known diets offer the best opportunities to determine TDFs and have often been applied to organisms consuming easily homogenized diets such as nutritionally calibrated pellets, invertebrates, and marine prey sources10,11. However, TDFs that are calculated for animals that eat whole prey in the wild are largely based on studies that subsampled a single tissue (muscle, blood, or hair) from these diet items as a proxy for fully homogenized whole prey12–14. For example, the TDF for snowy owls (Bubo scandiacus ) was determined using leg muscle tissue from mice of known origin13, though owls also digest organs and skin that make up a considerable digestible portion of the prey before regurgitating indigestible components (e.g., keratin structures and bone) in a pellet15. Modification of mammalian bones may also occur during digestion within the owl, suggesting part of the bone is also assimilated 16. In some species of Falconiformes , bones are completely digested and not found in the pellets15. Understanding the proportion of body tissues digested and their contributions to overall isotopic value of diet is essential to calculating TDFs of their consumer.
Isotopes of carbon and nitrogen are incorporated differently within prey tissues due to metabolic processes. Tissues that are involved in lipid synthesis (e.g., the liver) have lower δ 13C values as a result of kinetic isotope effects occurring during the conversion of pyruvate to acetyl coenzyme A during lipid synthesis17, while δ15N values are influenced by metabolic interconversions of amino acids, protein synthesis, or protein breakdown, and tissues involved in these processes, such as the liver, often have higherδ 15N values18,19. Samples that can be obtained noninvasively (hair, feathers, feces, blood, etc.) offer methodological advantages, but it is unknown to what extent they represent the whole animal isotopic composition. Whole-body values include fat bodies and organs surrounded by fat pads, such as the liver and heart, that may not be reflected in the keratin hair tissue in mammals. In rats, δ 15N differences as large as 2.9‰ were found between the brain and muscle tissue, andδ 13C differences up to 1.6‰ were found between the brain and spleen tissue of an individual. Similar variation has also been observed in laboratory mice and domestic rabbits19,20. Additionally, many carnivores do not digest the keratin-based hair or feather tissues of prey they consume11, so these tissues may not contribute to the overall isotopic value of the diet. Thus, it remains uncertain if the use of a single tissue from prey is an accurate proxy for whole-body isotope composition for animals that eat prey whole.
In this study, our objective was to determine if keratin tissue is representative of the whole body. First, we compared the isotope values of hair to the homogenized whole body of four mammalian species (rats, mice, rabbits, and guinea pigs), and feathers to the whole body of one avian species (quail). Second, this study examines the isotopic value of homogenized whole organism with and without outer covering (hair or feathers), as these materials are often fully or partially undigested by carnivores, and the degree to which they are assimilated is not well understood. This step will determine to what degree the hair or feather tissue influences the absolute values of the whole body when homogenized.