Discussion
We examined if hair or feathers can serve as proxies of whole-body isotope values. We found that hair is not an accurate representation for whole-body isotopic values in mammals, as theδ 13C values of hair differed significantly from the whole-body samples of mammals, though it may be accurate for birds. Because the offset was consistent among the four mammal species tested, we recommend a mathematical correction of hair isotope values for animals who consume mammalian prey whole. We also emphasize the need for further examination into which prey tissues should be analyzed to determine TDFs accurately.
We expected to observe differences between whole body and keratin outer covering because the rate and extent that carbon and nitrogen are incorporated varies among tissues of an individual organism24. We suspect thatδ 13C values of non-lipid-extracted bird feathers may be more reflective of whole-body values due to the oil they use to preen their feathers. The oil is high in cholesterol25 and may lower theδ 13C value of feathers to be more similar to that of whole body (which was also not lipid extracted); though we only tested one species (quail). Species with higher fat stores, such as those found in colder climates, may not follow this trend.
The nature of keratin as a structural protein may make it more reflective of the whole-body δ 15N values, and tissues that tend to have the largest differences inδ 15N values from hair proportionally do not contribute substantially to the overall whole-body value in mammals19. The mechanism to eliminate nitrogenous waste differs between mammals and birds, which excrete dilute urine and uric acid, respectively, which may contribute to the differences found between the offset of whole body and keratin outer coverδ 15N values between these two taxa26. Though there was a statistically significant difference between feather and whole body δ 15N values (0.32 ‰), the magnitude of this difference may be limited in its ecological significance compared to the variability that occurs inδ 15N values with trophic position. For example, the TDF for nitrogen has often been assumed to be 3.4 ‰4, though a recent meta-analysis found ranges of Δ15N from −3.3‰ to 9.7‰ with large variations between tissue and species types27.
Many studies assume that muscle represents the bulk of digestible and assimilated material consumed, which is often not the case in wild carnivores. This disparity was noted in a study of the diet of spotted hyena (Crocuta crocuta ) when the authors noted that assuming ingestion of only muscle may have led to an under-representation of more C3-reliant prey in their model due to the exclusion of bone, a 13C-enriched tissue that comprises a noteworthy portion of hyena diets28. The assumption of muscle consumption might be violated by other predators that seek out specific organs. For example, killer whales (Orcinus orca ) selectively feed on the livers of broadnose sevengill sharks (Notorynchus cepedianus )29, wolves (Canis lupus ) may shift their intake from largely muscle to organs in times of abundant prey30, and polar bears (Ursus maritimus ) consume large proportions of blubber in their preferred prey, ringed seals (Pusa hispida )31. In these cases, quantifying the isotopic values of what is consumed and in what proportions is warranted.
In conclusion, we suggest that future studies reconsider the assumption that non-invasively sampled tissues are representative of whole-body isotopic values. Often, tissues are prioritized for sampling when the collection methods are less invasive. In other cases, it may be logistically challenging to fully homogenize whole prey items, and it is not realistic in studies of fossilized remains or in studies of species that are very large, threatened or endangered, or have life histories that make them challenging to capture. Therefore, an alternative to using the whole-body values is to use a mathematical correction or offset from the available tissue. In our study, we calculated aδ 13C offset of -1.93 ± 0.37‰ between hair and muscle in mammalian prey that should be applied to calculate TDFs for consumers that consume the whole body. We suggest further studies of additional bird species to corroborate feathers as a suitable proxy for whole body isotope values.