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.