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 13Ctissue −δ 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.