Diet-tissue Fractionation Research Articles

Trophic Enrichment Factors of Carbon and Nitrogen Isotopic Ratios (Δ13C and Δ15N) in Four Marine Ciliates.

Understanding the magnitude and causes of isotopic fractionation between organisms and their dietary resources is crucial for gaining knowledge on stable isotope ecology. However, little is known regarding the diet-tissue fractionation values of marine ciliates, which play a critical role in the reconstruction of microbial food webs. In the present study, we conducted experiments on two benthic (Pseudokeronopsis pararubra and Protocruzia labiata) and two pelagic (Strombidium sulcatum and Uronemella filificum) marine ciliates, where they were fed with isotopically constant foods (Chaetoceros calcitrans and Isochrysis galbana) under laboratory culture conditions to determine their carbon and nitrogen isotopic fractionation values (Δ13C and Δ15N). The stable isotope values (δ13C and δ15N) of ciliates for all experiments rapidly increased after the initial feeding, with half-lives ranging from 6.1 to 23.0h for δ13C and from 3.1 to 24.9h for δ15N. The Δ13C and Δ15N for all ciliates represented significantly positive enrichments, with overall mean fractionations of 0.6±0.2 and 1.2±0.4, respectively. Irrespective of the dietary type, both Δ13C and Δ15N were very similar for the same ciliate species. These results suggest that Δ13C and Δ15N for marine ciliates are similar to those found in common marine organisms with very little food-dependent variation. Overall, quantifying the specific isotopic fractionation of marine ciliates is expected to provide fundamental information on the trophic transfer of carbon, nitrogen, and energy flow through the microbial pathway in marine ecosystems.

Diet‐tissue fractionation of δ15N by consumers from streams and rivers

Variation in the diet‐tissue fractionation of stable nitrogen isotopes (Δ15N) is a major source of uncertainty in mixing model outputs and the calculation of trophic level in food web studies in aquatic systems. Using δ15N and δ13C of algae and consumers collected from a broad range of streams and rivers in Australia and New Guinea, we calculated Δ15N using a gradient approach, and compared these estimates with those from the literature. Riverine invertebrates and fishes from this region have δ15N diet‐tissue fractionation at the low end of the range of values reported for literature summaries, but different trophic groups had different Δ15N estimates. Source‐consumer regressions based on δ13C had lower slope estimates and lower R2 values compared with those based on δ15N. This implies that although consumers on average obtain a portion of their organic carbon from higher plants, they derive most of their organic nitrogen from N‐rich algae. Use of appropriate Δ15N for riverine consumers will lead to more satisfactory results with isotope mixing models, and the uncertainty associated with Δ15N estimates can be incorporated in the latest iterations of these models that are probabilistic rather than deterministic.

Tissue turnover in ovine muscles and lipids as recorded by multiple (H, C, O, S) stable isotope ratios

Multiple stable isotope ratios (δ 2H, δ 13C, δ 18O and δ 34S) were measured in muscle, muscle lipids and lipid fractions collected from 28 lambs, subjected to a diet-switch and raised on two energy allowances (EAs), to determine tissue turnover and diet-tissue fractionation. The diet-muscle fractionations prior to the diet-switch were estimated to be −44.0‰, +1.9‰ and 0‰ for H, C and S, respectively, while the drinking water was demonstrated to be the main source of muscle O and thus δ 18O variation. The diet-intra-muscular lipid fractionations prior to the diet-switch were estimated to be −172.7‰, −1.3‰ and −11.5‰ for H, C and O, respectively. The C half-lives of muscle were determined to be 75.7 and 91.6 days for animals receiving the high and low EA, respectively. Extracting temporally resolved pre-slaughter dietary information from meat by analysing bulk muscle, muscle lipids and muscle lipid fractions appeared to be not practicable due to possible incomplete turnover of lipids.

Diet: tissue stable isotope fractionation of carbon and nitrogen in blood plasma and whole blood of male reindeer Rangifer tarandus

Estimates of diet derived from stable isotope analyses are sensitive to the accuracy of corrections made for diet-tissue fractionation. In particular, diet-tissue fractionation in reindeer Rangifer tarandus may be expected to differ significantly from the generic values often used in stable isotope dietary calculations, given the known values obtained from other ungulates and the complex digestive system and nutrient recycling characteristic of the species. We fed domestic reindeer a homogenous artificial diet of known isotopic value in order to directly determine diet-tissue isotopic fractionation of carbon and nitrogen, the main elements used in stable isotope dietary analyses. Diet-tissue fractionation values for blood plasma were +3.5 ± 0.1‰ (δ13C) and +4.2 ± 0.3‰ (δ15N). Diet-tissue fractionation values for whole blood were +3.7 ± 0.2‰ (δ13C) and +2.5 ± 0.3‰ (δ15N). Metabolic turnover rates were clearly sufficient for complete tissue replacement over the period of artificial feeding for blood plasma, but may not have been so for whole blood, especially for δ15N. Our values, except for whole blood δ15N, differ considerably from the generic values often used in dietary studies and interspecific comparisons of trophic niche. The results underline the importance of obtaining as specific as possible fractionation values for the species, tissue, and in some cases sex and physiological status of animals under examination, and the potential problems associated with assuming ‘generic’ fractionation values when comparing species, especially where digestive processes are dissimilar.

An evaluation of deuterium as a food source tracer in temperate streams of eastern Canada

Stable H isotope ratios (D/H, expressed as dD) hold promise as an additional tool for elucidating food sources for consumers in aquatic ecosystems. We tested the applicability of dD as a food source tracer in streams of New Brunswick, Canada. First, we analyzed dD and d 13 C in biofilm and terrestrial leaves and compared signal-to-noise ratios (variability within sources vs variability between sources) between the 2 elements. Signal-to-noise ratios were roughly similar, and 23 of the 31 sites had isotopically distinct food sources based on dD compared to 20 of 31 based on d 13 C. Second, we used mixing models to estimate % aquatic H and % aquatic C for benthic invertebrates at a subset of sites. Of 16 samples, only 1 had d 13 C that was outside the range of the 2 food sources (yielding % aquatic C. 100%), but 12 of the 16 samples had dD outside the range of the food sources, a result suggesting confounding effects of water and lipids on total body H content. Last, we analyzed dD in laboratory-reared consumers (brook trout and water striders) and in their diet before and after lipid extraction to estimate diet-tissue fractionation. Large differences between consumer and diet were apparent before lipid extraction, but no significant differences were found after lipid extraction. All of these measures indicate that dD could serve as a complementary, but not alternative, isotopic method for estimating food sources for consumers in streams. Further laboratory trials are needed to explore the influence of lipids on dD values.

Fractionation and metabolic turnover of carbon and nitrogen stable isotopes in black fly larvae

Diet-tissue fractionation factors and metabolic turnover rates of delta15N and delta13C were assessed in laboratory-reared black fly (Simulium vittatum IS-7) larvae fed isotopically distinct diets. Five treatments consisted of using food with different delta15N signatures throughout the experiments (19-26 days), a sixth shifted from a low to high delta15N signature diet (uptake) on day 14, and the last shifted from a high to low delta15N signature diet (elimination) on day 14. In the larvae, diet-tissue fractionation factors for delta13C, which were in steady state with food, ranged from -0.61 to 2.0, with a median of 1.87. The delta15N diet-tissue fractionation factors were mostly negative, ranging from +2.85 to -24.96 per thousand, with a single positive value from the elimination treatment in which larval delta15N did not achieve steady state with the food. Diet-tissue fractionation factors also had a significant negative relationship (r2 = 0.98) with delta15N values in the food suggesting that nitrogen diet-tissue fractionation factors are 15N concentration-dependent. The delta15N of shed head capsules and feces were enriched in 15N and could be mechanisms for elimination of 15N by the larvae. For delta15N, metabolic turnover values based on the Hesslein model were highly consistent (0.40 to 0.43 delta15N*day(-1)) between uptake and elimination phases and across experiments and were an order of magnitude greater than growth rates. The rapid turnover of nitrogen in black fly larvae, which was orders of magnitude greater than measured in vertebrates, makes them an excellent indicator of short-term changes in nitrogen inputs to aquatic systems.

Effects of body size and environment on diet-tissue δ13C fractionation in fishes

δ 13C data are often used in trophodynamic research where diet-tissue fractionation (Δ δ 13C) is assumed to be 0–1‰ per trophic level and unaffected by the size of animals or their environment. Variation in Δ δ 13C will influence conclusions about food sources, energy pathways and trophic level. To assess the effects of body size, age and environmental conditions on Δ δ 13C, European sea bass ( Dicentrarchus labrax) were reared on constant diets of dab ( Limanda limanda) or ( Ammodytes marinus) for 2years under natural environmental regimes. Bass were sampled approximately monthly to determine Δ δ 13C for muscle, heart and liver tissue and were 1.66‰, − 0.18‰, − 1.77‰ (sandeel diet) and 1.34‰, − 1.18‰, − 1.75‰ (dab diet) respectively. Arithmetic lipid correction increased Δ δ 13C to > 2‰ for muscle and liver. Δ δ 13C was dependent on body mass and experimental duration (age) and generally declined with weight or time even after correction for lipid content. For liver, increasing temperature increased Δ δ 13C. The Δ δ 13C estimates from this study were compared with all available published Δ δ 13C estimates for fish. Bass muscle Δ δ 13C was similar to previous estimates for fish white muscle Δ δ 13C (1.56 ± 1.10‰) and whole body Δ δ 13C (1.52 ± 1.13‰). Fractionations derived in this study, combined with those from the literature, support the use of diet-tissue fractionation values of between 1‰–2‰ for δ 13C, rather than the commonly used 0‰ − 1‰. For muscle Δ δ 13C, 1.5‰ is appropriate.

Isotope trophic-step fractionation of suspension-feeding species: Implications for food partitioning in coastal ecosystems

Pacific oysters ( Crassostrea gigas) and blue mussels ( Mytilus edulis) were reared on a microalgal ( Skeletonema costatum) diet to estimate their diet-tissue isotopic fractionation ( Δ). We established that the diet-tissue fractionation value for stable carbon ( δ 13C) and stable nitrogen ( δ 15N) were around + 2.0 and + 3.6 for 13C and 15N, respectively. Equilibrium isotopic signatures, as well as turnovers of carbon and nitrogen in whole tissues, were approximated by exponential decay models. Half-life values in bivalves were about two times longer for carbon (around 8.5days) than for nitrogen (14.5days). We then used species-specific fractionation values to re-estimate contributions of food sources to the diets of similar suspension-feeding species, for which standard fractionation values had been assumed in previous studies. We emphasize the importance of knowing specific isotopic fractionation values to understand inter-specific trophic relationships and food webs.

Stable isotope variability in tissues of the Eurasian perch Perca fluviatilis

Stable isotope analysis is frequently used as a complementary method of dietary analysis, to describe trophic relationships and assess food-web structure. These studies allow a precise determination, based on the calculation of a diet-tissue fractionation factor. The fractionation factor, determined for whole organisms or specific tissues, may vary substantially in natura. In the present study, δ 13C and δ 15N were assessed in lipid-free tissues (spleen, liver, viscera, scales, gills, spine, white muscle, brain) and in available energy reserves (proteins, glycogen, lipids) of Eurasian perch ( Perca fluviatilis) reared under controlled conditions and fed for 4 months with the same artificial diet. Some discrepancies in δ 15N and δ 13C data were observed among tissues, respectively up to 3.43‰ and 2.54‰ for δ 15N and δ 13C. The 15N signature in organs depends on their metabolic activity. Despite a significant δ 13C enrichment from feed to tissues, the lipids in spine, liver and viscera exhibit a certain stability.

Effects of nutritional restriction on nitrogen and carbon stable isotopes in growing seabirds

When using stable isotopes as dietary tracers it is essential to consider effects of nutritional state on isotopic fractionation. While starvation is known to induce enrichment of (15)N in body tissues, effects of moderate food restriction on isotope signatures have rarely been tested. We conducted two experiments to investigate effects of a 50-55% reduction in food intake on delta(15)N and delta(13)C values in blood cells and whole blood of tufted puffin chicks, a species that exhibits a variety of adaptive responses to nutritional deficits. We found that blood from puffin chicks fed ad libitum became enriched in (15)N and (13)C compared to food-restricted chicks. Our results show that (15)N enrichment is not always associated with food deprivation and argue effects of growth on diet-tissue fractionation of nitrogen stable isotopes (Delta(15)N) need to be considered in stable isotope studies. The decrease in delta(13)C of whole blood and blood cells in restricted birds is likely due to incorporation of carbon from (13)C-depleted lipids into proteins. Effects of nutritional restriction on delta(15)N and delta(13)C values were relatively small in both experiments (delta(15)N: 0.77 and 0.41 per thousand, delta(13)C: 0.20 and 0.25 per thousand) compared to effects of ecological processes, indicating physiological effects do not preclude the use of carbon and nitrogen stable isotopes in studies of seabird ecology. Nevertheless, our results demonstrate that physiological processes affect nitrogen and carbon stable isotopes in growing birds and we caution isotope ecologists to consider these effects to avoid drawing spurious conclusions.

Applications, Considerations, and Sources of Uncertainty When Using Stable Isotope Analysis in Ecotoxicology

Stable isotope analysis (SIA) has become a powerful tool for ecotoxicologists to study dietary exposure and biomagnification of contaminants in wild animal populations. The use of SIA in ecotoxicology continues to expand and, while much more is known about the mechanisms driving patterns of isotopic ratios in consumers, there remain several considerations or sources of uncertainty that can influence interpretation of data from field studies. We outline current uses of SIA in ecotoxicology, including estimating the importance of dietary sources of carbon and their application in biomagnification studies, and we present six main considerations or sources of uncertainty associated with the approach: (1) unequal diet-tissue stable isotope fractionation among species, (2) variable diet-tissue stable isotope fractionation within a given species, (3) different stable isotope ratios in different tissues of the animal, (4) fluctuating baseline stable isotope ratios across systems, (5) the presence of true omnivores, and (6) movement of animals and nutrients between food webs. Since these considerations or sources of uncertainty are difficult to assess in field studies, we advocate that researchers consider the following in designing ecotoxicological research and interpreting results: assess and utilize variation in stable isotope diet-tissue fractionation among animal groups available in the literature; determine stable isotope ratios in multiple tissues to provide a temporal assessment of feeding; adequately characterize baseline isotope ratios; utilize stomach contents when possible; and assess and integrate life history of study animals in a system.

Effects of body size and environment on diet-tissue δ15N fractionation in fishes

Nitrogen stable isotope natural abundance data are often used in trophodynamic research. The assumed nitrogen diet-tissue fractionation (Δ δ 15N) determines conclusions about trophic level, potential food sources and ontogenetic diet shifts. Δ δ 15N is usually assumed to be 3.0–3.4‰ per trophic level and unaffected by the size or age of animals or their environment. To assess the effects of body size, experimental duration and environmental conditions on fish tissue Δ δ 15N, two populations of European sea bass ( Dicentrarchus labrax) were reared on constant diets of dab ( Limanda limanda) muscle or sandeel ( Ammodytes marinus) for 2 years under natural light and temperature regimes. Bass were sampled at approximately monthly intervals to determine Δ δ 15N for muscle, heart and liver tissue. Mean values of Δ δ 15N were 3.83‰, 3.54‰, 2.05‰ (sandeel diet) and 3.98‰, 3.32‰, 1.95‰ (dab diet) for muscle, heart and liver tissue respectively. The assumption that fractionation was independent of body mass was upheld for muscle and heart tissue, but not for liver. Time effects on muscle Δ δ 15N were explainable by a sinusoidal function with a period of 1 year and wave height ∼ 0.3‰. Time resulted in increases in heart δ 15N and decreases in liver δ 15N which were small compared to background variation, equating to 1/6 of a trophic level over 2 years, and unlikely to have great significance in ecological studies. Heart and liver δ 15N were also affected by temperature probably reflecting the metabolic functions of these tissues and their associated rates of turnover. However in heart the explanatory power of temperature appeared tied to that of time. Although the Δ δ 15N for bass muscle on both diets approached 4‰, the Δ δ 15N values from this study, when combined with those from the literature, suggest that where fish species specific data are not available, a mean Δ δ 15N for fish muscle of 3.2‰ should be applied (mean white muscle Δ δ 15N = 3.15). The literature based mean Δ δ 15N for whole fish was lower than that of white muscle suggesting that a separate Δ δ 15N (2.9‰) should be applied when sampling whole fish.

Variation in ^|^delta;13C and ^|^delta;15N among different tissues of three estuarine bivalves: implications for dietary reconstructions

The stable isotope approach to dietary reconstructions is based on the consistent isotopic fractionation between diet and consumer. Despite the rapidly increasing use of stable isotopes in studies on bivalves, classical fractionation values (0–1‰ for δ13C and 3–4‰ for δ15N) are used in the literature irrespective of the tissue analyzed and of the sample preparation method. We measured δ13C and δ15N for lipid-removed and lipid-containing tissues (adductor muscle, foot, mantle lobe, siphon, labial palp, gill lamella, midgut gland, and male and female gonad) of three suspension feeding bivalves (Crassostrea gigas, Cyclina sinensis and Ruditapes philippinarum) to provide an essential calibration for the diet-tissue fractionation. There were similar tissue-specific patterns in the δ13C and δ15N, which rank adductor muscle>other tissues>midgut gland or female gonad. Removal of lipids raised the δ13C and δ15N for all tissues of the three species. Acid treatment of the total soft tissues of R. philippinarum had no significant effects on the isotopic compositions. Diet-tissue fractionations for R. philippinarum were estimated based on the known fractionation values for the lipid-containing total soft tissues. Lipid removal and associated rinsing raised not only the 13C fractionation by 0.4–1.5‰ but also the 15N fractionation by 0–0.8‰, resulting in large fractionation values outside of the range of currently accepted 13C and 15N fractionation. The present study demonstrates that isotopic analyses are affected by the choice of tissue type and sample preparation method.

High fidelity: isotopic relationship between stream invertebrates and their gut contents

AbstractBenthic macroinvertebrate families were sampled along 3 rivers in New Brunswick, Canada. Stable isotopes of C and N were compared between body tissue and gut contents of individuals. δ13C and δ15N of body tissue and gut contents were strongly correlated (r = 0.94 and 0.93, respectively) over a wide range of δ values. In nonpredators, only minor fractionation of δ13C and δ15N was observed. In predators, diet–tissue fractionation of 13C was minor, but 15N fractionation that may have been related to diet quality (N content) was observed. The influence of diet quality on N-isotope fractionation was inconsistent in direction and strength among families. Our results suggest that subjecting primary consumers to gut clearance prior to processing for stable-isotope analysis is unnecessary, but the guts of predators should be removed before processing.

Blood Isotopic (δ13C and δ15N) Turnover and Diet-Tissue Fractionation Factors in Captive Dunlin (Calidris Alpina Pacifica)

AbstractAvian studies are often interpreted using dual (e.g. 13C, 15N) isotope models, assuming turnover of both isotopes occur at similar rates, but only a few studies have quantified turnover rates for more than one of those isotopes simultaneously. To test the generality of previous turnover and fractionation estimates and assumption of synchronous C and N patterns of turnover rates, we captured Dunlin (Calidris alpina pacifica) wintering in the Fraser River Delta, British Columbia, and derived isotopic turnover rates and diet-tissue fractionation factors by experimentally manipulating diet. Birds (n = 15) were initially fed a terrestrially derived diet (mean δ13C: −24.7‰, mean δ15N: 3.5‰) for 54 days. A treatment group (n = 11) was then switched to a marine-derived diet (mean δ13C: −18.3‰, mean δ15N: 13.7‰); a control group (n = 4) was maintained on the terrestrial diet for a further 59 days. An exponential model described patterns of isotopic turnover for 13C and 15N, and turnover rates and half-lives of the two isotopes were correlated, confirming the assumption of synchronous patterns of turnover for those isotopes. The half-lives for 13C and 15N in Dunlin whole blood were 11.2 ± 0.8 days and 10.0 ± 0.6 days, respectively, and are among the lowest values obtained to date for wild birds. Variation in turnover rate among individuals was not related to indices of body condition.

BLOOD ISOTOPIC (δ13C AND δ15N) TURNOVER AND DIET-TISSUE FRACTIONATION FACTORS IN CAPTIVE DUNLIN (CALIDRIS ALPINA PACIFICA)

Abstract Avian studies are often interpreted using dual (e.g. 13C, 15N) isotope models, assuming turnover of both isotopes occur at similar rates, but only a few studies have quantified turnover rates for more than one of those isotopes simultaneously. To test the generality of previous turnover and fractionation estimates and assumption of synchronous C and N patterns of turnover rates, we captured Dunlin (Calidris alpina pacifica) wintering in the Fraser River Delta, British Columbia, and derived isotopic turnover rates and diet-tissue fractionation factors by experimentally manipulating diet. Birds (n = 15) were initially fed a terrestrially derived diet (mean δ13C: −24.7‰, mean δ15N: 3.5‰) for 54 days. A treatment group (n = 11) was then switched to a marine-derived diet (mean δ13C: −18.3‰, mean δ15N: 13.7‰); a control group (n = 4) was maintained on the terrestrial diet for a further 59 days. An exponential model described patterns of isotopic turnover for 13C and 15N, and turnover rates and half-liv...

Feeding level and individual metabolic rate affect delta 13C and delta 15N values in carp: implications for food web studies.

Stable isotope analyses are often used to calculate relative contributions of multiple food sources in an animal's diet. One prerequisite for a precise calculation is the determination of the diet-tissue fractionation factor. Isotopic ratios in animals are not only affected by the composition of the diet, but also by the amount of food consumed. Previous findings regarding the latter point are controversial. As stable isotope analyses have often been used to investigate aquatic food webs, an experiment with carp (Cyprinus carpioL.) was carried out to test the influence of the feeding level and individual metabolic rate on delta(13)C and delta(15)N values of the whole body. After an initial phase, 49 carp were assigned randomly to four groups and fed the same diet at different levels for 8 weeks. For 15 fish, the energy budget was determined by indirect calorimetry. Feed and individual fish were analysed for their proximate composition, gross energy content and delta(13)C and delta(15)N values. delta(13)C and delta(15)N values differed significantly at different feeding levels. While delta(13)C values of the lipids and delta(15)N values decreased with increasing feeding rate, delta(13)C values of the lipid-free matter showed a non-linear pattern. Data obtained from fish held in the respirometric system revealed a relationship between delta(13)C values and the percentage retention of metabolizable energy. Our results show that reconstructing the diets of fish from the isotopic ratios when the feeding level and individual metabolic rates are unknown would introduce an error into the data used for back-calculation of up to 1 per thousand for both delta(13)C and delta(15)N values and may have substantial effects on the results of calculated diets. As other workers have pointed out, the development and application of stable isotopes to nutritional ecology studies is a field in its infancy and gives rise to erroneous, misleading results without nutritional, physiological and ecological knowledge.

The Role of Fruits and Insects in the Nutrition of Frugivorous Bats: Evaluating the Use of Stable Isotope Models1

We estimated the relative contribution of fruits and insects as sources of dietary protein in two species of Neotropical frugivorous bats (Artibeus jamaicensis and Sturnira lilium) using stable carbon and nitrogen isotope analyses. An insectivorous species (Pteronotus parnellii) was also included for comparison. We found constant patterns in stable carbon and nitrogen isotope composition in blood that separated the two species of frugivorous bats from the insectivorous bat. When we used these isotopic values (combined with those of dietary fruits and insects) to estimate the percent contribution of fruits and insects to the diet of the bats, we obtained different results, depending on assumptions and model adopted. We tested models using both δ15N and δ13C results simultaneously and separately and further used diet-tissue fractionation factors of 3‰ for nitrogen and 1 and 3.5‰ for carbon. We found that a carbon-based model with a diet-blood enrichment factor of 3.5‰ produced the most parsimonious results. The model estimated that A. jamaicensis and S. lilium obtained most of their protein requirements from fruits, whereas P. parnellii fed mostly on insects. No sexual or seasonal variations in the diet of the two frugivorous species were detected. We found no evidence that the diet of sexually active females differed from that of nonsexually active females in the two species of frugivorous bats. We suggest that future studies better define isotopic fractionation between diet and tissues of bats using captive rearing and controlled diets.

Stable carbon and nitrogen isotopic fractionation between diet and tissue of captive red fox: implications for dietary reconstruction

The amount of isotopic fractionation (change in isotope ratios) between diet and animal tissues is generally poorly known and may be affected by trophic position. Diet-tissue fractionation of stable-carbon and -nitrogen isotopes was measured in several tissues of red foxes (Vulpes vulpes) raised on a commercial pellet feed. Stable carbon isotopic fractionation in red fox was positive for all tissues and was greatest in fur (2.6‰), intermediate in muscle (1.1‰), and least in liver and blood fractions (0.4-0.6‰). These carbon isotope fractionation values were greater than those previously measured for mammalian herbivores but were similar to values for marine mammals in most tissues. Little variation in stable nitrogen isotopic fractionation occurred among tissues, except in the blood fractions. Nitrogen isotopic fractionation was much higher in blood serum (4.2‰) than in liver, muscle, and fur (3.3-3.5‰). Cellular fractions of blood had the lowest fractionation values (2.6‰). There was a significant age effect in nitrogen- but not in carbon-isotopic fractionation. Subadult foxes (<1 year) were significantly enriched in 15N compared with adult foxes for fur, muscle, and liver (no blood was collected from adults). The cause of this enrichment is unclear, but it may be related to the higher rate of protein synthesis and catabolism in growing animals. This study is the first to report isotopic fractionation values for a terrestrial mammalian carnivore. Such estimates are necessary to interpret stable-isotope patterns in wild carnivores.

Reconstructing Avian Diets Using Stable-Carbon and Nitrogen Isotope Analysis of Egg Components: Patterns of Isotopic Fractionation and Turnover

Because eggs are composed of nutrients that are ultimately derived from the diet of adult females, the relative abundance of naturally-occurring stable isotopes of carbon and nitrogen in eggs should be related to those in their diet and this may form the basis of a method for tracing diets. Before such dietary reconstructions can be established, however, it is necessary to know how isotopic signatures change (or fractionate) from the diet during the synthesis of various egg components. In this study, stable-carbon and nitrogen isotopic fractionation factors between diet and whole yolk, lipid-removed yolk, albumen, and shell membranes of eggs of captively-raised Mallards (Anas platyrhynchos), Japanese Quail (Coturnix japonica), Prairie Falcons (Falco mexicanus), Peregrine Falcons (Falco peregrinus), and Gyrfalcons (Falco rusticolis) were established. In addition, carbon isotope fractionation patterns between diet and eggshell carbonate and yolk lipid were determined for quail, Mallards and falcons. On average, yolk, albumen and membranes were enriched in 15 N relative to diet for all birds by 3.4‰, a value typical of processes associated with protein synthesis. For quail and Mallards, albumen, membranes, and shell carbonate were enriched in 13 C relative to diet by 1.5, 3.6, and 14.9‰, respectively, and whole yolk was depleted in 13 C by 2.6‰ due to the presence of yolk lipids. Falcons showed lower carbon diet-tissue fractionation for all tissues (albumen: +0.9, whole yolk: -1.9, membranes: +2.7, carbonate: +11.2‰) and this may be due to a greater reliance on dietary lipids vs. carbohydrates for the synthesis of egg components or on fractionation differences associated with fermentation of carbohydrates. Patterns of isotopic fractionation between diet and Mallard eggs were not influenced by clutch order. A diet-switch experiment using quail indicated that albumen, shell membrane and shell carbonate values reflect diet integrated over 3-5 days and yolk over eight days prior to laying. Because birds were fed ad libitum, these data provide baseline data on fractionation patterns for birds optimally mobilizing nutrients from diet to egg components. These values may differ for those wild birds that rely more heavily on endogenous reserves.