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 parts per thousand), intermediate in muscle (1.1 parts per thousand), and least in liver and blood fractions (0.4-0.6 parts per thousand). 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 parts per thousand) than in liver, muscle, and fur (3.3-3.5 parts per thousand). Cellular fractions of blood had the lowest fractionation values (2.6 parts per thousand). There was a significant age effect in nitrogen- but not in carbon-isotopic fractionation. Subadult foxes (< 1 year) were significantly enriched in N-15 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.