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First are diet-tissue discrimination factors (sometimes referred to as trophic fractionation). The use of stable isotopes in food web studies is predicated on an understanding of how the isotopic composition of animal diet and tissues are related. Natural variability in the stable isotopic ratios of carbon, nitrogen, and sulfur (δ 13C, δ 15N, δ 34S) are widely used in animal ecology, including studies of animal migration, food webs, trophic position estimation, and food source reliance. Our study highlights several strong and general patterns, though accurate prediction of isotopic half-life from readily available variables such as animal body mass remains elusive. While isotopic half-life can be approximated using simple allometric relationships for some taxa and tissue types, there is also a high degree of unexplained variation in our models. For endothermic groups, relationships with body mass were weak and model slopes and intercepts were heterogeneous. Half-life for ectotherms can be approximated as: ln (half-life) = 0.22*ln (body mass) + group-specific intercept n = 261, p<0.0001, r 2 = 0.63. For ectotherms, different taxa-tissue combinations had similar allometric slopes that generally matched predictions of metabolic theory. Half-life was longest in ecotherms, followed by mammals, and finally birds. Half-life generally increases with animal body mass, and is longer in muscle and blood compared to plasma and internal organs.
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Here, we collect previously published half-life estimates, examine how half-life is related to body size, and test for tissue- and taxa-varying allometric relationships. Yet no studies have evaluated or summarized the many individual half-life estimates in an effort to both seek broad-scale patterns and characterize the degree of variability. There have been a large number of experimental isotopic diet shift studies aimed at quantifying animal tissue isotopic turnover rate λ (%·day -1, often expressed as isotopic half-life, ln(2)/λ, days). Yet uncertainty relating to the time period integrated by isotopic measurement of animal tissues can confound the interpretation of isotopic data. Stable isotopes of carbon, nitrogen, and sulfur are used as ecological tracers for a variety of applications, such as studies of animal migrations, energy sources, and food web pathways.