Stable Isotope Turnover and Half-Life in Animal Tissues: A Literature Synthesis

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. Yet uncertainty relating to the time period integrated by isotopic measurement of animal tissues can confound the interpretation of isotopic data. 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 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. 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. Half-life generally increases with animal body mass, and is longer in muscle and blood compared to plasma and internal organs. Half-life was longest in ecotherms, followed by mammals, and finally birds. For ectotherms, different taxa-tissue combinations had similar allometric slopes that generally matched predictions of metabolic theory. Half-life for ectotherms can be approximated as: ln (half-life) = 0.22*ln (body mass) + group-specific intercept; n = 261, p<0.0001, r2 = 0.63. For endothermic groups, relationships with body mass were weak and model slopes and intercepts were heterogeneous. 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. 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.

碳、氮、硫的稳定同位素（stable isotopes）作为生态示踪剂，被广泛应用于动物迁徙、能量来源及食物网通路等相关研究。然而，动物组织同位素测定所整合的时间周期相关的不确定性，可能会干扰同位素数据的解译。已有大量实验性同位素饮食转换研究，旨在量化动物组织的同位素周转速率λ（单位：%·天⁻¹，通常以同位素半衰期表示为ln(2)/λ，单位：天）。但目前尚无研究对海量单一半衰期估算值进行评估或汇总，以探究其宏观尺度上的分布规律并刻画变异程度。本研究收集了已发表的半衰期估算数据，分析了半衰期与动物体质量的相关性，并检验了不同组织和类群间的异速生长关系。研究发现，半衰期通常随动物体质量增加而延长，且肌肉与血液中的半衰期长于血浆和内脏器官。变温动物（ectotherms）的半衰期最长，其次为哺乳类，鸟类最短。对于变温动物而言，不同类群-组织组合的异速生长斜率相近，且整体符合代谢理论的预测。变温动物的半衰期可近似表示为：ln(半衰期) = 0.22×ln(体质量) + 类群特异性截距；样本量n=261，p<0.0001，决定系数r²=0.63。对于内温动物（endotherms）类群，其与体质量的相关性较弱，模型的斜率与截距均存在异质性。尽管针对部分类群和组织类型，可通过简单的异速生长关系近似估算同位素半衰期，但本研究模型中仍存在大量未被解释的变异。本研究揭示了若干显著且普适性的规律，但仅通过动物体质量等易获取变量精准预测同位素半衰期仍颇具挑战。