Fisk Aaron T, Sash Kim, Maerz John, Palmer William, Carroll John P, Macneil M Aaron
Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA 30602-2152, USA.
Rapid Commun Mass Spectrom. 2009 Jan;23(2):319-26. doi: 10.1002/rcm.3880.
Metabolic turnover rates (m) of delta(15)N and delta(13)C were assessed in different tissues of newly hatched captive-raised corn snakes (Elaphe guttata guttata) fed maintenance diets consisting of earthworms (Eisenia foetida) that varied substantially in delta(15)N (by 644 per thousand) and delta(13)C (by 5.0 per thousand). Three treatments were used during this 144 day experiment that consisted of the same diet throughout (control), shifting from a depleted to an enriched stable isotope signature diet (uptake), and shifting from an enriched to depleted stable isotope signature diet (elimination). Values of delta(13)C in the liver, blood, and muscle of the control snakes reached equilibrium with and were, respectively, 1.73, 2.25 and 2.29 greater than in their diet, this increase is called an isotopic discrimination factor (Deltadelta(13)C = delta(13)C(snake) - delta(13)C(food)). Values of delta(15)N in snake tissues did not achieve equilibrium with the diets in any of the exposures and thus Delta(15)N could not be estimated. Values of metabolic turnover rates (m) for delta(13)C and delta(15)N were greater in liver than in muscle and blood, which were similar, and relative results remained the same if the fraction of (15)N and (13)C were modeled. Although caution is warranted because equilibrium values of stable isotopes in the snakes were not achieved, values of m were greater for delta(13)C than delta(15)N, resulting in shorter times to dietary equilibrium for delta(13)C upon a diet shift, and for both stable isotopes in all tissues, greater during an elimination than in an uptake shift in diet stable isotope signature. Multiple explanations for the observed differences between uptake and elimination shifts raise new questions about the relationship between animal and diet stable isotope concentrations. Based on this study, interpretation of feeding ecology using stable isotopes is highly dependent on the kind of stable isotope, tissue, direction of diet switch (uptake versus elimination), and the growth rate of the animal.
在以蚯蚓（赤子爱胜蚓）为维持日粮饲养的圈养孵化的玉米蛇（锦蛇指名亚种）的不同组织中，评估了δ¹⁵N和δ¹³C的代谢周转率（m）。这些蚯蚓的δ¹⁵N（相差644‰）和δ¹³C（相差5.0‰）差异很大。在这个为期144天的实验中使用了三种处理方式，包括全程使用相同的日粮（对照）、从贫化稳定同位素特征日粮转换为富集稳定同位素特征日粮（摄取）以及从富集稳定同位素特征日粮转换为贫化稳定同位素特征日粮（消除）。对照蛇肝脏、血液和肌肉中的δ¹³C值与日粮达到平衡，分别比日粮中的值高1.73、2.25和2.29，这种增加称为同位素分馏因子（Δδ¹³C = δ¹³C（蛇）-δ¹³C（食物））。在任何一种暴露情况下，蛇组织中的δ¹⁵N值都未与日粮达到平衡，因此无法估算Δ¹⁵N。肝脏中δ¹³C和δ¹⁵N的代谢周转率（m）值高于肌肉和血液中的值，肌肉和血液中的值相似，如果对¹⁵N和¹³C的比例进行建模，相对结果保持不变。尽管由于蛇体内稳定同位素未达到平衡值，需要谨慎对待，但δ¹³C的m值大于δ¹⁵N的m值，这导致日粮转换时δ¹³C达到日粮平衡的时间更短，并且对于所有组织中的两种稳定同位素，消除过程中的值大于摄取过程中日粮稳定同位素特征的转换值。对摄取和消除转换之间观察到的差异的多种解释，引发了关于动物和日粮稳定同位素浓度之间关系的新问题。基于这项研究，使用稳定同位素解释摄食生态学高度依赖于稳定同位素的种类、组织、日粮转换方向（摄取与消除）以及动物的生长速率。
