Voigt Christian C, Matt Felix
Institute for Zoo and Wildlife Research, Evolutionary Ecology Research Group, Postfach 601103, 10252 Berlin, Germany.
J Exp Biol. 2004 Apr;207(Pt 10):1741-8. doi: 10.1242/jeb.00929.
We estimated the effect of nitrogen stress on the nitrogen isotope enrichments in wing membrane and blood of two nectar-feeding bats (Glossophaga soricina and Leptonycteris curasoae) by offering a nitrogen-poor diet with a high delta(15)N and delta(13)C. Before the experiment, bats were sustained on a normal diet with a low delta(15)N and delta(13)C. Under this first food regime, the fractionation of nitrogen isotopes averaged 3.1 per thousand delta(15)N for blood and 4.4 per thousand delta(15)N for wing membrane, which was almost twice as high as the corresponding fractionation of carbon isotopes. After switching to the nitrogen-poor diet, the enrichment of heavy isotopes increased for both elements in all tissues under study. The recently published estimates of half-life of carbon isotopes indicated a low turnover rate of carbon in wing membrane and blood and an almost constant half-life over varying losses of body mass. The estimates of half-life of nitrogen were two to six times higher than those of carbon. We argue that this discrepancy was caused by the mixing of nitrogen isotopes from internal and external sources. The mixing effect was probably negligible for carbon as the amount of ingested carbon outweighed the amount of mobilized carbon from internal sources. A correlation between the estimated turnover rates of nitrogen and losses of body masses was probably obscured by the additional fractionation of nitrogen isotopes in catabolic animals. We conclude that the interpretation of nitrogen isotope data of free-ranging animals is difficult when the animal's diet is changing to a critical nitrogen content.
我们通过提供一种具有高δ(15)N和δ(13)C的低氮饮食，来估计氮胁迫对两种食蜜蝙蝠（灰长吻蝠和库拉索长舌蝠）翼膜和血液中氮同位素富集的影响。在实验前，蝙蝠以低δ(15)N和δ(13)C的正常饮食维持生存。在这种第一种食物条件下，血液中氮同位素的分馏平均为每千分比δ(15)N 3.1，翼膜中为每千分比δ(15)N 4.4，这几乎是相应碳同位素分馏的两倍。在转换到低氮饮食后，所研究的所有组织中两种元素的重同位素富集都增加了。最近发表的碳同位素半衰期估计表明，翼膜和血液中碳的周转率较低，并且在体重变化时半衰期几乎恒定。氮的半衰期估计比碳高两到六倍。我们认为这种差异是由来自内部和外部来源的氮同位素混合造成的。对于碳来说，混合效应可能可以忽略不计，因为摄入的碳量超过了从内部来源动员的碳量。氮的估计周转率与体重损失之间的相关性可能被分解代谢动物中氮同位素的额外分馏所掩盖。我们得出结论，当动物的饮食转变为临界氮含量时，对自由放养动物的氮同位素数据进行解释是困难的。
