Nitrogen stress causes unpredictable enrichments of 15N in two nectar-feeding bat species.

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.