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First published online April 8, 2004
Journal of Experimental Biology 207, 1741-1748 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00929

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

Christian C. Voigt^1,* and Felix Matt²

¹ Institute for Zoo and Wildlife Research, Evolutionary Ecology Research Group, Postfach 601103, 10252 Berlin, Germany
² Institute of Zoology II, University Erlangen-Nürnberg, Staudtstr. 5, 91058 Erlangen, Germany

^* Author for correspondence (e-mail: voigt{at}izw-berlin.de)

Accepted 6 February 2004

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 ¹⁵N and ¹³C. Before the experiment, bats were sustained on a normal diet with a low ¹⁵N and ¹³C. Under this first food regime, the fractionation of nitrogen isotopes averaged 3.1 ¹⁵N for blood and 4.4 ¹⁵N 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.

Key words: nitrogen isotope, nitrogen stress, fractionation, mixing, Glossophaga soricina, Leptonycteris curasoae

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