Abstract
Stable isotopes are widely used in ecosystem studies to decipher trophic relationships between species. Using stable isotopes as trophic markers requires knowledge of the stable isotope ratios of consumer and putative prey, and of trophic discrimination factors (TDFs), the difference between the stable isotope ratios of predator and prey. TDFs differ among species, tissue sampled and the consumer’s physiological state. We tested whether consumer ontogeny also influences its stable isotope ratios by measuring the δ13C and δ15N values, and their associated TDFs, of African penguin (Spheniscus demersus) blood components (whole blood, red blood cells, plasma) among four age classes (chicks, fledglings, juveniles and adults). Blood was sampled from captive penguins concurrently with their diet. Ontogenetic effects on TDFs were complex, depending on the blood component and isotope considered. Blood components differed in their sensitivity to consumer physiology, but TDFs for both carbon and nitrogen were generally lower for the younger age classes. Age class influenced the stable isotope compositions of whole blood and red blood cells with chicks and fledglings exhibiting significantly lower δ13C and δ15N values than juveniles and adults. Plasma showed no effect of ontogeny. The results indicate age class affects stable isotope ratios and TDFs through ontogenetic changes in physiology. Therefore, when uncertainty concerning TDFs exists in studies of wild animals, stable isotope analysis should preferably be combined with other dietary techniques.
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The datasets generated during and analysed during the current study are available from the corresponding author on reasonable request.
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Acknowledgements
This work is based upon research supported by the South African Research Chairs Initiative of the Department of Science and Technology and the National Research Foundation (Grant number: 64801). This work forms part of the MSc thesis of Isabel Micklem who was funded by the National Research Foundation of South Africa. The authors thank Southern African Foundation for the Conservation of Coastal Birds (SANCCOB) team for the use of their captive penguins and all their help during sampling. The authors are indebted to Ian Newton and John Lanham from the University of Cape Town for timeously running the stable isotope analyses. The authors thank the Associate Editor and anonymous reviewers whose comments greatly improved the manuscript.
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Micklem, I.A., Connan, M., Stander, N. et al. Influence of ontogeny on stable isotope ratios and trophic discrimination factors of African penguin (Spheniscus demersus) tissues. Mar Biol 168, 96 (2021). https://doi.org/10.1007/s00227-021-03877-0
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DOI: https://doi.org/10.1007/s00227-021-03877-0