Nutrition, physiology, and stable isotopes: New information from fasting and molting penguins

Stable isotopes are increasingly used in animal ecology, but little attention has been paid to the underlying physiological processes accounting for changes in N-15/N-14 and C-11/C-12 ratios, for example, the influence of protein balance on delta 15N values. We investigated a "professional" faster, the King Penguin (Aptenodytes patagonicus), to test the effect of long-term food deprivation on the isotopic signature of tissues that can be nondestructively sampled, i.e., blood and feathers. Fasting for 25 days induced a tissue N-15 enrichment, thus leading to a moderate increase in the apparent trophic levels of penguins. As expected, N-15 enrichment was higher in tissues with high protein turnover rates (e.g., plasma, 0.70 parts per thousand than in those with low turnover rates (e.g., blood cells, 0.24 parts per thousand. Fasting decreased the delta(13)C value of plasma, which was due to an increase in its lipid content, as indicated by a concomitant rise in plasma C/N ratio. Finally, food deprivation induced a N-15 enrichment in keratin (1.68 parts per thousand), as indicated by the lower nitrogen signature for portions of new feathers that were synthesized at sea than for those parts grown on land, thus illustrating the different pathways for resource allocation (dietary vs. endogenous reserves) in molting birds. The study also emphasized the usefulness of collecting whole blood (or blood cells) in the field to overcome both the fasting and lipid effects observed in plasma.