Tissue-specific isotopic discrimination factors in gentoo penguin (Pygoscelis papua) egg components: Implications for dietary reconstruction using stable isotopes

Stable isotope analysis (δ13C and δ15N) is a useful tool when examining animal diets due to a general enrichment in heavier isotopes from prey tissues to the tissues of the predators consuming them. However, the amount of this enrichment, or discrimination, can vary among taxa and tissue type, limiting the use of stable isotope analysis when estimating diet composition. In this study we calculate the dietary isotopic discrimination factors of specific Pygoscelis penguin egg tissues, including eggshell, shell membrane, albumen and yolk, using a captive population of gentoo penguins fed known diets. We found that discrimination factors varied by isotope, tissue, and whether factors were calculated from whole fish or fish muscle. The observed variation in discrimination factors across tissues likely reflects differences in biochemical and metabolic processes during tissue synthesis. We validated the use of tissue discrimination factors derived in this study by independently reconstructing the diet composition of wild gentoo penguins at Cape Sherriff, Livingston Island, Antarctica using the δ15N values of eggshell organics and shell membrane. While eggshell organics and shell membrane from the same egg differed in raw δ15N values, modeling confirmed that these differences were due to tissue-specific isotopic discrimination. Furthermore, our results suggest that in 2006, female gentoo penguins at Cape Sherriff consumed a higher percentage of krill than fish during the egg-laying period.