Isotopic Discrimination between Food and Blood and Feathers of Captive Penguins: Implications for Dietary Studies in the Wild

Abstract

Using measurements of naturally occurring stable isotopes to reconstruct diets or source of feeding requires quantifying isotopic discrimination factors or the relationships between isotope ratios in food and in consumer tissues. Diet-tissue discrimination factors of carbon ((13)C/(12)C, or delta (13)C) and nitrogen ((15)N/(14)N, or delta (15)N) isotopes in whole blood and feathers, representing noninvasive sampling techniques, were examined using three species of captive penguins (king Aptenodytes patagonicus, gentoo Pygoscelis papua, and rockhopper Eudyptes chrysocome penguins) fed known diets. King and rockhopper penguins raised on a constant diet of herring and capelin, respectively, had tissues enriched in (15)N compared to fish, with discrimination factors being higher in feathers than in blood. These data, together with previous works, allowed us to calculate average discrimination factors for (15)N between whole lipid-free prey and blood and feathers of piscivorous birds; they amount to +2.7 per thousand and +4.2 per thousand, respectively. Both fish species were segregated by their delta (13)C and delta (15)N values, and importantly, lipid-free fish muscle tissue was consistently depleted in (13)C and enriched in (15)N compared to whole lipid-free fish. This finding has important implications because previous studies usually base dietary reconstructions on muscle of prey rather than on whole prey items consumed by the predator. We tested the effect of these differences using mass balance calculations to the quantification of food sources of gentoo penguins that had a mixed diet. Modeling indicated correct estimates when using the isotopic signature of whole fish (muscle) and the discrimination factors between whole fish (muscle) and penguin blood. Conversely, the use of isotopic signatures of muscle together with discrimination factors between whole fish and blood (or the reverse) leads to spurious estimates in food proportions. Consequently, great care must be taken in the choice of isotopic discrimination factors to apply to wild species for which no controlled experiments on captive individuals have been done. Finally, our results also indicate that there is no need to remove lipids before isotopic analysis of avian blood.

Resource partitioning within a tropical seabird community: new information from stable isotopes

“…Muscle samples were ground to a fine powder and muscle lipids were removed using cyclohexane. The low lipid content of whole blood does not typically necessitate lipid extraction (Cherel et al 2005). While overall statistically different (Kruskal-Wallis, H = 192.64, p < 0.0001), all mean values of C:N mass ratio encompassed a narrow range (3.1 to 3.5, always < 4) (see Tables 1-3).…”

“…15 N values and in feather Δ 15 N values (review in Cherel et al 2005), thus allowing comparisons of the nitrogen signatures between species to investigate their respective trophic positions. We assumed that Δ 15 N values were identical between chicks, immatures and adult birds because no experimental work tested the effect of growth per se on Δ 15 N in birds and modeling suggests that growing endotherms should show the same δ 15 N values as those of adults fed the same diet (Ponsard & Averbuch 1999).…”

Dietary Niche Shifts of Multiple Marine Predators under Varying Prey Availability on the Northeast Newfoundland Coast

“…To examine dietary niche position of predator groups in relation to capelin stable isotopic ratios, a best-fitting trophic discrimination factor was chosen for each group based on taxonomy and previous knowledge of diet. Values of 1.86‰ for δ 15 N and 0.46‰ δ 13 C for rockhopper penguins were applied to shearwater samples (Cherel et al, 2005), 3.1‰ for δ 15 N and −0.30‰ δ 13 C for ring-billed gull chicks (Hobson and Clark, 1992) were applied to gull chick samples, and 2.82‰ for δ 15 N and 1.28‰ δ 13 C for fin whales (Borrell et al, 2012) were applied to humpback whale samples.…”

“…For analysis, the cellular component of shearwater blood samples and whole blood of gull chicks were used. Whole blood and the cellular component have very similar stable isotopic ratios (Cherel et al, 2005), allowing direct comparison. Although, EDTA may affect the stable isotopic ratios of blood (Lemons et al, 2012), EDTA tends to stay in the plasma when blood is centrifuged, and thus the effect on the cellular component should be negligible (Käkelä et al, 2007).…”

Stable Isotope Research in Southern African Birds

“…Numerous other factors can affect diet-tissue fractionation and studies to date have addressed these issues for a range of animal species, mostly under controlled conditions in captivity (see Post, 2002;Vanderklift and Ponsard, 2003;Robbins et al, 2005). For example, fractionation factors may be affected by, i) prey type, ii) prey quality, iii) temperature, iv) form of nitrogen excretion, v) habitat type, vi) water stress, and vii) nutritional status (including differences in dietary C:N ratios) (Ambrose, 1991;Hobson and Clark, 1992a,b;Hobson et al, 1994.;Pinnegar and Polunin, 1999;Perkins and Speakman, 2001;Bearhop et al, 2002;Vanderklift and Ponsard, 2003;Pearson et al, 2003;Evans Ogden et al, 2004;Cherel et al, 2005;Robbins et al, 2005;Podlesak and McWilliams, 2006). In cases where we do not know more about discrimination factors related to these factors we may be unable to make clear interpretations concerning trophic positions for particular species or animal communities.…”