Influence of lipids on stable isotope ratios in mammal hair: highlighting the importance of validation

Abstract

Carbon (δ13C) and nitrogen (δ15N) stable isotope ratios are increasingly used in ecological studies to evaluate diet composition and trophic relationships. However, lipids may influence stable isotope ratios due to the depletion of 13C in adipose tissues relative to proteins and carbohydrates. δ13C values can be corrected by lipid extraction or normalization models. The aims of our study were to evaluate the effects of lipid extraction on stable isotope ratios in a terrestrial mammal, the caribou (Rangifer tarandus caribou), and to propose relevant lipid normalization models that are method‐ and tissue‐specific for δ13C values. We also evaluated whether four δ13C lipid normalization and correction models proposed in the literature were applicable to our study species. Stable isotope ratios were obtained for hair, plasma, and red blood cell samples of 44 caribou in the Gaspésie National Park (Québec, Canada). The effects of lipid extraction on stable isotope ratios were tested using a paired t‐test. A simple linear model was used to correct for the effects of lipid extraction and to assess its performance compared to that of published equations. Lipid content significantly influenced δ13C values in caribou hair. The four lipid normalization equations commonly used in the literature did not accurately predict δ13Clipid‐free values of caribou hair. Based on our results, we recommend controlling systematically for lipids in terrestrial systems and analyzing δ13C (lipid‐free) and δ15N (bulk) from two separate aliquots to reach a greater precision. We also recommend controlling for lipids in hair tissue. If not possible, we recommend using a lipid normalization model that is tissue‐, method‐, and species‐specific or applying a model that has been previously validated for the tissue and species of interest.

Diet composition and selection of Père David's deer in Hubei Shishou Milu National Nature Reserve, China

“…To further reduce the effect of lipids on stable isotopes, initially processed fur samples and tissue samples from the previous step were subjected to a 2:1 chloroform–methanol solution for further lipid extraction (Ehrich et al, 2011; Rioux et al, 2019). The mixture was shaken and stored at 4°C overnight (18 h; Folch et al, 1957).…”

Trophic niche partitioning between two prey and their incidental predators revealed various threats for an endangered species

“…Stable isotope signatures measured in tissues may be biased due to the variability in the lipid content of samples because lipids are more depleted in 13 C relative to protein and carbohydrate fractions (DeNiro & Epstein, 1977; McConnaughey & McRoy, 1979). We divided caribou hair samples into two parts to determine δ 13 C (lipid‐extracted) and δ 15 N (no lipid‐extracted) values separately to account for lipid effect on stable isotope signatures: one part of the subsamples received no further treatment prior to nitrogen isotope analysis, and the second part was lipid‐extracted prior to carbon isotope analysis (Kelly, 2000; Lesage et al, 2010; Post et al, 2007; Rioux et al, 2019). We conducted lipid extraction using the second part of powdered hair samples (to remove internal lipids) (Dunnett, 2005) and a solvent consisting of a mixture of chloroform and methanol (2:1 v/v) (Folch et al, 1957).…”

“…After three extractions, we dried samples by evaporation overnight, rinsed with distilled water, oven‐dried overnight at 50°C and powdered again. Due to methodological constraints, we used previously developed models of caribou normalization to correct the δ 13 C values of other animal hair for lipid content (equation 8 in Rioux et al, 2019).…”

Concentrations of urinary neopterin, but not suPAR, positively correlate with age in rhesus macaques

“…Urine and feces provide avenues to collect physiological markers of health non-invasively. These include urinary or fecal measurement of glucocorticoid metabolites (Ostrowski et al, 2006;Jaimez et al, 2012;Mohlman et al, 2020), urinary C-peptide of insulin (Emery Thompson and Knott, 2008;Higham et al, 2011;Sacco et al, 2021), isotopes (Deschner et al, 2012;Vogel et al, 2012;Rioux et al, 2019), fecal parasite load (Gillespie and Chapman, 2006;Nalubamba et al, 2012;Liu et al, 2022), and thyroid hormones (Gobush et al, 2014;Cristóbal-Azkarate et al, 2016). These measures are generally indicative of energy mobilization and allocation and may be linked to health, while some may also be indirectly related to immune system activation and function.…”