While definitely not a complete list, these points are commonly presented in the literature and therefore warrant discussion. There is no disciplinary standard when it comes to the decision on whether or not tissues should be lipid extracted prior to SIA. Most

studies on marine mammals cite the importance of lipid extraction when trying to interpret differences in δ13C values among tissues. The concentration of lipids, which have δ13C values that are up to 5‰ lower than associated proteins, varies among tissues. Thus lipid-rich Pifithrin �� tissues, such as liver, muscle and various blood components (e.g., serum and plasma), likely have lower δ13C values than lipid-poor tissues (e.g., hair, dentin, and whiskers). Interpreting differences in δ13C values among tissues can be difficult, since they can either be due to systematic tissue-specific differences in lipid concentration or temporal changes in ecology, or a combination of these possibilities. The δ15N values of lipids are not significantly different than associated proteins because lipids are primarily composed of carbon, oxygen, and hydrogen and only contain small amounts of nitrogen in cell walls and lipoprotein membranes (Lehninger 1982). Lipid extraction

is especially important in the interpretation of experiments designed to determine trophic Histone Methyltransferase inhibitor or tissue-specific discrimination factors (Hobson et al. 1996, Kurle 2002, Kurle and Worthy 2002, Lesage et al. 2002, Zhao et al. 2006, Stegall et al. 2008). Kurle (2002) found significant differences in δ13C values of serum and plasma in comparison to red blood cells (RBCs) in captive northern fur seals and attributed this to differences in the amount of lipid present in each blood component. In comparison to RBCs, total lipids are higher in plasma and serum because these components contain serum albumin, which is a major carrier of fatty acids

in the blood (Lehninger 1982). Furthermore, serum does not contain fibrinogen and many other clotting proteins (Schier et al. 1996), and thus has a higher lipid to protein ratio than plasma or RBCs, which explains why serum typically has lower δ13C values than plasma (Kurle 2002; Orr et al. 2009). Despite these mechanistic hypotheses, Stegall triclocarban et al. (2008) found no significant difference in δ13C values between lipid extracted (LE) and nonlipid extracted (NLE) serum from wild Steller sea lion pups and juveniles. Interestingly, this study also found no differences in δ13C values between LE milk, the assumed dietary source for pups, and NLE or LE serum. The tissue-to-diet discrimination patterns for three species of phocid seals reported in Lesage et al. (2002) are confounded by the fact that none of the pinniped tissues analyzed in the study were lipid extracted. As a result, these authors conclude that lipid extraction should be routine when measuring lipid-rich tissues or with tissues in which lipid content may vary with changes in diet or nutritional status.