in review). While see more indispensable amino acids must be derived from diet and are thus directly routed, it is known that dispensable amino acids may be synthesized de novo from other carbon containing compounds

(Howland et al. 2003, Jim et al. 2006). These results suggest that it may not be appropriate to lipid-extract prey samples when using isotopes to examine diet in consumers that consume lipid-rich foods, such as many marine mammals and seabirds. When samples have not been lipid extracted but C/N ratios are available, δ13C values can be corrected for lipid content using different algorithms (McConnaughey and McRoy 1979). This method allows one to choose an absolute difference between Obeticholic Acid in vivo pure protein and lipid and makes the assumption that pure protein has a theoretically derived atomic C/N ratio. While results of these studies are mixed with respect to the effects of lipid extraction on tissue δ13C values, we suggest that future studies minimize these confounding factors by using an accepted protocol to remove lipids from all samples. We offer a few simple rules as a guide when deciding how marine mammal tissues and associated prey should be prepared for SIA. Overall, our suggestions

are based on the type of consumer tissue(s) analyzed, which for marine mammals often depends on logistical considerations related to

sample availability. For consumers, samples should be prepared such that pure protein or pure lipid is analyzed. For example, protein-rich tissues known to contain a considerable amount of lipids (e.g., skin, muscle, internal organs, plasma, serum, and bone collagen) should be lipid-extracted prior to SIA. In contrast, whole blood (RBCs) and metabolically inert tissues constructed of keratin (e.g., fur and vibrissae) or tooth collagen (e.g., dentin) do not require lipid extraction because they do not contain considerable lipids. Lipid extraction is not necessary for studies focused 上海皓元 on deeper time scales where tooth hydroxyapatite (e.g., enamel) is the only trustworthy substrate. In regards to prey, it would be ideal to perform isotopic analyses of lipid extracted (LE) and nonlipid extracted (NLE) subsamples from individual prey samples when possible. At the very least, isotopic differences between LE and NLE subsamples should be characterized for any lipid-rich prey type (>15% lipids on a dry basis) in situations where consumers are eating a significant portion (>50% edible biomass) of such prey. This is especially important when analyzing consumer tissues that reflect bulk diet, such as bioapatite or lipid.