Tissue-specific carbon isotope patterns of amino acids in southern sea otters

The measurement of stable isotope values of individual compounds, such as amino acids (AAs), has become a powerful tool in animal ecology and ecophysiology. As with any emerging technique, questions remain regarding the capabilities and limitations of this approach, including how metabolism and tissue synthesis impact the isotopic values of individual AAs and subsequent multivariate patterns. We measured carbon isotope (δ¹³C) values of essential (AA_ESS) and nonessential (AA_NESS) AAs in bone collagen, whisker, muscle, and liver from ten southern sea otters (Enhydra lutris nereis) that stranded in Monterey Bay, California. Sea otters in this population exhibit high degrees of individual dietary specialization, making this an excellent dataset to explore differences in AA δ¹³C values among tissues in a wild population. We found the δ¹³C values of the AA_NESS glutamic acid, proline, serine, and glycine and the AA_ESS threonine differed significantly among tissues, indicating possible isotopic discrimination during tissue synthesis. Threonine δ¹³C values were higher in liver relative to bone collagen and muscle, which may indicate catabolism of threonine for gluconeogenesis, an interpretation further supported by correlations between the δ¹³C values of threonine and its gluconeogenic products glycine and serine in liver. This intraindividual isotopic variation yielded different ecological interpretations among tissues; for 6/10 of the sea otter individuals analyzed, at least one tissue indicated reliance on a different primary producer source than the other tissues. Our results highlight the importance of gluconeogenesis in a carnivorous marine mammal and indicate that metabolic processes influence AA_ESS and AA_NESS δ¹³C values and multivariate AA δ¹³C patterns.