Abstract
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 (AAESS) and nonessential (AANESS) 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 AANESS glutamic acid, proline, serine, and glycine and the AAESS 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 AAESS and AANESS δ13C values and multivariate AA δ13C patterns.
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Data availability
All data and R code referenced in this manuscript are available at https://github.com/AlexiBesser/Sea-Otter-Tissue-AA-d13C.git.
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Acknowledgements
We thank the Sea Otter Stranding Network for collection of sea otter tissue samples and Tim Tinker for information and advice. We thank everyone in the Newsome Lab for their support and Viorel Atudorei and Laura Burkemper at UNM-CSI for their excellence in and out of the lab. Thank you to Eva Stricker and Oliver Shipley for intellectual support.
Funding
This work was supported by awards from the NSF Graduate Research Fellowship Program to EES (DGE-1418062) and ACB (DGE-1939267). EES was also supported by UNM-CSI and a North Pacific Research Board Graduate Fellowship. ALR was supported by UNM-CSI.
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ALR and EAES conceived and designed the study. ALR, EAES, and ACB performed isotopic and statistical analyses. ALR wrote the manuscript with contributions and editorial advice from EAES, ACB, and SDN.
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Communicated by Pawel Koteja.
We are among the first to compare amino acid carbon isotope values among animal tissues. We found tissue metabolic activity alters isotopic values and significantly affects ecological interpretations.
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Robinson, A.L., Elliott Smith, E.A., Besser, A.C. et al. Tissue-specific carbon isotope patterns of amino acids in southern sea otters. Oecologia 204, 13–24 (2024). https://doi.org/10.1007/s00442-023-05505-8
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DOI: https://doi.org/10.1007/s00442-023-05505-8