Age, body size, and amino acid drivers of δ2H variation in the Cocos booby

Published: 24 August 2026| Version 1 | DOI: 10.17632/snsm5sx9j5.1
Contributors:
Kaycee Morra,
,
,
,

Description

Hydrogen isotope ratios (δ2H) are widely used to study animal movement and food web energetics. However, unexplained, non-geographically derived δ2H variation can limit the accuracy and resolution of δ2H-based interpretations. We examined potential causes of δ2H variation–including age, body size, and amino acid (AA) factors–in a non-migratory Cocos booby (Sula brewsteri) colony around Islas Marietas, Mexico (20°41′N, 105°36′W) in 2016-2018. We also explored the applicability of novel AA δ2H data as high-resolution tracers for biosynthesis and food web dynamics in the marine realm. Among the 87 birds in our study, the δ2H values of plasma from chicks (<45 days old) and feathers from juveniles (grown from ~40-70 days old) both increased with age by >25‰, and plasma δ2H correlated with body size in both chicks and adults. Proline δ2H values exhibited a striking range (over several hundred per mille), were the predominant AA driver of patterns in bulk plasma δ2H values (r2> 0.6 for chicks, > 0.8 for adults), and correlated with body size and chick age. Based on our dataset, we predict that Threonine δ2H values may indicate marine versus terrestrial resource use, Alanine δ2H values may reliably reflect environmental water, and Serine δ2H values may retain a marine basal food signal in carnivores. We suggest future studies carefully consider the influence of body size and age while interpreting δ2H datasets. Furthermore, we contend that AA δ2H data hold an untapped cache of ecological and physiological information that should be exploited to better understand δ2H propagation through food webs and refine the utility of this isotopic tool.

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Seabird, Amino Acid, Stable Isotope, Hydrogen Isotope

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