Can the carbon and nitrogen isotope values of offspring be used as a proxy for their mother’s diet? Using foetal physiology to interpret bulk tissue and amino acid δ15N values

摘要

The measurement of bulk tissue nitrogen (δ15N) and carbon isotope values (δ13C) chronologically along biologically inert tissues sampled from offspring can provide a longitudinal record of their mothers’ foraging habits. This study tested the important assumption that mother–offspring stable isotope values are positively and linearly correlated. In addition, any change in the mother–offspring bulk tissues and individual amino acids that occurred during gestation was investigated. Whiskers sampled from southern elephant seal pups (Mirounga leonina) and temporally overlapping whiskers from their mothers were analyzed. This included n?=?1895 chronologically subsampled whisker segments for bulk tissue δ15N and δ13C in total and n?=?20 whisker segments for amino acid δ15N values, sampled from recently weaned pups (n?=?17), juvenile southern elephant seals (SES)??2?years old (n?=?23) and adult female SES (n?=?17), which included nine mother–offspring pairs. In contrast to previous studies, the mother–offspring pairs were not in isotopic equilibrium or linearly correlated during gestation: the Δ15N and Δ13C mother–offspring offsets increased by 0.8 and 1.2‰, respectively, during gestation. The foetal bulk δ15N values were 1.7?±?0.5‰ (0.9–2.7‰) higher than mothers’ δ15N values before birth, while the foetal δ13C increased by ~1.7‰ during gestation and were 1.0?±?0.5‰ (0.0–1.9‰) higher than their mothers’ δ13C at the end of pregnancy. The mother–offspring serine and glycine Δ15N differed by ~4.3‰, while the foetal alanine δ15N values were 1.4‰ lower than that of their mothers during the third trimester of pregnancy. The observed mother–offspring δ15N differences are likely explained by shuttling of glutamate–glutamine and glycine–serine amongst skeletal muscle, liver, placenta and foetal tissue. Foetal development relies primarily on remobilized endogenous maternal proteinaceous sources. Researchers should consider foetal physiology when using offspring bulk tissue isotope values as biomarkers for the mother’s isotopic composition as part of monitoring programmes.