Abstract
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) < 2years 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.
Highlights
The interaction between the nutritional ecology and reproduction of organisms regulates their population dynamics (e.g. Gardner and Grafen, 2009; Bergman et al, 2019)
The carbon (δ13C) and nitrogen (δ15N) stable isotope values of animal tissues are useful for reconstructing diet composition as well as inferring broad-scale patterns in habitat use and movement (Newsome et al, 2010; Ohkouchi et al, 2017; Pagani-Núñez et al, 2017). δ13C and δ15N values of a consumers’ tissue(s) reflect its diet but are offset in a predictable fashion due to physiologically mediated processes associated with nutrient assimilation and excretion (DeNiro and Epstein, 1976)
This study evaluates the assumption that bulk tissue δ13C and δ15N values measured along the length of phocid pup whiskers grown in utero predictably reflect the isotopic composition of their mothers (e.g. Lerner et al, 2018)
Summary
The interaction between the nutritional ecology and reproduction of organisms regulates their population dynamics (e.g. Gardner and Grafen, 2009; Bergman et al, 2019). Sampling approaches that can provide longitudinal dietary data are required and are especially insightful for species with complex life histories that include extensive movement between foraging and breeding grounds (Young et al, 2015; Nielsen et al, 2018) Such approaches must consider that the capture and handling of free-ranging large animals for dietary investigations are notoriously challenging. Isotopically heavier 15N is preferentially retained in the whole-body nitrogen pool because deamination of amino acids tends to remove 14N that is excreted via urine and faeces (DeNiro and Epstein, 1976; Post, 2002) Such diet-to-tissue isotopic offsets ( 13Cdiet–tissue or 15Ndiet–tissue) are commonly called trophic discrimination factors (TDFs) and are required for the use of mixing models to quantify dietary inputs (Parnell et al, 2013)
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