Milk isotopic values demonstrate that nursing fur seal pups are a full trophic level higher than their mothers

• Published in: Rapid Communications in Mass Spectrometry
• Main Authors: Cherel, Yves, Hobson, Keith A., Guinet, Christophe
• Other Authors: Centre d'Études Biologiques de Chizé - UMR 7372 (CEBC), Université de La Rochelle (ULR)-Centre National de la Recherche Scientifique (CNRS)-Institut National de Recherche pour l’Agriculture, l’Alimentation et l’Environnement (INRAE), Air Quality Research Division Toronto, Environment and Climate Change Canada
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2015
• Subjects: [SDE]Environmental Sciences; Antarctic; The Antarctic; Antarc*
• Online Access: https://hal.archives-ouvertes.fr/hal-01274143; https://doi.org/10.1002/rcm.7243

International audience RATIONALE: In mammals including humans, mother-to-offspring transfer of nutrients has been the focus of several isotopicstudies. Measurement of δ13C and δ15N values were mainly conducted on easily sampled tissues such as blood and hair thatallow the calculation of apparent discrimination factors (Δ13C and Δ15N) between offspring and maternal tissues. Quantifyingreal Δ13C and Δ15N values requires the measurement of the δ13C and δ15N values of milk, the exclusive food of newborns.Surprisingly, little isotopic information is available on milk and its biochemical components (lipids and proteins).METHODS: Paired blood and milk samples from 10 lactating females and their pups were collected from two otariidspecies, the Antarctic and subantarctic fur seals. Tissue δ13C and δ15N values were measured using continuous-flowisotope ratio mass spectrometry (CFIRMS) on maternal and offspring blood, and on whole milk, lipid-free milk and milklipids, thus allowing the calculation and comparison of apparent (maternal blood to offspring blood) and real (lipid-freemilk to offspring blood) Δ13C and Δ15N values.RESULTS: In both fur seal species, the apparent Δ13C values averaged ~0.0‰. Lipid-free milk was slightly 13C-depletedcompared with both maternal and pup blood and it was strongly 13C-enriched (~6.3 ‰) compared with milk lipids. Incontrast, the apparent and real Δ15N values averaged 1.2–1.4 and 2.6–3.0‰, respectively, the differences being explainedby the ~1.5 ‰ lower milk δ15N values than those of maternal blood.CONCLUSIONS: In fur seals, the low apparent Δ15N translated into a higher real Δ15N value, amounting to a full trophiclevel, which is in agreement with the almost never verified hypothesis that 15N differences between mothers and theiroffsprings should reflect one complete trophic level. The study highlights the need to measure milk isotopic values todisentangle the nutritional mother-to-offspring relationships.