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

Yves Cherel (1) , Keith A. Hobson (2) , Christophe Guinet (1)

1 CEBC - Centre d'Études Biologiques de Chizé - UMR 7372
2 Air Quality Research Division [Toronto]

RATIONALE: In mammals including humans, mother-to-offspring transfer of nutrients has been the focus of several isotopic studies. Measurement of δ13C and δ15N values were mainly conducted on easily sampled tissues such as blood and hair that allow the calculation of apparent discrimination factors (Δ13C and Δ15N) between offspring and maternal tissues. Quantifying real 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 otariid species, the Antarctic and subantarctic fur seals. Tissue δ13C and δ15N values were measured using continuous-flow isotope ratio mass spectrometry (CFIRMS) on maternal and offspring blood, and on whole milk, lipid-free milk and milk lipids, thus allowing the calculation and comparison of apparent (maternal blood to offspring blood) and real (lipid-free milk 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-depleted compared with both maternal and pup blood and it was strongly 13C-enriched (~6.3 ‰) compared with milk lipids. In contrast, the apparent and real Δ15N values averaged 1.2–1.4 and 2.6–3.0‰, respectively, the differences being explained by 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 trophic level, which is in agreement with the almost never verified hypothesis that 15N differences between mothers and their offsprings should reflect one complete trophic level. The study highlights the need to measure milk isotopic values to disentangle the nutritional mother-to-offspring relationships.

Domaines: Sciences de l'environnement

Yves Cherel, Keith A. Hobson, Christophe Guinet. Milk isotopic values demonstrate that nursing fur seal pups are a full trophic level higher than their mothers. Rapid Communications in Mass Spectrometry, 2015, 29, pp.1485-1490. ⟨10.1002/rcm.7243⟩. ⟨hal-01274143⟩