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

RATIONALE In mammals including humans, mother-to-offspring transfer of nutrients has been the focus of several isotopic studies. Measurement of δ(13) C and δ(15) N values were mainly conducted on easily sampled tissues such as blood and hair that allow the calculation of apparent discrimination factors (∆(13) C and ∆(15) N) between offspring and maternal tissues. Quantifying real ∆(13) C and ∆(15) N values requires the measurement of the δ(13) C and δ(15) N 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 δ(13) C and δ(15) N 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) ∆(13) C and ∆(15) N values. RESULTS In both fur seal species, the apparent ∆(13) C values averaged ~0.0 ‰. Lipid-free milk was slightly (13) C-depleted compared with both maternal and pup blood and it was strongly (13) C-enriched (~6.3 ‰) compared with milk lipids. In contrast, the apparent and real ∆(15) N values averaged 1.2-1.4 and 2.6-3.0 ‰, respectively, the differences being explained by the ~1.5 ‰ lower milk δ(15) N values than those of maternal blood. CONCLUSIONS In fur seals, the low apparent ∆(15) N translated into a higher real ∆(15) N value, amounting to a full trophic level, which is in agreement with the almost never verified hypothesis that (15) N 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.