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

• Main Authors: Yves Cherel, Keith A Hobson, Christophe Guinet
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Antarctic; The Antarctic; Antarc*
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.1037.6826; http://www.cebc.cnrs.fr/publipdf/2015/CRCMS29_2015.pdf

RATIONALE: In mammals including humans, mother-to-offspring transfer of nutrients has been the focus of several isotopic studies. Measurement of δ 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) Δ 13 C and Δ 15 N values. RESULTS: In both fur seal species, the apparent Δ 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 Δ 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 Δ 15 N translated into a higher real Δ 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.