Nitrogen and carbon isotope fractionation between mothers, neonates, and nursing offspring

Stable isotope signatures of lactating females and their nursing offspring were measured on 11 species, including herbivores, carnivores, hibernators, and non-hibernators. We hypothesized that: (1) nursing offspring would have stable isotope signatures that were a trophic level higher than their mothers, and (2) this pattern would be species-independent. The plasma of adult females had a '¹⁵N enrichment over their diets of 4.1ǂ.7‰, but offspring plasma had a mean '¹⁵N enrichment over maternal plasma of 0.9ǂ.8‰ and no C enrichment (0.0ǂ.6‰). The trophic level enrichment did not occur between mother and offspring because milk was depleted in both '¹⁵N (1.0ǂ.5‰) and '¹³C (2.1ǂ.9‰) relative to maternal plasma. Milk to offspring plasma enrichment was relatively small ('¹⁵N enrichment of 1.9ǂ.7‰ and '¹³C enrichment of 1.9ǂ.8‰) compared to the trophic level enrichment between the adults and their diets. While some species did have significant differences between the isotope signatures of mother and offspring, the differences were not related to whether they were hibernators or non-hibernators, carnivores or herbivores. Investigators wanting to use stable isotopes to quantify weaning or other lactation processes or diets of predators when both adults and nursing offspring are consumed must first establish the parameters that apply to a particular species/environment/diet combination.