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

Stable isotope signatures of lactating females and their nursing offspring were measured on I I species, including herbivores, carnivores, hibernators , and non-hibernators. We hypothesized that: (1) nursing offspring would ha ve stable isotope signatures that were a trophic level higher than their mo thers, and (2) this pattern would be species-independent. The plasma of adu lt females had a delta N-15 enrichment over their diets of 4.1 +/-0.7 parts per thousand, but offspring plasma had a mean delta N-15 enrichment over m aternal plasma of 0.9 +/-0.8 parts per thousand and no C enrichment (0.0 +/ -0.6 parts per thousand). The trophic level enrichment did not occur betwee n mother and offspring because milk was depleted in both delta N-15 (1.0 +/ -0.5 parts per thousand) and delta C-13 (2.1 +/-0.9 parts per thousand) rel ative to maternal plasma. Milk to offspring plasma enrichment was relativel y small (delta N-15 enrichment of 1.9 +/-0.7 parts per thousand and delta C -13 enrichment of 1.9 +/-0.8 parts per thousand) compared to the trophic le vel enrichment between the adults and their diets. While some species did h ave significant differences between the isotope signatures of mother and of fspring, 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 o f predators when both adults and nursing offspring are consumed must first establish the parameters that apply to a particular species/environment/die t combination.