The known, the unknown and the unknowable: weaning times from archaeological bones using nitrogen isotope ratios

Empirical observations of δ15N of bone collagen by age at death from 56 archaeological sites (n = 1560) document an increase over the adult mean at ages ∼0-2 years. These observations are generally consistent with a hypothesis that posits a difference in trophic level between the nursing infant and the mother; however, using these data to reconstruct weaning ages is problematic. The assumptions used to determine age of weaning are reviewed; uncertainty in the isotopic trophic offsets, high scatter due to low sample numbers, errors in the age determination of infants, and how representative the samples are for the whole population are possible contributors to uncertainty in determining weaning times from archaeological bones. Other possible explanations for these age-related isotopic differences have generally not been considered in the archaeological literature. Factors bearing further investigation are the possibility of developmental (nondietary) differences in tissue isotopic composition, incorporation of non-protein nitrogen in milk and the effects of the gut microbiome.