Isotopic Evidence of Weaning Behaviors from Farming Communities of the Peruvian North-Central Coast: Insights into the Demography of the Transition from the Middle to the Late Formative Periods (500–400 B.C.)

Previous studies on settlement patterns have suggested that the prehistoric farmers who inhabited the Peruvian north-central coast during the transition between the Middle and Late Formative Periods (500–400 B.C.) experienced considerable population growth. In this study we assess the mean weaning age of the population recovered at the Quebrada Chupacigarro cemetery in the middle valley of Supe (2739–2153 cal B.P.), using stable isotopes, to test if weaning practices are compatible with this scenario. Stable nitrogen (δ¹⁵N) isotopes from bone collagen of 34 individuals (18 subadults and 16 adults) were analyzed by age using a cross-sectional approach with the WARN (Weaning Age Reconstruction with Nitrogen isotope analysis) method. Furthermore, stable carbon (δ¹³C) isotopes were assessed to identify age-related dietary differences between juveniles and adults. The results show that the introduction of supplementary foods occurred around 6 months of age (95% CI: 0.0–1.2 years; 65% CI: 0.1–0.9 years), while complete weaning was likely achieved at 2.6 years (95% CI: 1.2–4.2 years; 65% CI: 1.9–3.5 years). The findings suggest a relatively late age of end of weaning, not necessarily compatible with higher fertility rates. The resultant increased fertility in Quebrada Chupacigarro is plausible only under the hypothetical failure of postpartum infertility mechanisms, modulated by other potential factors from the context, such as culture-driven weaning practices.