Abstract Studies on settlement patterns suggested that the fisher-hunter-gatherers who constructed the shellmounds (or sambaquis) scattered along the Brazilian Southeastern coast must have experienced considerable population growth by living in rich coastal settings. In this paper we assess, using stable isotopes, the weaning patterns and subadult diets of the sambaqui Jabuticabeira II (1214–830 cal BC to 118–413 cal AD) to test: 1) if weaning strategies are compatible with scenarios of high population density and 2) if there are evidences of dietary sex differences and sex-biased parental investment. Stable isotope data (δ15N, δ13Ccol, and δ13Cap) of 106 samples from 60 individuals (adults and subadults) were analyzed by combining bone cross-sectional and tooth serial-sectioning approaches to simulate a longitudinal study. Various analytical methods were used to describe weaning processes by identifying age-related changes in the diet of juveniles compared to those of adults. The isotopic results show that although exclusive breastfeeding length is variable, the introduction of a supplementary diet occurs around 6 months of age, whereas complete weaning was achieved for most children at approximately 2–3 years of age (more probably ~2.3 years of age). Little variability was observed in weaning and post-weaning diets. Differences between adult males and females suggest sex-biased dietary peculiarities. However, no clear cut difference in parental investment strategies favoring boys could be found. Because a weaning completion age of 2.3 years (and the presumed inter-birth interval that corresponds to) can be associated with slow to moderate growth, our reconstruction of weaning patterns and subadult diets of this sambaqui group partially supports a scenario compatible with high population growth.