Stable Nitrogen Isotope Analysis of Dentine Serial Sections Elucidate Sex Differences in Weaning Patterns of Wild Chimpanzees (Pan troglodytes)

Offspring provisioning is one of the most energetically demanding aspects of reproduction for female mammals. Variation in lactation length and weaning strategies between chimpanzees (Pan troglodytes), our closest living relative, and modern human societies have been reported. When and why these changes occurred is frequently debated. Our study used stable nitrogen isotope data of tooth root dentine from wild Western chimpanzees (Pan troglodytes verus) in Tai National Park, C^ote d’Ivoire, to quantify weaning in these chimpanzees and explore if infant sex plays a role in maternal investment. We analyzed serial sections of deciduous lateral incisor root dentine from four Tai chimpanzees to establish the d15N signal of nursing infants; we then analyzed serial sections of first permanent mandibular molar root dentine from 12 Tai chimpanzees to provide quantitative d15N data on weaning in this population. Up to 2 years of age both sexes exhibited dentine d15N values 2–3% higher than adult female Tai chimpanzees, consistent with a nursing signal. Thereafter a steady decrease in d15N values consistent with the onset, and progression, of weaning, was visible. Sex differences were also evident, where male d15N values decreased at a significantly slower rate compared to females. Confirmation of sex differences in maternal investment among Tai chimpanzees, demonstrates the viability of using isotope analysis to investigate weaning in non-human primates. Additionally, assuming that behaviors observed in the Ta€? chimpanzees are illustrative of the ancestral pattern, our results provide a platform to enable the trajectory of weaning in human evolution to be further explored.