Histological, Trace Element, and Stable Isotopic Reconstruction of Dental Development, Diet, and Weaning in Ugandan Chimpanzees (Pan troglodytes schweinfurthii): Implications for Studies of Hominin Life History

Abstract

Understanding the origins of the modern human life history profile, or the scheduling of energy tradeoffs between growth, somatic maintenance, and reproduction, is a key issue in biological anthropology. In order to determine when, and in what contexts, the modern human life history profile evolved, we must look to our hominin ancestors in the fossil record to reconstruct their diets and life histories, as well as to our closest living relatives, the chimpanzees, for clues to the links between observed life history variables, such as weaning age, and hard tissue evidence for changes in diet and developmental pace that may also preserve in the fossil record. This project investigates the relationships between the dental development, the weaning process, and the diets of wild Ugandan chimpanzees (Pan troglodytes schweinfurthii) using three different, but interrelated methods. The first study uses histological analysis to compare dental formation and emergence variables between individuals from several chimpanzee communities, and results suggest that the chimpanzees from the Ngogo community have delayed dental development and life history pace relative to other communities of this subspecies (Pan troglodytes schweinfurthii). It is also found that the range of ages at first molar emergence across these wild chimpanzee communities overlaps with that previously documented for captive chimpanzees. A final finding is that a growth spurt during first molar root formation of the Ngogo chimpanzees in this study likely coincides with the age at which this tooth comes into functional occlusion, slightly preceding inferred ages at weaning completion documented by other researchers using a fecal isotope study of Ngogo infant chimpanzees. The second study examines the relationship between trace elemental distributions in chimpanzee dental tissues and structural indicators of developmental pace established in the previous study, to assess whether the first molar root growth spurt shows evidence of aligning with weaning completion. This work uses laser ablation inductively coupled plasma-mass spectrometry (LA ICP-MS) to collect calcium-normalized barium (Ba/Ca) values from the first molars used in the previous study, and finds that the distribution of this apparent proxy for dietary change within the enamel and dentine of these chimpanzees’ teeth supports the link between the timing of the root growth spurt and weaning completion proposed in the first study. The third study attempts to disentangle the numerous sources of variability in chimpanzee dietary inputs, which manifest in their bodily tissues, and which inform studies of dietary reconstruction, paleoecology, and life history for extinct taxa, including fossil hominins. Stable isotope analyses of carbon (δ13C), oxygen (δ18O), and nitrogen (δ15N) are conducted on bone, teeth, and hair of the chimpanzees used in the previous two studies, and the variation in these isotopic outputs within an individual’s tissues, between the different tissues, and between individuals is in some cases great enough that this could obscure sources of dietary variation in fossil taxa to whom these same methods are applied. Another finding is that isotopic variation in sources of dietary inputs, either from different dietary items themselves, or from the same items growing in different environments, could confound interpretations of fossil hominin diets. Taken together, these three studies use structural markers of root growth rate and chemical markers of dietary changes to develop a proxy for weaning completion in chimpanzees, and present a refined diet-to-enamel δ13C offset value to be used in future primate dietary reconstructions.