Stable isotope records of nursing and weaning: A case study in elephants with implications for paleobiological investigations

Abstract

Nursing and weaning are behaviors that are central to the growth of offspring and success of mammal populations. The duration of nursing and timing of weaning are essential elements of a species' life history, but they are also somewhat plastic; weaning age increases during intervals of drought or other dietary stress. Thus, weaning age provides a proxy for nutritional stress in an extinct population, provided there is a way to determine weaning age for long-dead individuals. In growth records of proboscidean tusks, stable isotope compositions have been used to investigate nursing and weaning, but a taxon-specific framework for interpreting isotope data has been lacking. Here, we present results of a longitudinal study of stable isotopes in a nursing dam-calf pair of captive African elephants. The data include behavioral observations and hair growth rates, as well as stable isotope ratios in elephant feed and drinking water, in plasma and milk from the dam, and in tail hair from both individuals. Serial changes in the stable isotope ratios of nitrogen (δ15N), carbon (δ13C), oxygen (δ18O), and hydrogen (δ2H) reflect the transitions from placental nutrition in utero to exclusive nursing immediately after birth, and then to final weaning when the calf was six years old. Three of the four isotope systems reveal enrichment in the calf with respect to the dam prior to parturition and all show this relative enrichment during early nursing. At the end of nursing, final weaning was marked by changes in two of the systems: the convergence of dam and calf δ13C records and the abrupt onset of relative depletion in calf 15N following a year of elevated δ15N in both individuals. These patterns provide criteria for recognizing records of nursing and weaning contained in fossil tusks, providing grounds for assessing stress in prehistoric populations.