Stable isotopes reveal the effects of maternal rank and infant age on weaning dynamics in wild chacma baboons

Abstract

Maternal strategies reflect the trade-off between offspring needs and maternal ability to invest, a concept described by the evolutionary theory of parent–offspring conflict. In mammals this conflict has often been investigated by studying weaning, the transition from maternal milk consumption to dietary independence. An investigation of individual variation in weaning can provide information on the adaptive significance of maternal strategies in relation to social and biological variables. We analysed stable nitrogen isotopes of hair samples collected from 22 mother–infant dyads in a wild population of chacma baboons, Papio ursinus, in conjunction with behavioural data on suckling, to explore the temporal dynamics of weaning, as well as the extent and determinants of individual variation in these dynamics. The weaning pattern suggested by isotope values and behavioural data were congruent. The difference between infant and mother stable nitrogen isotope values decreased faster with age in infants of low-ranking mothers, which suggests a faster progression towards weaning, perhaps due to subordinate females experiencing lower resource availability and so being less able to bear the costs of lactation over prolonged periods. Additionally, within-infant variation in stable nitrogen isotope values showed an increase with age (which was not detectable between infants), potentially highlighting the nutritional costs that weaning imposes on offspring. Our combination of isotope analysis and behavioural data from a wild population provides insight into the evolution of maternal strategies. In particular, it suggests that the quantity of care a mother can provide is affected by her rank, with subordinate females possibly not able to lactate for as long and perhaps benefiting from weaning earlier.