Wild Chimpanzees Show a Decrease in Pant Grunting over Their First 6 Years of Life.

Data from a large cross-sectional sample of wild chimpanzee mother-infant dyads yield evidence that young chimpanzees' pant grunting unfolds nonlinearly over the early developmental period. Though infants begin pant grunting early, and mothers' rates did not decrease, infant pant grunting declined as infants aged through infancy. Mother-infant dyadic pant grunting discordance therefore increased over infancy, with some discordance observed at even the earliest ages. In half of 90 observed instances involving infants ranging in age from 2 weeks to 69 months, only one member of the mother-infant dyad pant grunted; infants' pant grunting was not influenced by their mother's age, their position on their mother's body at the time of the greeting, or the dominance status of the male greeted. Male infants were more likely to pant grunt than female infants. We discuss the developmental trend in the context of infants' increasing independence, changing social motivations, and male-dominated social hierarchy.