The development of a greeting signal in wild chimpanzees.

Adult chimpanzees produce a unique vocal signal, the pant-grunt, when encountering higher-ranking group members. The behaviour is typically directed to a specific receiver and has thus been interpreted as a 'greeting' signal. The alpha male obtains a large share of these calls, followed by the other adult males of the group. In this study, we describe the development of pant-grunting behaviour from the first grunt-like calls of newborn babies to the fully developed pant-grunts in adults. Although babies produce grunts from very early on, they are not directed to others until about 2 months of age. Subsequently, socially directed grunting steadily increases in frequency to peak around 7 months of age, but then decreases again to reach a nadir in older infants and juveniles, while the specificity in use increases. During adolescence, grunt production increases again with grunts given most frequently to socially relevant individuals. As young chimpanzees are closely affiliated to their mothers for the first decade of their lives, we also compared the grunting patterns of mothers and their offspring, which revealed some influences in pant-grunt production. In conclusion, the acquisition of pant-grunting behaviour in chimpanzees is a long-lasting process with distinct developmental phases in which social influences by the mother and other group members are likely to play a role.