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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 pantgrunting 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.

International audienceThis paper first reviews data collected from 1976 to 2013 regarding the life histories of members of the main E1 study group of bonobos (Pan paniscus) in Wamba. The E1 group exhibited strong tendencies toward female dispersal and male residence during the entire study period, thereby exemplifying the typical characteristics of a male-philopatric and female-dispersal society. This pattern did not change after the abandonment of artificial provisioning. We then present two new cases of immigrant females, focusing especially on social association patterns, dominance relationships, and affiliative interactions during the approximate 2.5 years from the time of their immigration to their first birth. These females began engaging in social grooming with resident females immediately after their arrival but rarely did so with adult males, suggesting that they regard social bonding with females as more important than that with males. They also emigrated at a young age and frequently engaged in social play. Indeed, social bonding established through frequent social play may be related to the development of socially symmetrical relationships, which are the basis for the egalitarian bonobo society. Intragroup competition for food and mates was unlikely explanations for the tendency toward female dispersal. However, male residence and the risk of father–daughter incest may encourage female transfer among bonobo

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Safety in numbers is thought to be the principal advantage of living in groups for many species. The group can only provide protection against predators, however, when group cohesion is maintained. Vocalisations are used to monitor inter-individual distances, especially under conditions of poor visibility, but should be avoided in the presence of predators. Mentally tracking the movements of silent and invisible group members would allow animals foraging in dense vegetation to stay close to their group members while reducing the use of vocal contact. We tested the socio-spatial cognitive abilities of wild vervet monkeys (Chlorocebus pygerythrus) by comparing their reactions to plausible and implausible displacements of group members simulated by sound playbacks. Our methods are comparable to those used in studies of 'object permanence' and 'invisible displacements' of inanimate objects. Our results show that vervets can track the whereabouts of invisibly and silently moving group members, at least over short periods of time.