Rocking gesture as communicative behavior in the wild pygmy chimpanzees in Wamba,central Zaire

The pygmy chimpanzee rocks its upper body in a variety of contexts, while the common chimpanzee rocks in only 3 situations; courtship, charging display and frustration. In the pygmy chimpanzee, behaviors involving rocking except for charging display are intentional communicative behaviors which bring about various body contact interactions. Each interaction pattern corresponds to each information set consisting of participant age-sex combination, additional signal and group situation. Rocking without any other gestures functions as a sign of tolerance or appeasement to access. This suggests that communicative behaviors involving rocking have a kind of segmentive structure, i.e., rocking represents access while other additional information sets represent the interaction pattern, the character of which is analogous to linguistic communication. Rocking in most interactions possibly derived from rocking in courtship behavior which appears to be a ritualized form of frustrated rocking, while other forms of rocking may possibly be of different origins.