ピグミーチンパンジーの音声コミュニケーションに表れる社会的特性

抄録

This study describes how the vocal repertoire of wild pygmy chimpanzees reflects their social relationships. The data were collected from the El and E2 groups at Wamba, in central Zaire.
The pygmy chimpanzee vocal repertoire in Wamba was divided into six categories based on the vocal sounds and social contexts. Note that this repertoire lacks vocalizations emitted during dyadic interactions. Directed vocalizations were emitted by a single individual, and others rarely responded.
The most important category in pygmy chimpanzee vocal repertoire is “Hooting” as contact call. However, the individual vocal variation was so small that it was hard to identify pygmy chimpanzees by their vocalizations only. They were also hard to identify by their vocalizations becauses all members of a party emitted loud Hooting simultaneously. Synchronization or turn-taking during vocal chorus was rare.
The contact calls of the pygmy chimpanzee of Wamba has two particular social features. (1) Many individuals can participate at the same time or the equivalent way using the same vocalizations. In these situations, neither dominant-subordinate relationships nor other relations between particular two individuals become evident. (2) Information involved in the communication can be obtained just by hearing the vocalization of others. Each participant may select two opposite behaviors according to these two features either (1) vocalizing with others or (2) listening to the vocalization of others. Pygmy chimpanzees seem to attach more importance to (1) than (2); they prefer joining vocalization to listening.
This characteristic of pygmy chimpanzee vocal communication may reflect a unique character of their society. The pygmy chimpanzee group seldom disperses into small parties, and moves in a large single party including almost all members of the group. From their social interactions, no particular affinitive relationship except between mother and offspring could be discerned. This pattern of pygmy chimpanzee vocal communication may indicate that their tendency to gather in a large party is not due to the network of affinitive dyadic social relationships, but to their preference to “gather” itself; each member of the group interacts equally with almost all members of a group.