OVERVIEW

Chimpanzee social interactions involve a rich repertoire of body movements and postures, autonomic indicators of arousal, vocal and nonvocal sounds, and facial expressions. Scores of regularly observed expressive physical behaviors have been described for wild chimpanzees, and comprehensive catalogs continue to grow as new behaviors are identified, because they are either rare, site-specific, or recently invented. Similarly, building on a basic repertoire of calls described in early field studies, researchers continue to distinguish new vocalizations as acoustic analyses reveal subtle variants within previously delineated call categories. Nonvocal sounds, made by striking, scratching, or otherwise manipulating body parts or objects in the environment, are commonly produced and offer seemingly limitless opportunities for innovation.

Expressive behaviors can be categorized according to the sensory modality they involve, that is, either olfactory, tactile, visual, or auditory. They are considered communicative “signals” insofar as they appear to have evolved to convey information to other individuals and to mediate social interactions. When signals are perceived through only a single sensory channel, they are comparatively easy to investigate. This is the case, for example, with loud vocalizations produced and monitored by individuals who are not in sight of one another. However, most chimpanzee interactions involve multimodal combinations of signals directed toward individuals that are in view. For example, it is common to observe a female approach and ultimately touch a male while she simultaneously crouches, bobs her head, and utters submissive vocalizations. Such multimodal events are more challenging to interpret, since it is difficult in practice to disentangle the functional consequences of their individual or variously combined components.

Animal communication is a social phenomenon, and analysis of communicative events necessarily requires consideration of both signalers and receivers. Classic descriptions of animal communication define the “message” of a signal as the information content of the signal itself. In this view, messages primarily consist of information about the sender. Potential information might include species, social group, individual identity, sex, status, physical condition, or motivational state. The “meaning” of a signal is its message combined with information available from the context of its production, and it is operationally defined in terms of the receiver's behavioral response. By combining message with context, receivers then have information available to predict the sender's probable imminent behavior and respond appropriately.