CHIMPANZEE FOOD GRUNTS ARE DIRECTED AT SPECIFIC INDIVIDUALS: PRECURSORS FOR TRIADIC COMMUNICATION?

摘要

If non-human primates (primates) were capable of vocalising to inform a receiver about an external entity, such intentional triadic communication would represent an important element of continuity with human language. Whilst dyadic communication between a signaller and receiver is common in the animal kingdom (Hurford 2007), triadic communication that involves a signaller, receiver and an external referent is relatively rare in animal communication systems, yet it is abundant in human language. In order to produce a call with the intent to inform a receiver about an external event, at least four different behavioural or cognitive elements are required: (1) production of a functionally referential call (context specific and indexical for receivers); (2) voluntary control over call initiation; (3) direction of calls at specific receivers and (4) understanding and manipulation of the mental states of receivers. Despite recent attempts (e.g. Crockford et al., 2012), there is currently no convincing evidence that primates produce calls to manipulate the mental states of others. Previous studies on a variety of species have, however, shown that primates may have some of the other necessary elements (e.g. Seyfarth et al., 1980; Pierce 1985, Hopkins et al., 2007). However, if these are to act as prerequisites for intentional triadic communication, they need to occur within a single calling system, rather than across different contexts and species. In this study we address this issue and test experimentally whether chimpanzee rough grunts, which function to refer to food (Slocombe & Zuberbühler, 2005), are produced selectively, indicating voluntary control, and whether they are directed at specific individuals. These are necessary prerequisites for a system capable of actively informing others about external events.