Regulation of vocal output by chimpanzees finding food in the presence or absence of an audience

Chimpanzees (Pan troglodytes) give both food barks and pant hoots upon encountering food and regulate their calls based upon such factors as food quantity, quality, and possibly divisibility. Although it has been determined that several species, both primate and non-primate, regulate their food calls based upon the presence or absence of an audience, this has not been systematically explored with chimpanzees. Group-housed chimpanzees were given access to either large or small quantities of food when they had either visual access to companions (Audience condition) or were visually isolated (No Audience condition). We predicted that chimpanzees would call more for larger quantities of food and more in the presence of an audience. As expected, food calling was greater for large quantities of food than small quantities. The effect of an audience was more complex. A visible audience increased the rate of food calling for a large, sharable quantity of food, yet decreased the rate for a small, non-sharable quantity. These results are consistent with the hypothesis that chimpanzee vocal expression is sensitive to social context, and that they are able to regulate the information made available to others in accordance with predicted future interaction.