The influence of observer presence on gestural behaviors by chimpanzees (Pan troglodytes)

Previous reports of the influence of observer presence on the display of gestures by chimpanzees and other apes have been hampered by relatively small samples or confounded presentation of food and an experimenter. The present study presented each of 35 chimpanzees with four conditions in constant order: (a) both food and experimenter present, (b) food absent, experimenter present, (c) food present, experimenter absent, and (d) both food and experimenter present. Seventeen subjects exhibited 23 gesture during the course of the experiment. The gestures were not randomly distributed in time [Cochran's Q (3, N = 17) = 13.78, p < .05]: gestures were concentrated in the two conditions with the experimenter present. Seventeen of the 23 gestures exhibited were in the presence of the experimenter, and the remaining three gestures were exhibited within 2 seconds prior to the arrival of the experimenter (i.e., upon the experimenter's approach). Nearly twice as many subjects gestured when both food and the experimenter were present than when only the experimenter was present. When no food was present, gestures tended to be directed toward the experimenter, whereas when both food and an observer were present, gestures were evenly split between being directed at the food and being directed at the experimenter. Thus, referential communication is exhibited by captive chimpanzees without explicit training.