Abstract: We conducted three studies to examine whether the four great ape species (chimpanzees, bonobos, gorillas, and orangutans) are able to use behavioral experimenter-given cues in an object-choice task. In the subsequent experimental conditions subjects were presented with two eggs, one of which contained food and the other did not. In Study 1 the experimenter examined both eggs by smelling or shaking them, but only made a failed attempt to open (via biting) the egg containing food. In a control condition, the experimenter examined and attempted to open both eggs, but in reverse order to control for stimulus enhancement. The apes significantly preferred the egg that was first examined and then bitten, but had no preference in a baseline condition in which there were no cues. In Study 2, we investigated whether the apes could extend this ability to cues not observed in apes so far (i.e., attempting to pull apart the egg), as well as whether they made this discrimination based on the function of the action the experimenter performed. Subjects significantly preferred eggs presented with this novel cue, but did not prefer eggs presented with a novel but functionally irrelevant action. In Study 3, apes did not interpret human actions as cues to food-location when they already knew that the eggs were empty. Thus, great apes were able to use a variety of experimenter-given cues associated with foraging actions to locate hidden food and thereby were partially sensitive to the general purpose underlying these actions.

Abstract: Many primate species reliably track and follow the visual gaze of conspecifics and humans, even to locations above and behind the subject. However, it is not clear whether primates follow a human's gaze to find hidden food under one of two containers in an object-choice task. In a series of experiments six adult female chimpanzees followed a human's gaze (head and eye direction) to a distal location in space above and behind them, and checked back to the human's face when they did not find anything interesting or unusual. This study also assessed whether these same subjects would also use the human's gaze in an object-choice task with three types of occluders: barriers, tubes, and bowls. Barriers and tubes permitted the experimenter to see their contents (i.e., food) whereas bowls did not. Chimpanzees used the human's gaze direction to choose the tube or barrier containing food but they did not use the human's gaze to decide between bowls. Our findings allowed us to discard both simple orientation and understanding seeing-knowing in others as the explanations for gaze following in chimpanzees. However, they did not allow us to conclusively choose between orientation combined with foraging tendencies and understanding seeing in others. One interesting possibility raised by these results is that studies in which the human cannot see the reward at the time of subject choice may potentially be underestimating chimpanzees' social knowledge.

Abstract: Chimpanzees and young children understand much about what other individuals have and have not seen. This study investigates what they understand about their own visual perception. Chimpanzees, orangutans, and 2.5-year-old children were presented with a finding game in which food or stickers were hidden in one of two or three tubes. We varied whether subjects saw the baiting of the tubes, whether subjects could see through the tubes, and whether there was a delay between baiting and presentation of the tubes to subjects. We measured not only whether subjects chose the correct tube but also, more importantly, whether they spontaneously looked into one or more of the tubes before choosing one. Most apes and children appropriately looked into the tubes before choosing one more often when they had not seen the baiting than when they had seen the baiting. In general, they used efficient search strategies more often than insufficient or excessive ones. Implications of subjects' search patterns for their understanding of seeing and knowing in the self are discussed.

Abstract: Stumptailed macaques, Macaca arctoides, are characterized by high levels of postconflict affiliative contacts between opponents. We investigated the occurrence of postconflict affiliative contacts between opponents and third parties that were not involved in the original conflict. We collected 10-min focal observations during postconflict and control periods in which we recorded all aggressive and affiliative behaviours between opponents and third parties. We distinguished three types of third parties depending on the relationship with the focal animal: own kin, opponent's kin and individuals unrelated to both opponents. We analysed the interactions with third parties separately, while distinguishing two classes of affiliative behaviours: (1) allogrooming and contact sitting and (2) sociosexual behaviours (e.g. genital inspection). The macaques showed differences between postconflict and control periods in their affiliative contacts with third parties. Aggressors received more postconflict grooming and contact sitting from their opponents' kin, received more sociosexual behaviour from their own kin and unrelated individuals, and directed more sociosexual behaviour to unrelated individuals. Victims received and directed less postconflict grooming from and towards their own kin. They received more postconflict sociosexual behaviour from all partners except their own kin and directed more sociosexual behaviour to all partners except the opponent's kin. This study establishes the occurrence of multiple postconflict triadic affiliation in stumptailed macaques, and is the first to show that victims receive contacts from third parties in a cercopithecine species, a behaviour previously described only in chimpanzees. It also highlights the importance of analysing the different affiliative behaviours separately in postconflict situations. Otherwise, many of the patterns we report, especially those involving victims, would have been missed.

Abstract: Capuchin monkeys were tested in five experiments in which two individuals competed over food. When given a choice between retrieving a piece of food that was visible or hidden from the dominant, subordinate animals preferred to retrieve hidden food. This preference is consistent with the hypotheses that either (1) the subordinate knew what the dominant could and could not see or (2) the subordinate was monitoring the behaviour of the dominant and avoiding the piece of food that it approached. To test between these alternatives, we released subordinates with a slight head start forcing them to make their choice (between a piece of food hidden or visible to the dominant) before the dominant entered the area. Unlike chimpanzees, Pan troglodytes, subordinates that were given a head start did not preferentially approach hidden pieces of food first. Therefore, our experiments provide little support for the hypothesis that capuchin monkeys are sensitive to what another individual does or does not see. We compare our results with those obtained with chimpanzees in the same paradigm and discuss the evolution of primate social cognition. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.