Chimpanzees (Pan troglodytes) strategically manipulate their environment to deny conspecifics access to food

Abstract

Humans modify their environment to grant or prevent others' access to valuable resources, for example by using locks. We tested whether sanctuary-living chimpanzees (N = 10) would flexibly modify their environment to either allow or deny a dominant conspecific access to a shared food source by giving them the option to change a food reward's pathway prior to releasing it. The food could end up in one of two locations: one was accessible to both the subject and a dominant conspecific, the other one was only accessible to the subject. We further manipulated the extent of inhibitory control needed for modifying the pathway by varying the subjects' starting position. Our subjects reoriented the pathway competitively to monopolize food but changed the pathway less often in trials with high inhibitory demands. We further show how inhibitory task demands in a social context influence chimpanzees' future planning. Our results show that chimpanzees will strategically manipulate their environment to maximize their own and deny a dominant conspecific access to food.

Figure 1

Schematic drawing of a testing session trial. This shows a mutual side trial in which the subject would have to move to the other side (high inhibitory demands) to reorient the seesaw. If the subject directly releases the food, it will end up in the location that is mutually accessible to the subject and dominant conspecific (left green area). Delivering the reward to the solo location (right green area) requires the subject to first move to the opposite room without pulling the release rope, reorient the seesaw to the right by pulling the rope attached to the right side of the seesaw (1), move back into the tunnel to now pull the food release rope (2), and subsequently collect the food from the solo food location (3).

Figure 2

Schematic drawing of the four types of experimental trials. Example with the dominant conspecific on the left side. Each subject completed four sessions consisting of 12 trials each with 3 trials of each type. The order of trials and location of the dominant were pseudo-randomized and counterbalanced across sessions. Solo side and mutual side refer to the location the seesaw is directed to at the beginning of a trial. Low and high inhibitory demands for reorienting the seesaw are determined by the starting position of the subject, i.e., whether they must change the side and pass the food release rope to reorienting the seesaw.

Figure 3

Percentage of reorienting the seesaw for different initial seesaw orientations at the beginning of a trial (mutual side vs. solo side) and inhibitory demands (high vs. low) operationalized by the subjects’ starting position.

Figure 4

Picture of apparatus setup. Taken from the video camera’s point of view in the hallway. Subjects could move between the left room in the front and right room in the front using the overhead tunnel. Pulling the rope attached in the middle of the subject’s overhead tunnel would release food that would then fall on the seesaw and slide down. The picture shows the seesaw oriented to the right side. In this constellation, released food would fall to the right side by default; however, the orientation of the seesaw could be manipulated by pulling the rope on the left side, which would change the reward pathway to the left side. The apparatus was placed in-between the rooms of the subject and the room in which a dominant conspecific was placed during test sessions. Food that would slide down the seesaw to the right side would be accessible from the close room on the right side and the adjacent room on the right side and vice versa on the left side.