Problem solving in the presence of others: how rank and relationship quality impact resource acquisition in chimpanzees (Pan troglodytes)

Abstract

In the wild, chimpanzees (Pan troglodytes) are often faced with clumped food resources that they may know how to access but abstain from doing so due to social pressures. To better understand how social settings influence resource acquisition, we tested fifteen semi-wild chimpanzees from two social groups alone and in the presence of others. We investigated how resource acquisition was affected by relative social dominance, whether collaborative problem solving or (active or passive) sharing occurred amongst any of the dyads, and whether these outcomes were related to relationship quality as determined from six months of observational data. Results indicated that chimpanzees obtained fewer rewards when tested in the presence of others compared to when they were tested alone, and this loss tended to be greater when paired with a higher ranked individual. Individuals demonstrated behavioral inhibition; chimpanzees who showed proficient skill when alone often abstained from solving the task when in the presence of others. Finally, individuals with close social relationships spent more time together in the problem solving space, but collaboration and sharing were infrequent and sessions in which collaboration or sharing did occur contained more instances of aggression. Group living provides benefits and imposes costs, and these findings highlight that one cost of group living may be diminishing productive individual behaviors.

Figure 1. The tower.

The tower was constructed of steel mesh and rods that allowed the chimpanzees to see clearly inside the tower. Food was only accessible from the top of the tower if the hanging tray was raised to the top without tipping the tray such that all the food fell to the bottom of the tower. Along the bottom edges of the tower were steel panels to discourage chimpanzees from attempting (in vain) to access fallen food from the ground. The tower was baited through a pipe that extended from the side of the tower to the outside of the room. The size of the chimpanzee is to scale relative to the tower (drawn from photo of adult chimpanzee BOB).

Figure 2. Change in Rewards Obtained from Solo Sessions when in the Presence of Higher and Lower Ranked Individuals.

The number of rewards obtained with higher and lower ranking partners was subtracted from the number of rewards obtained in solo sessions. Subjects from Group 3 and 4 are represented by open and closed circles, respectively. Statistics were calculated only on subjects tested with both higher and lower ranking partners in Group 4 (Wilcoxon signed ranks test, W = 21, N = 6, P = 0.031).

Figure 3. Dyadic Association Index & Time Shared on Tower.

The y-axis shows the duration in seconds in which both chimpanzees were located on top of the tower during dyadic sessions. The x-axis shows the twice-weight association index generated from six months of focal follows. Dyads that showed at least one occurrence of cooperative problem solving are represented by a square (with the exception of the dyad LOU & BUF because no AI was available). In both social groups, there was a significant positive correlation between AI and time shared on the tower (Mantel tests, Group 3: r_S = 1, N = 6, P<0.01; Group 4: r_S = 1, N = 8, P<0.01).