Captive chimpanzee foraging in a social setting: a test of problem solving, flexibility, and spatial discounting

Abstract

In the wild, primates are selective over the routes that they take when foraging and seek out preferred or ephemeral food. Given this, we tested how a group of captive chimpanzees weighed the relative benefits and costs of foraging for food in their environment when a less-preferred food could be obtained with less effort than a more-preferred food. In this study, a social group of six zoo-housed chimpanzees (Pan troglodytes) could collect PVC tokens and exchange them with researchers for food rewards at one of two locations. Food preference tests had revealed that, for these chimpanzees, grapes were a highly-preferred food while carrot pieces were a less-preferred food. The chimpanzees were tested in three phases, each comprised of 30 thirty-minute sessions. In phases 1 and 3, if the chimpanzees exchanged a token at the location they collected them they received a carrot piece (no travel) or they could travel ≥10 m to exchange tokens for grapes at a second location. In phase 2, the chimpanzees had to travel for both rewards (≥10 m for carrot pieces, ≥15 m for grapes). The chimpanzees learned how to exchange tokens for food rewards, but there was individual variation in the time it took for them to make their first exchange and to discover the different exchange locations. Once all the chimpanzees were proficient at exchanging tokens, they exchanged more tokens for grapes (phase 3). However, when travel was required for both rewards (phase 2), the chimpanzees were less likely to work for either reward. Aside from the alpha male, all chimpanzees exchanged tokens for both reward types, demonstrating their ability to explore the available options. Contrary to our predictions, low-ranked individuals made more exchanges than high-ranked individuals, most likely because, in this protocol, chimpanzees could not monopolize the tokens or access to exchange locations. Although the chimpanzees showed a preference for exchanging tokens for their more-preferred food, they appeared to develop strategies to reduce the cost associated with obtaining the grapes, including scrounging rewards and tokens from group mates and carrying more than one token when travelling to the farther exchange location. By testing the chimpanzees in their social group we were able to tease apart the social and individual influences on their decision making and the interplay with the physical demands of the task, which revealed that the chimpanzees were willing to travel farther for better.

Behavioral research as physical enrichment for captive chimpanzees

“…The experiment—the “Token Exchange” study—was a voluntary research study in which the apes could take small plastic tokens (10 cm long lengths of 2.5 cm diameter white PVC pipe) to different locations within their exhibit to exchange with researchers for small differentially valued food rewards (as determined through food‐preference testing conducted prior to the commencement of the study): high‐value grapes and medium‐value carrot pieces [Hopper et al, ]. The foods used were approved by veterinary and nutrition staff and constituted components of the chimpanzees’ daily food allowance, rather than additional rewards.…”

“…The location where the token hoppers were hung remained the same throughout the three phases. Full details of the methods of this experiment and the differences between the three phases are described in Hopper et al [], and we provide an overview in Figure for clarity.…”

“…The minimum distance to travel from location A to C was 15 m. Phase 3 was identical to Phase 1. For further details, please see Hopper et al [].…”

“…To address this question, we evaluated the impact of a behavioral research study on the activity levels of a group of captive chimpanzees. In a previously‐published study, we investigated how a group of zoo‐housed chimpanzees weighed the relative benefits and costs of travelling to obtain food [Hopper et al, ]. This was a voluntary activity for which subjects could take PVC tokens to different locations at the perimeter of their exhibit and exchange them with researchers for small differentially‐valued food rewards, such that they always had to travel farther to exchange tokens for more desirable foods.…”

Chimpanzee food preferences, associative learning, and the origins of cooking

“…We also know that chimpanzees (Hopper, Kurtycz, Ross, & Bonnie, 2015) and capuchin monkeys (Evans & Westergaard, 2006) will transport items to exchange for better items or, in the case of the monkeys, use them as tools to obtain higher value foods. Thus, although it was shown that the chimpanzees preferred cooked to raw potato, which supports previously research, it was not additionally shown that they understood (or thought) that the cooked potato was a transformed version of the raw potato.…”