Zoo-Housed Chimpanzees Can Spontaneously Use Tool Sets But Perseverate on Previously Successful Tool-Use Methods

Tool use is documented in both wild and captive chimpanzees, but the creation of tool sets (e.g., two, or more, tools used in a sequence to solve a task), seems to be less common. This has raised the question of whether tool sets are a culture-dependent trait (CDT), or can be re-innovated independently, and thus fall within chimpanzees’ Zone of Latent Solutions (ZLS). To test this, we provided a group of zoo-housed chimpanzees with a novel task that mimicked the wild conditions, required a tool set to solve, and all necessary materials to do so. We therefore enhanced the necessary conditions for the tool-set behavior, but without providing demonstrations of tool-set manufacture or use. We baited an artificial termite mound with six ketchup-filled tubes, each with a 45-degree bend and with a cellophane barrier across the top. The chimpanzees needed a rigid tool to pierce the barrier and then a flexible tool to reach the reward. Overall, chimpanzees did use both tool types in order, thus demonstrating that this tool-set form is not a CDT. Yet, few attempts were successful, and the majority were not made using a tool set. This suggests that the behavioral pattern did not stabilize, which may have been due to unintentional difficulty created by the opacity of our task. Overall, our study showed that the form of tool sets can be within the ZLS of chimpanzees, but future studies need to determine what stabilizes the behavioral patterns in the wild.