Extractive foraging of underground storage organs (USOs) is believed to have played an important role in human evolution. This behavior is also present in wild chimpanzees (Pan troglodytes), who sometimes use tools in the task. Despite the importance of studying this behavior in chimpanzees to model how early hominins may have used tools in the context of USO excavation, it remains to be directly observed due to the chimpanzees lack of habituation in the two study sites that yielded evidence of tool-use in USO excavation. Until now, no studies in captivity had been conducted to learn how chimpanzees could excavate underground food. The present experiment was designed to provide captive chimpanzees with opportunities to use tools in the excavation of artificially-placed underground food at their semi-naturally forested enclosure. The study was conducted independently with two groups of chimpanzees living at the Kristiansand Zoo, in Kristiansand, Norway. The experiment had three phases: food was placed inside holes that were 1) left open, 2) filled with regular soil, and 3) filled with clay. Materials to be used as tools were provided once during the study. The chimpanzees predominantly excavated the buried fruits manually. They used one hand to excavate soil and used both hands, alternating right and left, to excavate clay. The chimpanzees rarely used tools to excavate regular soil, while more often used tools to excavate naturally compacted soil (below the depth where the fruits were placed) and clay. In general, tool-use increased with the hardness of the soil type. The chimpanzees were selective in their choice of materials to be used as tools, preferring long and heavy sticks from trees. Even though they were observed to manufacture tools in other contexts, they were never seeing to make tools for the excavation of underground food. Only one instance of tool modification occurred. The chimpanzees gathered their own tools from the enclosure: these tools were similar in physical characteristics and material to the ones they selected from the provided materials. Some tools remained in the study area and were reused in different days. The tools that were reused more frequently were transported more. In the beginning of the study, tools were only used as investigatory probes. But later, the chimpanzees succeeded in using tools for excavation by incorporating different actions: perforate, pound, dig, shovel, and enlarge. Some individuals seemed to acquire the actions through their own trial and error, while others seemed to learn through observation of skilled individuals. It was found that excavation was not a single action, but a series of different actions all performed (manually or with tools) with the goal of extracting the underground food. Tool actions emerged sequentially and independently in the two study groups: probe, perforate, pound, dig, and shovel. Mastering one action seemed to facilitate the invention of the following action. The implications of the present study for the behavior of wild chimpanzees are discussed
