Date Published: May 15, 2019
Publisher: Public Library of Science
Author(s): Alba Motes-Rodrigo, Parandis Majlesi, Travis Rayne Pickering, Matthias Laska, Helene Axelsen, Tanya C. Minchin, Claudio Tennie, R. Adriana Hernandez-Aguilar, Michael D. Petraglia.
It is hypothesized that tool-assisted excavation of plant underground storage organs (USOs) played an adaptive role in hominin evolution and was also once considered a uniquely human behavior. Recent data indicate that savanna chimpanzees also use tools to excavate edible USOs. However, those chimpanzees remain largely unhabituated and we lack direct observations of this behavior in the wild. To fill this gap in our knowledge of hominoid USO extractive foraging, we conducted tool-mediated excavation experiments with captive chimpanzees naïve to this behavior. We presented the chimpanzees with the opportunity to use tools in order to excavate artificially-placed underground foods in their naturally forested outdoor enclosure. No guidance or demonstration was given to the chimpanzees at any time. The chimpanzees used tools spontaneously in order to excavate the underground foods. They exhibited six different tool use behaviors in the context of excavation: probe, perforate, dig, pound, enlarge and shovel. However, they still excavated manually more often than they did with tools. Chimpanzees were selective in their choice of tools that we provided, preferring longer tools for excavation. They also obtained their own tools mainly from naturally occurring vegetation and transported them to the excavation site. They reused some tools throughout the study. Our new data provide a direction for the study of variables relevant to modeling USO extractive foraging by early hominins.
It has been hypothesized that plant underground storage organs (USOs) were key dietary resources for African Pliocene and Pleistocene hominins who lived in dry and/or open environments [1–3]. Data on hominin craniodental anatomy [4–7], dental microwear [8–11], enamel chemistry [10, 12–16], and archaeology  are consistent with this hypothesis, indicating that USOs were likely to have been important in the diet of at least some species in the genera Australopithecus, Paranthropus, and Homo. Further, ecological studies on the abundance  as well as on mechanical properties of USOs  that grow in environments similar to those reconstructed for some hominin species, show that USOs were potentially an important food supply for hominins in such environments. In addition, ethnographic data from contemporary human hunter-gatherers (Homo sapiens) living in dry habitats demonstrate that USOs are staple foods in the diet of many of these populations [20–23]. However, despite this evidence, the answers to the questions of how USO extractive foraging developed in early hominins and how USOs could have played an adaptive role in hominin evolution remain elusive.
All but one adult female chimpanzee succeeded in excavating underground food in at least one of the experiments conducted, and the majority of the chimpanzees also did so with tools (seven out of ten and seven out of nine in Experiments 1 and 2, respectively). This is an especially interesting observation, considering that the chimpanzees were naïve with regard to excavating at the onset of this study. The only other study that investigated the excavating behavior of a previously naïve great ape, reported that four out of seven bonobos at the Bonobo Hope Sanctuary (Des Moines, USA) and two out of eight at the Wuppertal Zoo (Wuppertal, Germany) used tools to excavate buried food at least once . Thus, both naïve captive chimpanzees and naïve captive bonobos spontaneously used tools for excavating underground food.