Abstract Stone tool use by wild chimpanzees of West Africa offers a unique opportunity to explore the evolutionary roots of technology during human evolution. However, detailed analyses of chimpanzee stone artifacts are still lacking, thus precluding a comparison with the earliest archaeological record. This paper presents the first systematic study of stone tools used by wild chimpanzees to crack open nuts in Bossou (Guinea-Conakry), and applies pioneering analytical techniques to such artifacts. Automatic morphometric GIS classification enabled to create maps of use wear over the stone tools (anvils, hammers, and hammers/ anvils), which were blind tested with GIS spatial analysis of damage patterns identified visually. Our analysis shows that chimpanzee stone tool use wear can be systematized and specific damage patterns discerned, allowing to discriminate between active and passive pounders in lithic assemblages. In summary, our results demonstrate the heuristic potential of combined suites of GIS techniques for the analysis of battered artifacts, and have enabled creating a referential framework of analysis in which wild chimpanzee battered tools can for the first time be directly compared to the early archaeological record.

Citation: Benito-Calvo A, Carvalho S, Arroyo A, Matsuzawa T, de la Torre I (2015) First GIS Analysis of Modern Stone Tools Used by Wild Chimpanzees (Pan troglodytes verus) in Bossou, Guinea, West Africa. PLoS ONE 10(3): e0121613. https://doi.org/10.1371/journal.pone.0121613 Academic Editor: Elsa Addessi, CNR, ITALY Received: September 26, 2014; Accepted: February 3, 2015; Published: March 20, 2015 Copyright: © 2015 Benito-Calvo et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited Data Availability: All relevant data are within the paper and its Supporting Information files. Funding: Funding by the Leverhulme Trust (IN-052) is gratefully acknowledged. The research in Guinea was supported by a MEXT grant #20002001 and #24000001, JSPS-U04-PWS to TM, FCT-Portugal (SFRH/BD/36169/2007), and a Dissertation Fieldwork Grant from the Wenner-Gren Foundation for Anthropological Research to SC. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.

Introduction The use of stone tools to crack open nuts by chimpanzees in West Africa has received considerable attention by primatologists [1–3], and the evolutionary implications of this behavior have been widely discussed [4–6]. Parallels between chimpanzee tool use and the archaeological record have been drawn [7–12], and in recent years the need for systematic comparisons between the two data sources has been widely recognized [13–16]. Modern humans that still use stone tools are also powerful analogs for understanding the evolution of technological behaviors [17]. There are, however, recent arguments stating the importance of also using Pan troglodytes as a model for understanding the origins of technology [18–20]. Early hominins and modern chimpanzees share some relevant features (e.g., brain size, arboreal adaptations, likely use of percussive technology) [21–23]. In addition, chimpanzees, regardless of variability in habitat type, group size, presence of predators, hunting behavior, etc, are all tool users [24]. More importantly, they have the largest and most complex repertoire of tool use apart from humans [25], with sequential use of tools [26] and use of tool-composites [11]. Stone tools used by wild chimpanzees have normally been associated with the presence of specific surface features such as depressions or concavities [4]. Pounding tools with similar features have been reported in archaeological assemblages such as Gesher Benot Ya´aqov (Israel), where presence of pitted stones has been associated with nut cracking activities [27]. Pitted stones have also been described in Olduvai Beds III and IV (Tanzania), although these have been linked to bipolar knapping activities [28] rather than to nut cracking. Similar tools have been also described in Upper Paleolithic and Mesolithic sites [29–31] as well in Holocene sites where, for example, Australian assemblages show that pitted stones can also be associated with shell fish processing [32]. An avenue for stone tool-use comparisons between chimpanzee and human ancestors is the application of archaeological perspectives to the West African chimpanzee stone tool assemblages, pioneered by Mercader et al [33–34]. More recent approaches have combined studies of such material culture with a direct observation of wild chimpanzee behavior [10–11], [35], in order to establish direct links between artifact patterns, site formation and chimpanzee technological behavior. These pioneering works have paved the way for the use of an archaeological perspective to the study of chimpanzee stone tool use, in which nonetheless systematic analysis of battered artifacts is still lacking [36]. Recent progress on the study of battered stone tools in archaeological contexts [36–39] is leading to innovative approaches in this new field of research. However, despite the increasing amount of data available in Primatology and Archaeology, and the growing awareness of its relevance for understanding the evolution of technology in the human lineage (see [40–41] for a new round of analytical approaches to percussive technologies), the lack of systematic analysis of modern chimpanzee stone tools still precludes direct comparisons between the technological records of human and non-human primates. The aim of this study is to present the first systematic analysis of modern nut-cracking tools (anvils/hammers) through the application of new GIS techniques to identify, grade and quantify damage patterns in the artifacts used by wild chimpanzees in Bossou (Guinea-Conakry). In addition, we present a variety of new analytical methods that will allow further quantitative cross-comparisons between human and non-human assemblages, and which will contribute to establishing a contextual framework in which the co-evolution of stone-tool technology in the human and chimpanzee lineages can be better understood.

Materials Study site The village of Bossou, in Guinea, West Africa, harbors a population of wild chimpanzees (Pan troglodytes verus) that have been systematically studied since 1976 [3], [42]. Bossou is one of eight long-term field sites for the study of chimpanzee behavior [15]. The core range of this community (comprised by 13 individuals during this data collection period) lies within the small forest of Bossou, and is between 5 and 7 km2. Bossou chimpanzees are especially known for making and using a large variety of tools [3], [25] and, more specifically, for the habitual use of a pair of movable stones (anvil and hammer stone) to crack open oil palm nuts (Elaeis guinneensis), which are abundantly available [3], [43–44]. There are only two West African sites where stone tool use by chimpanzees has been the focus of systematic studies: Bossou in Guinea and Taï in Ivory Coast. The chimpanzee community at Bossou represents a unique case study to examine the role of percussive technology in the evolution of technology: 1) this population does not customarily use boulders or wood as tools. The nut-cracking activities rely mostly on the use of movable stone tools [11]; 2) Stone tool dimensions are relatively ‘standardized’ [10], partially due to the species of nut that is cracked, i.e. the oil-palm, which is relatively soft [45]. Thus, tools are of small to medium size, and both the hammer and anvil along with the nuts are often transported to the nut-cracking sites [35]. The outdoor laboratory and the experimental procedure In 1988, a so-called ‘outdoor laboratory’ was initiated in the Bossou forest (Latitude 7°38'52.07"N, Longitude 8°30'17.95"W, WGS84), which is a part of the public National Research, with the permission of the Direction Nationale de la Recherche Scientifique et Technique (DNRS) and the Institute de Recherche Environnementale de Bossou (Republic of Guinea). This has been running for 26 years, providing exceptional insights concerning the development of tool use behavior and of the experimental assemblages used by these chimpanzees. All research involving wild chimpanzees was non-invasive and strictly adheres to ethics guidelines detailed by the Association for the Study of Animal Behaviour. This study (introduction of nuts and stones) is approved by the committee for the Ethical guideline of studying wild primates of the Primate Research Institute of Kyoto University (2013). Kyoto University has been directing all research conducted in this field site, in collaboration with the Guinea authorities since 1976. The ‘Outdoor Laboratory’ is a small cleared area on the top of a ‘sacred’ hill, within the core range of the wild chimpanzee community. Here, researchers place a matrix of numbered stones along with seven piles of nuts around the matrix, and wait behind a screen of vegetation for the chimpanzees to visit (Fig. 1 and see [10–11], [43], for more details of the outdoor methods). Each year, the nut cracking “experimental season” varies between December and February–this falls within the dry season and the peak of chimpanzee nut-cracking behavior [46]. The sample of stone tools analysed for this study was used by the wild chimpanzees during the nut-cracking season of December 2008 to February 2009, when 47 experimental sessions were conducted (totaling 34 h 48 min) using two species of nuts: Elaeis guineensis and Coula edulis, the latter being a non-locally available nut that has been introduced to this community a decade ago [35]. PPT PowerPoint slide

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larger image TIFF original image Download: Fig 1. Bossou ‘outdoor laboratory’. A) View of the experimental area in the Forest of Bossou, Guinea. Researchers record the nut-cracking behavior every year, using several video cameras while staying behind a screen of vegetation, c. 20m distance from the wild chimpanzees using tools. B) Female using a stone hammer and anvil to crack open nuts. Note the assortment of stones on the right side which is provided by the researchers, along with the piles of nuts. https://doi.org/10.1371/journal.pone.0121613.g001 For the purpose of this study, the analysed sample of tools is a sub-set of anvils and/or hammer stones selected from the experimental assemblage (N = 4). Another small sample of stone tools (N = 2) used by the same chimpanzee population in a monitored natural nut-cracking site of the forest (Fig. 2) was analysed to compare with results from the experimental assemblage. Stone tools analysed were of local raw materials (amphibolite and African iron oxide). PPT PowerPoint slide

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larger image TIFF original image Download: Fig 2. View of a natural nut-cracking site in Bossou forest, with tools 4 and 5 analysed in this study. https://doi.org/10.1371/journal.pone.0121613.g002 Criteria to select stone tools for this study were: 1) artifacts that had indisputably been used for pounding activities by Bossou chimpanzees. 2) Objects representative of the raw materials usually available in the natural nut-cracking sites (e.g., African iron oxide and amphibolite). 3) Artifacts with clear battering marks susceptible of being analysed digitally and microscopically.

Methods Each battered surface was digitized, 3D scanned (3D laser scanner Next Engine) and analysed separately (8 battered surfaces out of 6 artifacts), following the GIS methods outlined in S1 Text. Identification of damage patterns in Bossou chimpanzee stone tools involved visual mapping [40] and automatized morphometric classifications. GIS morphometric classification [47–51] lacks initial input from the analyst, and grouping is based on an automatized GIS identification of variables, derived from 3D models scanned from the Bossou tools. In order to assess the validity of both techniques, visual mapping and morphometric classifications of the Bossou percussive tools were conducted independently, in such a way that the analyst of the morphometric classification had no access to the original artifacts but was provided with the 3D models suitable for the GIS study. Morphometric GIS analysis Resulting Digital Elevation Models (DEM) of each scanned tool face (N = 8) were used for surface morphometric analysis of topographic attributes calculated with GIS (ArcGIS 10.1 and SAGA 2.1). Several Digital Surface Models (DSM) were calculated from elevation data (S1 Text), such as primary and secondary derivates (i.e. slope, aspect and curvatures), hillshading models, or topographic profiles [48–49]. These DSM were used for a first basic morphometric analysis of the stone tools surfaces. Subsequently, roughness [52–58], topographic position [50], [59], and relative depth DSM were calculated (S1 Text), in order to identify and interpret use-wear features. Roughness was used to estimate polish areas. We applied three different roughness indices: Terrain Ruggedness Index (TRI), Vector Ruggedness Measure (VRM), and the 3D/2D area ratio (see details in S1 Text). In addition, topographic position index (TPI) and relative depth models were applied to map and characterize surface depressions, in order to identify concavities associated with percussive activities. Visual mapping and GIS spatial pattern analysis Following protocols outlined by de la Torre et al [40], digital images were geo-referenced in a local Cartesian system using ArcGIS 10.1. Macroscopically-identified percussive marks were outlined over the images, and indexes such as area, perimeter, and distribution and size of the areas covered by percussion marks (S1 Text), were calculated to produce a spatial pattern of the use wear distribution along the tools.

Conclusions Our results demonstrate the potential of morphometric analysis, especially using variables such as roughness methods (especially VRM index), TPI index, and relative depth models. These were calculated through automatic morphometric classification of 3D models in order to identify use wear patterns in stone tools, and validated through the visual mapping of battered areas. These variables can be calculated automatically, providing an accurate and objective way to analyze morphological features and their spatial distribution in archaeological, experimental and primatological stone tools. Furthermore, these methods are applicable not only to meso-scale 3D scans, but also to microtopographic models, and can be used to establish patterns of tool damage and use wear in lithic assemblages. The newly emerged field of Primate Archaeology aims establishing analytical foundations to interpret primate behavior from an archaeological viewpoint [6], [45], [62–64], but much work is still needed to develop appropriate comparative protocols with the archaeological record. This paper has presented the first systematic GIS analysis of stone tools used by modern wild chimpanzees during nut-cracking activities, and in doing so has also provided innovative analytical techniques that can be applied to the early human archeological record, therefore prompting comparisons. Further ongoing research is focused on the comparison between the microscopic and technological analysis of the assemblage and the behavioral data collected chimpanzee nut-cracking (Carvalho et al. in prep). The morphometric classification of battering use wear, validated through blind testing by visual inspection, provides an enhanced method for the study of material culture and will strengthen the links between the behavior of extant primates and the archaeological record of our early human ancestors.

Acknowledgments We are grateful to the Direction Nationale de la Recherche Scientifique et Technique and the Institute de Recherche Environnementale de Bossou (Republic of Guinea), for granting us permission to carry out this research in Bossou. We thank the local guides at Bossou, République de Guinée for invaluable support and field assistance.

Author Contributions Conceived and designed the experiments: SC TM. Performed the experiments: SC. Analyzed the data: AB-C AA IDLT. Contributed reagents/materials/analysis tools: AB-C AA IDLT. Wrote the paper: AB-C SC AA IDLT.