Distance-decay effect in stone tool transport by wild chimpanzees

Abstract

Stone tool transport leaves long-lasting behavioural evidence in the landscape. However, it remains unknown how large-scale patterns of stone distribution emerge through undirected, short-term transport behaviours. One of the longest studied groups of stone-tool-using primates are the chimpanzees of the Taï National Park in Ivory Coast, West Africa. Using hammerstones left behind at chimpanzee Panda nut-cracking sites, we tested for a distance-decay effect, in which the weight of material decreases with increasing distance from raw material sources. We found that this effect exists over a range of more than 2 km, despite the fact that observed, short-term tool transport does not appear to involve deliberate movements away from raw material sources. Tools from the millennia-old Noulo site in the Taï forest fit the same pattern. The fact that chimpanzees show both complex short-term behavioural planning, and yet produce a landscape-wide pattern over the long term, raises the question of whether similar processes operate within other stone-tool-using primates, including hominins. Where hominin landscapes have discrete material sources, a distance-decay effect, and increasing use of stone materials away from sources, the Taï chimpanzees provide a relevant analogy for understanding the formation of those landscapes.

Keywords: chimpanzees; distance-decay effect; primate archaeology; stone tools; transport.

Figure 1.

Position of inselbergs (black) and located hammerstones (grey) in the Taï National Park. The size of the grey circles (hammerstones) corresponds to the weight of the hammerstone material at a site. The two polygons represent the home range of the North and the South group. The cross symbol represents the location of the excavated Noulo chimpanzee site.

Figure 2.

(a) Assessing pit depth from Panda nut-cracking hammerstones using three-dimensional models. (1) Photograph (Sony Nex6); (2) three-dimensional scan (NextEngine laser scanner); (3) topographic model of the pitted area (Geographic Information System (GIS)). (b) Refit of broken hammerstone, each part was independently used as a hammer at two Panda cracking sites that were 37 m apart.

Figure 3.

Weight of stone tools as a function of the distance to the nearest inselberg. Each circle represents a stone tool (black circle: this study, cross: excavated stones from [42]). The dashed line shows the fitted model and the dotted lines the 95% confidence interval. (The excavated material was not included in the model and only placed on the graph for visual aid.)

Figure 4.

Use-wear pit depth as a function of the distance to the nearest inselberg. Each dot represents one stone tool. The dashed line shows the fitted model and the dotted lines the 95% confidence interval.

Figure 5.

Granite stone distribution in the chimpanzee home range in the Taï National Park. Available stone size is corrected for the area sampled in the three different ecological conditions (forest, inselberg, swamp). The horizontal line represents the minimum weight of a suitable Panda hammerstone (assessed through our sample size).