Chimpanzee accumulative stone throwing

Abstract

The study of the archaeological remains of fossil hominins must rely on reconstructions to elucidate the behaviour that may have resulted in particular stone tools and their accumulation. Comparatively, stone tool use among living primates has illuminated behaviours that are also amenable to archaeological examination, permitting direct observations of the behaviour leading to artefacts and their assemblages to be incorporated. Here, we describe newly discovered stone tool-use behaviour and stone accumulation sites in wild chimpanzees reminiscent of human cairns. In addition to data from 17 mid- to long-term chimpanzee research sites, we sampled a further 34 Pan troglodytes communities. We found four populations in West Africa where chimpanzees habitually bang and throw rocks against trees, or toss them into tree cavities, resulting in conspicuous stone accumulations at these sites. This represents the first record of repeated observations of individual chimpanzees exhibiting stone tool use for a purpose other than extractive foraging at what appear to be targeted trees. The ritualized behavioural display and collection of artefacts at particular locations observed in chimpanzee accumulative stone throwing may have implications for the inferences that can be drawn from archaeological stone assemblages and the origins of ritual sites.

Figure 1. Chimpanzee range map showing the geographic distribution of accumulative stone throwing populations.

The map shows the locations of all chimpanzee populations studied across Africa including the four PanAf temporary research sites (TRSs) where accumulative stone throwing behaviour was observed (white circles; 1: Boé, Guinea-Bissau; 2: Sangaredi, Guinea; 3: Mt. Nimba, Liberia; 4: Comoé GEPRENAF, Côte d’Ivoire). Chimpanzee accumulative stone throwing was not observed at all other research sites: PanAf TRS (light grey circles), PanAf TRS carried out at mid- to long-term chimpanzee research sites (dark grey circles) and PanAf TRS carried out at mid- to long-term research sites of habituated chimpanzees (black circles). See also Supplementary Table 1. (Map created by M. Arandjelovic using QGIS version 2.6.1: http://www.qgis.org/en/site/).

Figure 2. Photographs and stills of accumulative stone throwing behaviour and sites.

(a) Adult male chimpanzee tossing a stone; hurling a stone (Boé, Guinea-Bissau); and banging a stone (Comoé GEPRENAF, Côte d’Ivoire). (b) Boé, Guinea-Bissau landscape: stones accumulated in a hollow tree; a chimpanzee accumulative stone throwing site; and stones accumulated in-between buttress roots (see also Supplementary Movies 1–7).

Figure 3. Flow chart describing the behavioural elements observed in chimpanzee accumulative stone throwing.

Three key behaviours were common to all observations of adult (N = 63) chimpanzee accumulative stone throwing (grey rectangles): 1) picking-up and handling a rock, 2) pant hoot introduction and/or build up phase, and 3) throwing the stone. Other behaviours were only sometimes observed or were observed in combination with one another (italicized). *Only 50 videos contained audio, all of which recorded a pant hoot vocalization.