Prior to our study, there were reports and anecdotes of wild chimpanzees throwing and banging stones in Liberia, Guinea, and Guinea-Bissau (A.G. pers. obs.) in addition to occasional observations of habituated chimpanzees throwing rocks and other objects during male threat displays12. The first direct observations of stone throwing behaviour in association with the presence of accumulated stones at specific trees (henceforth “accumulative stone throwing”) were recorded by camera traps on March 24, 2011, at the Sangaredi PanAf temporary research site (TRS) in Guinea (Fig. 1; TRS #2). Following this observation, additional data collection procedures particular to this behaviour were incorporated into the PanAf protocol and administered to all TRSs across Africa to ensure comparable data across research sites (see Methods).

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/). Full size image

Chimpanzee accumulative stone throwing behaviour

Thirty-one TRSs located within the Pan troglodytes range were sampled between 2011 and 2015 for a period of 14–17 months. An additional three TRSs were on-going and studied for less than 14 months, for a total of 34 (see Supplementary Table 1). At four TRSs: (Boé, Guinea-Bissau; Sangaredi, Guinea; Mt. Nimba, Liberia and Comoé GEPRENAF, Côte d’Ivoire; Fig. 1) we found multiple hollow and/or buttressed trees exhibiting clear signs of wear with an accumulation of rocks at their base or inside the tree (Fig. 2). Using remote video camera traps, we subsequently filmed chimpanzees at each of these four TRSs approaching focal trees with a stone in their hand, or grabbing a stone from the base or from inside the tree’s hollow cavities, and then proceeding to throw it (N = 64 total stone throwing events; Table 1). We observed three particular variants of the behaviour: the rock was thrown at the tree using one or both hands (‘hurl’); hit repeatedly against the tree while the chimpanzee held it (‘bang’); or thrown into the hollow tree or a hollow groove formed by large buttress roots (‘toss’; Table 1; Supplementary Movies 1–7). The individuals observed in the camera trap footage were mainly adult males, but we also observed an adult female and a juvenile exhibiting the behaviour (Supplementary Movies 1 and 6). Common to all accumulative stone throwing observations exhibited by adults (N = 63) was the pant hoot vocalization, in particular the introduction and/or build-up phase25, which occurred after or while the individual picked up and handled the rock (Fig. 3). The pant hoot is a characteristic feature of the ritualized agonistic displays of adult male chimpanzees, which typically also involves piloerection, bipedal stance, hand and feet drumming on buttress roots of trees and, in some populations, is preceded by leaf-clipping25,26,27. Unfortunately, audio was recorded for only 50 of the 64 events captured on camera traps, so we may underestimate the variation in vocal behaviour accompanying accumulative chimpanzee stone throwing. We further observed that rock handling and throwing was sometimes accompanied by the individual swaying back and forth while bipedal and piloerect, and even leaf-clipping (Supplementary Movie 4; Fig. 3), all behaviours associated with a typical chimpanzee display27. When the rock was thrown, this was often, but not always, accompanied by the climax phase of the pant hoot consisting of scream elements and drumming with the hands or feet on the tree25,26. In some cases we do not have footage of the full series of behaviours since camera trap videos are limited in length (60 seconds), and cameras were triggered at varying times for each accumulative stone throwing event captured.

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). Full size image

Table 1 Data from 11 temporary research sites (TRSs) across West Africa where data were collected for 14–17 months between 2011 and 2014 to document the occurrence of chimpanzee accumulative stone throwing. Full size table

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. Full size image

The accumulative stone throwing behaviour was only observed in Western chimpanzees, Pan troglodytes verus (Fig. 1). Moreover, to our knowledge, the behaviour has not been observed at any of the 17 existing mid- to long-term chimpanzee research sites (LRS) across Africa20 (Fig. 1). The wear observed on the trunks and buttress roots of trees targeted by the accumulative stone throwing indicated that all of the active sites had been in use for some time. Stone tools appeared to be regularly reused: in 57 of 64 stone throws filmed, the individual picked up a rock from the base of the tree, and once from inside the tree. We also observed the same individual at the same tree repeatedly engaging in accumulative stone throwing (Table 1), suggesting individuals frequently revisit sites.

Raw material accumulation and availability

At all four TRSs where the behaviour was observed, stones had accrued around the base of the tree at each accumulative stone throwing site. In the few cases when rocks were found piled inside the hollows of trees, or nestled between the grooves of buttress roots, we counted between four and 37 stored rocks (Boé (Fig. 3), Comoé GEPRENAF, and Mt. Nimba). The average weight of the individual stones at Boé was 3.6 kg (range: 0.5–17 kg), at Mt. Nimba 2.06 kg (0.2–7.1 kg) and at Comoé GEPRENAF 0.98 kg (0.8–7 kg).