Interpreting phytoliths assemblages at chimpanzee (Pan troglodytes verus) nut-cracking sites in Bossou Forest, Guinea

Abstract

The nut-cracking behaviour of the chimpanzees of Bossou Forest has been long documented and studied in order to understand ultimate drivers for this form of durophagy by these apes. At sites in which they break open the nuts of the oil palm (Elaeis guineaeensis) on stone anvils with stone hammers, fragments of the tools as well as remnants of the nuts may be deposited into local sediments; however, they may become less visible at the macroscopic level as these sites are no longer used (become dormant). We build upon work that has been done to document this unique cultural heritage in West African chimpanzees by interpreting phytolith assemblages in sediments at active nut-cracking sites (used within the last two months). We compare these assemblages with those found in sediments of sites that have been dormant for ~10 years and sites where chimpanzees have not been observed to access and crack open oil palm nuts with stone tools. We predicted that larger assemblages of spheroid echinates, a phytolith associated with palms, would be found at active nut-cracking sites, however we found no statistical difference between total spheroid echinates (measured as total number found per gram of dry sediment) assemblages produced across active nut-cracking, dormant, or non-nut-cracking sites. This may have been due to small sample size (N= up to 6 sediment samples for each of the three sites) and so further sample collection and analyses are needed for inter-site comparisons. We also measured 2D area (sq micrometers), perimeter and maximum diameter of spheroid echinates found in nut endocarps (shells) and leaflets from an oil palm frond. Intra-specific differences from all three measurements were found with these phytoliths being larger in the nut endocarp than the leaflet. This has implications for (re)interpreting the assemblages of spheroid echinate phytoliths at the three sites to determine if there is a greater productivity larger-sized spheroid echinates that fall within the size range of those measured for the nut endocarp (mean maximum diameter of 14.9 micrometres versus 7.8 micrometers for those found in oil palm leaflet). Finally, we argue for the importance of initialising and continuing the interpretation of phytolith assemblages in local sediments that are located near sites where directed plant input by non-human taxa has occurred, in this case, chimpanzees cracking open oil palm nuts using stone tools. This is important to refine our interpretation of phytolith assemblages where human and non-human taxa who use complex foraging strategies share ecological spaces.