Stone handling behavior in Balinese long-tailed macaques
Most of the behavioral characteristics and environmental correlates of stone handling in Balinese long-tailed macaques are similar to those we found in Japanese macaques.
For example, stone handling behavior is routinely performed by most group members in the population of Balinese long-tailed macaques living in the Sacred Monkey Forest of Ubud, central Bali (whereas it is absent or rare in other populations on the island).
It is very likely that this behavioral practice is socially influenced, either directly via observational learning or indirectly when encountering behavioral artefacts (i.e., stone assemblages).
However, there are also differences in the stone handling behavioral patterns performed by Japanese macaques and Balinese long-tailed macaques. Amanda Pelletier is exploring such inter-species variation.
For example, percussive stone handling patterns (e.g., pound stone on stone, pound on surface, lift and drop, tap stone onto body parts) seem to be more frequent and more prevalent in Balinese long-tailed macaques than in Japanese macaques.
Also, it appears that combinatorial actions involving stones and other objects (e.g., wrapping stones inside leaves, or grinding plant materials with stones) are more frequent and more prevalent in Balinese long-tailed macaques than in Japanese macaques.
We hypothesize that such inter-species differences in stone play have implications for our understanding of the behavioral propensity to use stones as tools.
This video also illustrates how "conformity bias" ("Do as the majority does") may support the social learning of a solitary object play activity, like stone handling.
The perception-action theory (or affordance learning theory) posits that the non-instrumental manipulation of objects and the use of these objects as tools are developmentally and evolutionary linked.
Our current research project aims to combine observational and experimental studies to investigate how stone play may be a behavioral precursor to stone tool use.
More specifically, we hypothesize that stone handling may be
maintained proximately (as a culturally-enhanced and pleasurable playful activity) and represent in
some groups a reservoir of stone-directed behavioral diversity and complexity from
which stone tool use could emerge.
In Balinese long-tailed macaques, we predict that stone tool use is more likely to be elicited (by stone tool use tasks) in groups where stone handling is a well-established behavioral tradition than in non-stone handling groups. Indeed, the former would have the exaptive possibility of extending their behavioral repertoire to stone tool use.
In the long term, we will extend these analyses to Thai long-tailed, Taiwanese, and rhesus macaques through ongoing collaborations with other researchers (namely, Drs. M. Gumert, N. Gunst, M. Huffman, C. Nahallage, and H.-H. Su).
The overarching goal of this project is to generate scenarios about the evolutionary histories of stone play and stone tool use in the genus Macaca – the best-studied monkey taxon, featuring broad behavioral and ecological diversity – as a model for the comparative analysis of the developmental, mechanistic, and evolutionary processes that have shaped our own lithic culture during the course of human evolution.