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Jean-Baptiste Leca, Ph.D.

Associate Professor

Department of Psychology

University of Lethbridge

Canada


Stone handling project: Main results obtained

Like many other behaviors in Japanese macaques, stone handling is cultural behavior [REPORT] (see also Invited Commentary by Leca in [REPORT])

Through the analysis of a total of 4,530 observation hours, including 2,460 hours of video-recorded data, the combination of the comparative, longitudinal, and experimental approaches to studying stone handling behavior [REPORT] allowed us:

1) to establish the comprehensive repertoire of 45 stone handling patterns in Japanese macaques [REPORT], and show that stone handling is a structurally complex object-directed manipulative activity [REPORT]

... and a good candidate for the study of manual lateral biases (e.g., hand preferences) [REPORT]

(see also photos and videos of stone handling patterns)

2) to reveal substantial variation in stone handling between troops, referred to as stone handling cultures, with local variants being customary in some troops, and rare or even absent in others although they were ecologically possible, and performed by a majority of individuals in some troops, or only idiosyncratically in others [REPORT]

(see also Figure),

3) to show a geographic distribution of clear troop-dependent clusters of stone handling variants suggestive of the notion of cultural zones, based on inter-troop observation and possibly males transferring stone handling patterns when migrating from one troop to another [REPORT]

4) to rule out simple alternative explanations for such behavioral variability, such as genetic determinants or some obvious environmental differences like stone availability [REPORT]

5) to indicate that group size and composition in age classes, as well as group spatial cohesion may impact the prevalence of stone handling [REPORT]

6) to provide evidence for the role of social factors in the acquisition of the behavior and the maintenance of the tradition, which may involve not only direct social influences through the observation by naïve infants of their mothers as stone handling demonstrators, but also indirect social inputs through the stimulating effect of stone handling artifacts [REPORT]

7) to interpret some intra-group variability in the performance of stone handling from the viewpoint of ontogenetic constraints, with was a gradual increase in the number and complexity of stone handling patterns displayed by infants, which revealed a neuro-motor developmental phase of this behavior [REPORT]

8) to show that in several troops, this behavioral tradition has reached its transformation phase, with an increase in the stone handling repertoire and an expansion of the contexts in which stone handling is practiced, also referred to as “ratchet effect” or “cumulative culture” [REPORT 1]   [REPORT 2]

9) to argue that food provisioning by humans may be a key factor in the innovation and transformation phases of the stone handling tradition [REPORT]

10) to propose that in free-ranging provisioned troops, stone handling was an extension of foraging-like activity, misdirected on readily available objects (stones) instead of food items [REPORT 1]    [REPORT 2]

11) to suggest that under relaxed selective pressures on foraging, stone handling may simply serve the function of maintaining in some populations a set of behaviors that could evolve into tool-use [REPORT] and could even have implications for welfare and neurological health [REPORT]

12) to discuss the role of phylogenetic constraints and behavioral predispositions in the evolution of the stone handling culture in the genus Macaca

[REPORT 1]    [REPORT 2]