Indirect social influence in the maintenance of the stone-handling tradition in Japanese macaques, Macaca fuscata

Abstract

Comparative and longitudinal studies have shown that stone-handling (SH) behaviour, defined as the noninstrumental manipulation of stones by performing various behavioural patterns, is socially transmitted across generations as a cultural behaviour in Japanese macaques. We investigated experimentally how stimulus/local enhancement, a form of indirect social influence through the physical traces typically left in the environment by previous stone handlers, might trigger SH behaviour at the individual level, and thus could contribute to the maintenance of the SH tradition at the group level. Through the semicontrolled conditions of field experiments, conducted in the free-ranging provisioned Arashiyama E troop, in which the SH tradition has been well established for nearly three decades, our results supported the stimulus/local enhancement hypothesis. Most group members preferentially directed their SH behaviour towards typical physical traces of SH activity (piles of stones) over randomly scattered stones. Encountering SH artefacts enhanced the use of these particular stones for performing SH in that particular part of the environment. The common occurrence of such ‘play stations’ may favour the frequent reuse of the same stones over time and explain the transport of stones between and around SH artefacts. We provided evidence for the role of indirect social inputs on the long-term persistence of the SH tradition in Japanese macaques, through SH by-products, the stimulating effect of which can be delayed in time and separate in space from others. We discuss our findings from the perspective of socially mediated behaviours, conformity-enforcing responses and niche construction.