Dyadic response facilitation of object play in Balinese long-tailed macaques.

Social influence is at the core of the emergence and maintenance of behavioral traditions in various animal taxa. Response facilitation is a mechanism of social influence whereby observing a demonstrator performing a behavior temporarily increases the probability that the observer will perform the same behavior. We focused on stone handling (SH) behavior, a form of object-directed play routinely displayed by free-ranging long-tailed macaques (Macaca fascicularis) in Ubud, Bali, Indonesia. We tested whether the expression of SH was subject to dyadic response facilitation. We compared video-recorded focal samples of an individual immediately after they had witnessed a SH bout performed by a conspecific, and matched-control focal samples of the same witness in the absence of any surrounding SH bouts. We found converging evidence that SH was facilitated within pairs of individuals. First, SH occurred significantly more often and lasted significantly longer in the post-witnessing condition than in the matched-control condition. Second, a monkey initiated SH more rapidly in the former than in the latter, and this significant facilitation effect mainly occurred during the first two minutes after witnessing SH. By demonstrating that the expression of SH was socially mediated, we provided further support for the cultural nature of this behavior.