Albanese, V., Berardi, R., Llorente, M. et al. 2022. Mechanisms of expression of object play: A comparative study of stone handling in two captive groups of long-tailed macaques (Macaca fascicularis fascicularis). Behavioural Processes 203, 104774.

Stone handling (SH) is a form of solitary object play that is socially learned and culturally maintained. We studied two captive groups (Modena, N = 20; Padova, N = 20) of common long-tailed macaques housed in a sanctuary in Italy. Our research goal was two-fold: (1) establish the first SH repertoire in captive-raised long-tailed macaques, and (2) explain major differences in the expression of SH between the two study groups. Despite being of identical size and sharing similar environmental conditions, we found that SH was performed by most group members in Modena, whereas SH was absent in Padova. We aimed to explain this inter-group variation by exploring the role of proximate factors that are known to affect the occurrence of SH: demography, dominance, stone availability, activity budget, and food provisioning. The atypical age structure of Padova (i.e., no immature individuals) may have impaired the emergence of SH in this group. In Modena, we found no significant effect of hierarchical rank on SH frequency and duration and no temporal relationship between SH and feeding. Regarding the activity budget, SH filled in for a portion of affiliative and resting behaviours in Modena. Our findings lend support to the cultural nature of SH.