Of stones and monkeys: Testing ecological constraints on stone handling, a behavioral tradition in Japanese macaques

Japanese macaques are known to manipulate stones by displaying various seemingly functionless behavioral patterns, including carrying a stone, rubbing two stones together, or gathering several stones into a pile. This form of solitary object play called stone handling (SH) is a behavioral tradition in Japanese macaques, showing striking intertroop differences in frequency and form. Here, we evaluated two ecologically based hypotheses invoked to account for these differences. We hypothesized that the occurrence and form of SH would be affected by stone availability and the degree of terrestriality. We used standardized sampling methods to assess differences in SH and terrestriality among four captive and six free-ranging troops of Japanese macaques, and determine site-specific stone availability. Although we demonstrated that SH is almost exclusively a terrestrial activity, our comparative analyses showed that the number of stones readily available and the relative amount of time spent on the ground by the macaques were not associated with the intertroop differences in the occurrence of SH. Failure to accept the terrestriality and stone availability hypotheses suggests that the performance of SH and the motivation to engage in this activity are both more diverse and more complex than the direct links to time spent on the ground or the number of stones locally available. Other environmental influences and sociodemographic factors should be jointly considered to identify the sources of variation in SH, as a beginning to better understand the constraints on the appearance and subsequent diffusion of stone-use traditions in nonhuman primates.