Animals must make route choices every day when moving through their habitat while foraging. Choosing an optimal route can be cognitively costly, and primates and other animals have been shown to use simple heuristics, "rules of thumb", to make foraging route choices. We investigated the potential use of heuristics among foraging free-ranging Japanese monkeys (Macaca fuscata) during solitary foraging trials. We also investigated the potential influence of individual variables (age and sex) and social variables (presence in the central group, presence of potential inter- and intraspecific competitors), on the use of heuristics, route length and trial time. We used a multi-destination foraging experiment with 6 platforms in a (4 m × 8 m) Z-array, completed by 29 Japanese macaques in 155 runs at the Awajishima Monkey Center in Japan. Our results showed that the macaques chose routes consistent with heuristics (e.g. nearest neighbour heuristic 19.4%, convex hull heuristic 4.5%) and selected optimal routes (shortest path in 23.9% of the trials). We also identified a potential new heuristic that was used most frequently, that we termed the "sweep heuristic" (27.1% of trials), which we interpreted as a strategy to deal with competitive foraging trade-offs - choosing routes to prioritize not leaving isolated food pieces behind. Age was significantly related to trial time; juvenile macaques were faster than adults and young adults, using speed to gain access to resources. Solitary trials with conspecifics present took significantly longer routes. Our results suggest that contextual factors led to variation in Japanese macaque decision-making, and we suggest that the preferential use of a sweep heuristic may have been a response to high intragroup competition.
