Tree-shaking displays in a free-ranging, provisioned group of Japanese macaques (Macaca fuscata)

Tree- or branch-shaking displays have been reported in some nonhuman primate species, but large-sample studies addressing their possible functions are rare. The present study examined 842 and 72 tree-shaking (TS) displays by free-ranging male and female Japanese macaques (Macaca fuscata), respectively, recorded in and around the group's provisioned feeding site. In the mating season, TS displays performed by peripheral and non-group (i.e., non-central) males were almost always accompanied by loud vocalizations, whereas central males were less vocal. Also, non-central males rarely performed female-directed aggressive behaviours, whereas central males frequently did so. Aggression towards females by non-central males poses a potential cost, i.e. punishment by central males, whereas vocalizing to attract females appears to have few costs, if any, yet it might provide substantial reproductive benefits. By contrast, central males performed fewer TS displays with vocalizations as they were at less risk of getting punished for threatening or attacking females. In the non-mating season, almost all TS displays were performed by central males; non-central males were rarely observed in or around the feeding site in the non-mating season. TS displays in the non-mating season may help central males to reinforce their high-ranking positions. Most female TS displays were by the alpha female and her daughters in the non-mating season, their probable function also being to reinforce their high ranks.