Short-term separation from groups by male Japanese macaques: costs and benefits in feeding behavior and social interaction

To expand our understanding of fission-fusion behavior and determine its variability among primates, studies of both individual-based and group-based fission-fusion are necessary. We conducted a parallel tracking study of male and female Japanese macaques (Macaca fuscata yakui) during the non-mating season to clarify the general features of separate ranging by males of this species, an example of fission-fusion behavior, and to reveal its associated costs and benefits. Males frequently engaged in short-term separate ranging, leaving the company of females and ranging on their own for periods averaging 68 min in duration. However, the males did not venture outside the group's home range. When ranging separately from the group, males spent more time feeding, particularly on fruit, stayed longer in each feeding tree, and fed at a lower rate than when ranging with the group. These behavioral changes suggest that males can avoid within-group feeding competition by ranging alone. However, this behavior was also associated with higher traveling costs, and these separated males were more vulnerable to intergroup competition and had fewer opportunities for social interaction. The frequency of separate ranging was lower when highly clumped food plant species were the main food source. Lower-ranked males, who received more aggression when ranging with the group, exhibited a higher frequency of separate ranging. This behavioral flexibility with respect to group cohesion may allow males to reduce the costs of group living without completely losing the benefits. Specifically, by ranging alone, males may acquire sufficient feeding time without being disturbed by other group members. Conversely, when ranging with the group, males can access grooming partners and advantages in intergroup competition.