Short‐Term Separation From Groups by Male Japanese Macaques: Costs and Benefits in Feeding Behavior and Social Interaction

大谷洋介、澤田晶子、半谷吾郎

集団生活の利益・不利益と集団参加の意思決定についての知見を深めるため、ニホンザルオス個体の集団からの一時離脱行動について明らかにした。オスは平均68分間の短期的な離脱を繰り返し、離脱中にはほとんどの場合単独で遊動していた。離脱中はより長い時間採食し、採食に占める果実の割合が増加していた。また集団から離脱中は一本の木に長い時間滞在し、ゆっくりとした速度で採食を行っていた。これらの変化は、オスが離脱によって集団内採食競合を回避していたことを示唆する。一方で、オスは離脱時により多くの移動コストを支払い、また他個体との社会交渉の機会を喪失することが分かった。加えて、集団間の競合については離脱時のオスは脆弱になっていた。集団から離脱するかどうかというオスの選択は、食物の分布様式や社会的地位に影響を受けていた。今回明らかにされた一時離脱行動によって、オスは集団生活上の利益を完全には失うことなく、不利益を補償している可能性が示された。

American Journal of Primatology, http://onlinelibrary.wiley.com/doi/10.1002/ajp.22241/abstract

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.

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