Intraspecific Differences in Social Structure of the Japanese Macaques: A Revival of Lost Legacy by Updated Knowledge and Perspective

Abstract

The current socioecological models were developed to identify the type of food competition via ecological factors affecting female social relationships in primates (Wrangham 1980; van Schaik 1989; Isbell 1991; Sterck et al. 1997). There is a slight difference among the models (see Isbell and Young 2002 for details), but rough consensus in the following scenarios. The models predict that clumped or patchy food distributions bring about within-group contests over foods, which shape a linear and stable dominance hierarchy among females. If high-quality foods are clumped in intermediate-sized patches relative to the group size, a within-group contest would predominate. Only highly competitive females and her kin would be able to monopolize food resources, sometimes through coalition among kin (i.e., nepotism). Consequently, a linear and stable dominance hierarchy (i.e., despotic) and female philopatry would evolve among females [RN, or “Resident-Nepotistic,” in the terminology of Sterck et al. (1997)]. In contrast, if low-quality foods are highly dispersed or clumped in a patch large enough to accommodate all group members, a within-group scramble (not a within-group contest) would prevail, whereby all females could equally share the food resources without any coalition. As a result, a nonlinear and unstable dominance hierarchy (i.e., egalitarian) and female dispersal would evolve [DE, or “Dispersal-Egalitarian”; Sterck et al. (1997)]. Even under a weak within-group contest, however, female philopatric trait would evolve to defend such large food patches cooperatively against neighboring groups under severe between-group contest [RE or “Resident-Egalitarian,” Sterck et al. (1997)]. Moreover, van Schaik (1989) and Sterck et al. (1997) added a fourth category (RNT, or “Resident-Nepotistic-Tolerant”). When both types of contest were severe, dominant females would become tolerant to subordinates to derive cooperative forces from them at a group encounter although linear dominance hierarchy was found among females.