Size Matters in Primate Societies: How Social Mobility Relates to Social Stability in Tibetan and Japanese Macaques

Social mobility, defined as vertical movement of positional status for members in a society, has profound effects on social stability, dynamics, and evolution. In this chapter, we compared as case studies of intragenerational mobility in adult females of Tibetan (Macaca thibetana) and Japanese macaques (M. fuscata) to illustrate how social mobility can be measured in primate societies and how group size affects social mobility. Using dominance rank data (absolute ranks) collected from two long-term field projects in Mt. Huangshan, China, and on Koshima Island, Japan, we found that group size was over three times larger in Japanese macaques than in Tibetan macaques. The former also showed a higher level in the rate of rank change than the latter. Interestingly, due mainly to larger variation in the rate of rank change in Japanese macaques, social mobility measured by regression analysis showed no difference between the two species. However, when we used relative rank by factoring in group size, social mobility increased in Tibetan macaques but decreased in Japanese macaques when compared with absolute rank, leading to the counterintuitive result that social mobility was lower in Japanese macaques than in Tibetan macaques. Our results provide novel and unexpected insights into questions about social dynamics such as why group fission is so much more common than group fusion in primate societies.