Coefficient of genetic relationship and the probability of intragenealogical fission in Macaca mulatta

Abstract

1. The average degree of relatedness (r), based on Sewall Wright's (1922) coefficient of inbreeding (F), was calculated for 68 matrilineages of rhesus monkeys involved in four group fissions. The average degree of relatedness for each entire social group before and after fission was also calculated. Matrilineages involved in group fission were found to subdivide when the average degree of relatedness between members fell below the level of first cousins. Whole social groups fissioned when the average degree of relatedness between members fell below the level of second cousins. 2. These r values, though calculated only through the maternal line, are somewhat lower but overlap with those found by Chagnon (1975) for the average degree of relatedness in fissioning villages of Yanamamo Indians calculated through both maternal and paternal lines. 3. Two matrilineally organized human groups, which undergo fission frequently, show comparable patterns of group fission. The decay of group solidarity is apparently correlated with a decay in the average degree of relatedness among group members. 4. Both in humans and in macaques, seceding groups, though often socially subordinate, have a higher average degree of relatedness, and show greater group solidarity than either the mother group before fission or the larger fission product.