Group Fission in Barbary Macaques (Macaca sylvanus) at Affenberg Salem

Abstract

We analyzed eight group fissions occurring during a 20-year period in three groups of a free-ranging provisioned Barbary macaque population. The founder group fissioned four times within 3.5 years after transfer to the enclosure, indicating that external factors—new environment, more space, absence of other groups—facilitated group fissions. Two groups resulting from these fissions, split twice within 2.5 and 1 years, respectively, many years later. The process of fissioning lasted from a few months to almost 2 years. Fissions were preceded by peripheralization/subgrouping of mainly young adult males (8-10 years old), suggesting that male competition was the primary force for the fissions. The males were joined by middle- to low-ranking but not the lowest-ranking females. The resulting new groups were usually smaller than the groups in which the former α-matriline—old groups—stayed, and they were also more variable in size and sex ratio, suggesting that variable numbers of surplus individuals were expelled during fission. Mean adult sex ratios were similar in both groups after fission, indicating that the competitively superior males in the old groups (groups + α-matriline) could not increase their breeding opportunities. Female kin, even of large matrilinies, almost always stayed together during fission. Natal males strongly preferred to join the old groups, and this preference was most pronounced in juveniles and subadults. Hence, most natal males stayed with maternally related females, i.e., remained true natal males, if the females stayed in old groups. They were separated from female kin, i.e., became seminatal, if the females joined the new groups. These seminatal males did not differ from natal males with respect to matrilineal rank, but they had more female relatives, above all more close relatives (sisters), indicating that avoidance of mating with maternal kin was important for group choice. Despite joining the same group as female kin during fissioning, breeding opportunities of natal males (ratio of unrelated females/male) were not less than that of their seminatal peers, because natal males had fewer female relatives. Only a minority of both groups of males would have done better by joining the alternative group. Paternal relatives were distributed during fission by chance, and loss of patrilinies was therefore much less pronounced. We conclude that the rules governing social relationships among Barbary macaque males are less apt to cope with the high number of males resulting from provisioning, whereas the rules regulating social relationships of females living in a nepotistic, female-bonded society are very robust in this respect.