Adaptive benefits of group fission: evidence from blue monkeys

Abstract Group fission, when one group of animals splits permanently into two, signals the threshold at which group members decide their group is too large, and allows within-group comparisons of behavior and reproduction that illuminate determinants of group size. We examined whether living in too large a group increases within-group competition by analyzing changes to activity budgets, agonism rates, fertility, and infant survivorship of adult female blue monkeys (Cercopithecus mitis) before vs. after seven permanent fissions. Accounting for seasonal variation, we found that females spent less time feeding right before sub-grouping began than a year before fission or after fission. Agonism rates generally declined after fission. However, females did not experience behavioral effects of competition equally. Those in the larger post-fission group spent less time consuming fruit right before sub-grouping than right after, while females in the smaller group experienced lower rates of agonistic losses a year after fission than before. Conception probability was lower in the 2 yr preceding fission compared to other times. Infants born up to 2 yr before fission had lower survival than those born afterwards or during stable periods. Females that joined the smaller post-fission group had lower fertility and infant survival overall. These changes in behavior and reproduction suggest heightened resource competition before fission, which was reduced when groups split. However, depending on which group a female joined, it took longer for benefits of group fission to manifest. These findings indicate ecological constraints on group size and show that fission mitigates within-group competition.