Maternal rank acquisition in a primate with low aggression and coalition rates

Abstract

For many group-living animals, acquiring adult dominance rank is an important social transition. In species with maternal rank “inheritance,” juveniles appear to acquire their adult rank by receiving coalitionary support from kin and by altering patterns of aggressive behavior as they age. Previous studies of rank acquisition, however, have focused on species with high rates of aggression and coalitionary support. We studied rank acquisition in blue monkeys, a species with relatively low aggression and coalition rates, hypothesizing that individualistic changes in agonistic behavior would accompany rank acquisition while the presence of potential coalition partners would be relatively unimportant. Focusing on juvenile females in a wild population, we evaluated latency to reach maternal status and assessed how agonistic behavior changed with age. We also compared rank acquisition in orphaned vs. non-orphaned juveniles. Females from higher ranking matrilines and those in larger groups took longer to reach their maternal status and having more kin did not accelerate rank acquisition. As they aged, subjects were more likely to show aggression to individuals that they were expected to outrank as adults. Female juveniles also appeared to work their way up the hierarchy by overturning ranks with groupmates sequentially. Most orphans reached their matrilineal rank, though orphans were less likely to reach their matrilineal rank than non-orphans, suggesting that in some cases mothers do play an important role in rank acquisition. Orphans with more or high-ranking kin were not more likely to reach their matrilineal rank. These results support the hypothesis that age-related changes in agonistic behavior accompany rank acquisition in blue monkeys while the availability of kin as coalition partners often has little effect on the process. Behavioral mechanisms underlying maternal rank acquisition may vary between closely related species in conjunction with other patterns of social behavior, such as aggression and coalition rates. In group-living animals with matrilineal hierarchies, individuals are born low-ranking, but eventually acquire a rank similar to their mother’s. In species with high rates of aggression and coalition formation, juveniles reach their mother’s rank by receiving support from kin in aggressive interactions, and by selectively directing aggression to groupmates from lower ranking families. We studied maternal rank acquisition in blue monkeys, hypothesizing that the process might occur differently in a species with low rates of aggression and coalitions. Indeed, the availability of kin appeared to have limited effects on rank acquisition. Maternal rank acquisition was, however, associated with age-related changes in juvenile behavior. We concluded that in some species with matrilineal hierarchies, rank acquisition might occur relatively independent of support from kin.