Social ties drive post-fission group choice in blue monkeys.

Permanent group fissions present rare opportunities for socially philopatric individuals to select their groupmates. We hypothesized that animals prioritize maintaining beneficial social ties during fission. We assessed post-fission group choice in philopatric adult female blue monkeys by considering the strength and consistency of their social ties, dominance relations and relatedness with female peers, as well as their risk of infanticide and ties to the original group's resident male. Using a temporal network model, we assessed which females remained together (i.e. maintained ties) after fission. Females preferentially maintained ties to individuals with whom they had consistently strong affiliative ties before fission. More closely related individuals were likely to maintain ties only if they had similar risks of infanticide. Conditional logit models showed that females vulnerable to infanticide were more likely to join the post-fission group with the original group's resident male than the group without him. Overall, female blue monkeys appear to consider multiple types of relationships when structuring their post-fission groups, prioritizing consistent affiliative ties with other females and ties with a familiar male, and even separating from kin when infant offspring are vulnerable. The relationships prioritized during fission likely confer benefits, and females' choices illuminate how sociality influences decision-making.