Relationship quality affects fission decisions in wild spider monkeys (Ateles geoffroyi)

Fission–fusion dynamics are thought to be mainly a response to differential availability of food resources. However, social factors may also play a role. Here, we examined whether the quality of social relationships between group members affects fission decisions. During 21 months, we collected data on social interactions and fission events of 22 spider monkeys (Ateles geoffroyi) living in a community in the protected area of Otoch Ma'ax Yetel Kooh, Yucatan, Mexico. By entering seven indexes of social interactions into a principal component analysis, we obtained three components of relationship quality, which we labelled “compatibility,” “value” and “insecurity” given the relative loadings of the indexes. Our results showed that individuals were more likely to fission into the same subgroup with community members with whom they shared higher levels of compatibility and value and lower levels of insecurity. In addition, individuals preferred to fission into the same subgroup with same‐sex group members, as expected based on what is known for the species. Our findings highlight the role of social factors in fission decisions. Adjustments in subgroup size are based on multifaceted social preferences, incorporating previously unexamined aspects of relationship quality, which are independent from overall levels of affiliative interactions.