FISSION-FUSION DYNAMICS IN MAMMALS: LINKING ANIMAL MOVEMENT TO GROUP BEHAVIOR

Abstract

Mammals living in groups show temporal variation in spatial cohesion and membership of groups, a behavior known as fission-fusion dynamics (FFD). Changes in cohesion depend on the movement behavior of individuals, which is influenced by their social environment, among other factors. I aimed to answer two main questions: 1) how do cognitive abilities and environmental factors explain the variation in social systems of mammals with FFD? and 2) how are FFD related to movement behavior? To answer the first question, I built a dataset on social traits of mammals with FFD from published references and used multivariate analysis to uncover the patterns of variation in social systems. Variation resulted mostly from differences in group and subgroup sizes, and differences in social traits evidenced the presence of discrete categories of social organization that might represent distinct strategies of FFD. To assess the effects of brain size and resource availability on social traits, I used generalized estimating equations as a phylogenetic comparative method. Brain size affected most social traits in marine mammals, supporting predictions of the social brain hypothesis. Resource availability was poorly correlated with social traits in all terrestrial mammals, but it had different effects for herbivores and carnivores, suggesting that environmental constraints acting on FFD differ between trophic levels. To answer the second question, I assessed the predictive power of several movement metrics characterizing tracks of orangutans on FFD, using generalized boosted regressions. Tortuosity, speed, and the number of behaviors were strong predictors of group presence and size, while temporal changes in movement behavior were correlated with changes in cohesion. These findings highlight the potential use of individual movement data to predict aspects of FFD. Lastly, I used an agent-based model to explore the influence of different levels of specificity in recognition on grouping behaviors. Model results suggest that basic social behavioral rules influence FFD, and that more complex group dynamics, such as hierarchical group structures, only emerge in scenarios with high levels of recognition specificity. Overall, the model suggests that recognition abilities, which likely correlate with cognitive skills, may play an important role in the evolution of social systems.