UC Davis

Living in a group means not necessarily being able to do what you want, when you want. Social animals regularly confront consensus decisions where group members must collectively choose between mutually exclusive actions, such as where to go and when to move. Reaching consensus is often particularly difficult when group members have different needs and capabilities, leading to conflicts of interest over what to do and when to do it. In cohesive, heterogeneous social groups that must come consensus decisions to avoid group dissolution, some individuals ultimately have to compromise their preferred patterns of behavior, presumably at a cost to themselves. Making consensus decisions is a primary challenge of group-living, and how social species come to consensus is central to understanding the evolution of complex social systems like our own. A key challenge to studying how group decisions are made in the wild is determining the motivations driving individuals' decisions, and thus the link between individual needs and collective action. In this dissertation, I present a novel integration of two influential fields in behavioral ecology—optimal foraging theory and collective decision-making—to seek to overcome this challenge and understand how group-level actions emerge from the behavioral decisions of individuals. In chapter 1, I present a theoretical framework for using optimal foraging theory, specifically the marginal value theorem, as a tool for understanding individual preferences about when to leave a food patch, against which the behavior of groups can be compared under different decision-making regimes. Then, in chapters 2 and 3, I use empirical studies of cohesive social groups of wild white-faced capuchin monkeys (Cebus capucinus) on Barro Colorado Island, Panama to understand the drivers behind divergent foraging behaviors and preferences of group members, how these groups resolve conflicts of interest to come to consensus decisions, and who pays the costs of compromise. In chapter 2, I examine aspects of social and individual predictors of foraging behavior, showing that dominance rank, age class, and timing of access to a foraging patch all influence patterns of foraging behavior and efficiency. In chapter 3, I illustrate how foraging theory models can be applied to my empirical data on capuchin monkey collective foraging—very simply and with standard observational data collection techniques—to understand how social groups resolve conflict and reach consensus over foraging decisions. My dissertation demonstrates the benefit of taking a theory-driven approach to generate predictions about emergent collective phenomena that can be tested with simple field and experimental data across a range of systems. In doing so, foraging theory can help the field of collective behavior become a more predictive science.