Group breeding in vertebrates: linking individual- and population-level approaches

Abstract

Background: Why individuals breed in groups and why patterns of group breeding are so variable are long-standing questions in evolutionary ecology. Researchers have tended to study either population-level patterns such as breeding group size or else the decisions that individuals make when joining groups, but have rarely explicitly linked the two, using knowledge of individual decisions to predict larger-scale population patterns. Aim: We describe an integrated method designed to categorize and explain a diversity of vertebrate social systems, with a focus on colonial breeding. This approach places group breeding within an evolutionary context by first elucidating the process of group formation, identifying decision rules that individuals use when joining groups, and verifying their fitness consequences, then using these individual-level processes to predict and understand patterns of population-level variation. Results: To facilitate the identification of decision rules, we differentiate between groups that form as a consequence of resource patchiness, and groups that form because individuals derive social benefits from breeding in close association with conspecifics. We next demonstrate how these decision rules inform us about the maintenance of population-level patterns of group breeding, like group size variation. Worked examples: We illustrate this process-to-pattern approach with several empirical examples, highlighting its efficiency relative to the more traditional approach of defining pattern first. By identifying individual-based decision rules in four different vertebrate taxa, we demonstrate the power and utility of using individual-level decision making to explain larger-scale patterns of group living.