1 Introduction Many prey choose to live, forage, and reproduce in groups—this is one of the most readily- observed phenomena in biology. A common adaptive explanation for grouping behavior is that it aids in anti-predatory defense. For instance, Starlings ( Sturnus vulgaris ) are well known to forage in flocks in the presence of predators [1]. Shoaling fish, e.g. the eastern mosquitofish ( Gambusia holbrooki ), have been documented to identify predators more accu- rately in larger groups [2]. Ostriches ( Struthio camelus ) have been reported to experience anti-predatory benefits when foraging in groups [3]. Even when there is a correlation between grouping behavior and protection from predators, however, it is difficult to pin down what benefits actually select for the evolution of grouping behavior. Several such fitness benefits have been proposed. For example, grouping can improve group vigilance [4–7], reduce the chance of being encountered by predators [5, 8], dilute an individual’s risk of being attacked [9–14], enable an active defense against predators [15], or reduce predator attack efficiency by confusing the predator [16–20]. Other possible ben- efits not involving predation include improved mating success [21], increased foraging effi- ciency [22], and the ability for the group to solve problems that would be impossible to solve individually [23], for example through the division of labor [24]. With all of these interdependent factors potentially affecting the evolution of grouping, it is difficult to study the independent effects of each benefit in biological systems, let alone explore how they unfold over evolutionary time scales. However, recent research has shown that it is possible to explore the potential independent effects of each benefit by modeling them with digital models of evolution [25]. In previous work, we created several models to explore the predator confusion [20] and selfish herd [13, 14] hypotheses to find when these benefits do (and do not) independently select for grouping behavior. One advantage of these models is that once the independent effects of the various grouping benefits are understood, we can then combine the benefits into a single model to study their relative importance and separate the adaptive benefits (that select for the evolution of grouping) from the incidental 3