side effects of grouping. 

Here, we focus on anti-predator vigilance (i.e., the many eyes hypothesis) as a possible selective mechanism for the evolution of gregarious foraging behavior, and control for the in- fluence of the other benefits described above. First proposed using a mathematical model [4] and explored experimentally a year later [1], the many eyes hypothesis makes two key pre- dictions, both of which arise from the assumption that vigilance is costly because it imposes a trade-off with foraging efficiency: (a) individual prey vigilance will decline as a group size increases, and (b) because prey can more equitably divide the task of watching for predators in large groups, they will experience a fitness benefit from foraging more. Therefore, there will be a selective advantage for prey that forage in groups up to a certain group size. In the 40 years since its inception, these predictions have been examined in numerous species across hundreds of independent studies [2, 3, 26–28]. Furthermore, several game theoretical models have been applied to refine the predictions of when collective vigilance in foraging groups should evolve [29], and subsequently matched to experimental data [30]. 

These previous studies focus on the potential fitness consequences of vigilance in groups of animals, but they do not address the circumstances under which vigilance, and the advan- tages of being in a group with many watchful eyes, provides a sufficient selection pressure to favor group living, independent of other pressures. When considering the evolution of grouping behavior, it is vital to take into account both the benefits and costs imposed by the behavior [31]. To satisfy this requirement, a handful of researchers have recently turned to digital models to study the evolution of animal behavior [32–34]. These researchers use a digital model of evolution to evolve the behavior of a population of locally-interacting ani- mats, enabling them to explore the evolution of behavior in complex environments that are beyond the means of mathematical models [25, 35]. Additionally, these evolutionary model systems allow researchers to explicitly control for complicating factors, such as the dilution effect [27] and food density [36], that are commonly confounded with collective vigilance as factors benefiting group-living organisms. 

