5 10 25 50 Group Size 0.0 0.2 0.4 0.6 0.8 1.0 Vigilance 5 10 25 50 Group Size 0 200 400 600 800 1000 Fitness 5 10 25 50 Group Size 0.0 0.2 0.4 0.6 0.8 1.0 Grouping 5 10 25 50 Group Size 0.0 0.2 0.4 0.6 0.8 1.0 Vigilant and Grouped Homogeneous, Iteroparous Homogeneous, Semelparous Heterogeneous, Iteroparous Heterogeneous, Semelparous Figure 3: Treatment comparison when prey can choose to forage in groups. Al- lowing prey to decide whether they wish to be in the group produces very similar results compared to when they are forced to group. In homogeneous groups, prey choose to spend most of their time in the group. However, grouping breaks down (alongside vigilance) in heterogeneous groups of semelparous prey. This occurs despite there being no direct penalty assessed for choosing to group. Error bars indicate bootstrapped 95% confidence intervals over 100 replicates; some error bars are too small to be visible. predicted by the many eyes hypothesis: As group size increases, individuals are able to rely more on collective rather than individual vigilance and can in turn devote more of their own time to foraging. Since we use a relative attack rate that scales the predator’s attack frequency with group size, we can be sure that this phenomenon is due to group vigilance and not the dilution effect (fewer attacks per individual in larger groups) cited in other studies. We note that vigilance in the heterogeneous/semelparous treatment appears to evolve away almost entirely in a group size of 50. To explain why this trend might be due to something other than collective vigilance, we can instead look at trends in the fitness of the populations. Several interesting trends are seen when we look at the influence of group size on average group fitness. In both homogeneous treatments, there is a steady increase in fitness with in- creasing group size, suggesting that gregarious foraging behavior is under positive selection. We see no significant fitness increase with group size in the heterogeneous/iteroparous popu- 12