Figure 5: Fitness for prey with and without the option to forage in groups. In heterogeneous/semelparous groups, prey with the option to group have lower fitness than prey that are forced to group. Error bars indicate bootstrapped 95% confidence intervals over 100 replicates; some error bars are too small to be visible. 5 10 25 50 Group Size 0 50 100 150 200 Fitness Heterogeneous, Semelparous Individual Grouped Optional evolve vigilance behavior regardless of whether the prey are forced to group or not (Figure 4). Similarly, heterogeneous/semelparous populations only evolve vigilance behavior in smaller groups, whereas the advantage of collective vigilance is lost in larger groups. At larger group sizes, prey with the ability to choose whether or not to forage in heterogeneous/semelparous groups instead evolve lower levels of vigilance than required to protect the group (Figure 4), which results in a decrease in overall group fitness relative to prey that always forage in groups (Figure 5). In our final treatment, we investigate the impact of assessing a direct cost of foraging in a group (e.g., competition for food). Figure 6 shows that except in the homogeneous/semelparous treatment, an explicit grouping cost selects against gregarious foraging behavior even when the grouping penalty is small ( M = 1 . 0). Conversely, prey in the homogeneous/semelparous treatment maintain some level of gregarious foraging behavior even when the penalty for foraging in groups is extreme ( M = 1 , 000). Therefore, we conclude that in the presence of even a small penalty for foraging in a group and the absence of additional selection pressures that favor gregarious foraging (e.g., improved social status for sentinels), only the combina- tion of high genetic relatedness within the group and a semelparous reproductive strategy Explicit cost of grouping 15