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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). 

## Explicit cost of grouping 

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 

