Figure 2: Treatment comparison when prey are forced to forage in groups. Both group homogeneity and a semelparous reproductive strategy select for high levels of vigilance. However, only homogeneous groups experience an increase in fitness as group size increases. In contrast, vigilance behavior breaks down in larger, heterogeneous groups of semelparous prey. Error bars indicate bootstrapped 95% confidence intervals over 100 replicates; some error bars are too small to be visible. and the only trait that is evolving is the prey decision to be vigilant or not at every time step. Under these conditions, we find that prey living in homogeneous groups consistently evolve higher levels of vigilance than their counterparts living in heterogeneous groups (Figure 2). This suggests that organisms living in groups with high genetic relatedness are more likely to evolve cooperative strategies. Thus, in our model as in many natural systems, gregarious foraging is most favorable when genetic interests are aligned. Figure 2 also shows that semelparous prey are more likely to evolve vigilant strategies than iteroparous prey. This finding follows from the fact that semelparity selects more strongly for successful evasion of predator attacks, since prey death negates all previous foraging efforts. This effect is seen across both homogeneous and heterogeneous groups, indicating that semelparity is a strong enough selective pressure to act independently of group genetic composition. Importantly, prey vigilance does not evolve at all in the absence of predation (Figure S1), and gradually reducing the predation rate leads to a correspondingly gradual decrease in prey vigilance levels (Figure S2). Therefore, we know that the selection pressure imposed by predation is the primary driving force behind this evolved vigilance behavior. All three treatments that evolve any level of vigilance also see the prevalence of vigilance decrease as group size increases. This pattern is important because it matches the pattern 11 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 Homogeneous, Iteroparous Homogeneous, Semelparous Heterogeneous, Iteroparous Heterogeneous, Semelparous