Figure 4: Vigilance in prey with and without the option to forage in groups. In ho- mogeneous groups, prey with forced and optional grouping evolve similar vigilance behaviors. In contrast, individualistic (non-grouping) prey evolve vigilance behaviors that maximize in- dividual fitness. Meanwhile, individuals in heterogeneous/semelparous populations with the option to group evolve to be less vigilant than either of the other two treatments. Error bars indicate bootstrapped 95% confidence intervals over 100 replicates; some error bars are too small to be visible. of the commons [50, 51], where selfish actions that provide an individual short-term benefit lead to a decrease in overall group fitness. To explore this apparent tragedy of the commons scenario further, we directly compare vigilance and fitness values from the forced and optional grouping experiments alongside a third experiment where we force the population to forage and survive as individuals. Figure 4 shows that when given the choice to group in the homogeneous treatments, prey behavior closely mirrors the behavior observed when forced to forage in a group. This observation confirms the previous suggestion that collective vigilance in homogeneous groups provides a fitness benefit that positively selects for gregarious foraging behaviors. In contrast to the homogeneous populations, heterogeneous populations are much less likely to evolve gregarious foraging behaviors. Heterogeneous/iteroparous populations never 14 Tragedy of the commons in heterogeneous groups 5 10 25 50 Group Size 0.0 0.2 0.4 0.6 0.8 1.0 Vigilance Heterogeneous, Iteroparous 5 10 25 50 Group Size 0.0 0.2 0.4 0.6 0.8 1.0 Vigilance Heterogeneous, Semelparous 5 10 25 50 Group Size 0.0 0.2 0.4 0.6 0.8 1.0 Vigilance Homogeneous, Iteroparous 5 10 25 50 Group Size 0.0 0.2 0.4 0.6 0.8 1.0 Vigilance Homogeneous, Semelparous Individual Grouped Optional