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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- 

