model), semelparous prey in our model tend to devote most of their time to watching for predators. When given the opportunity to group with other prey and take advantage of col- lective vigilance, semelparous prey are actually able to spend less time being vigilant. Thus, when semelparous prey evolve lower levels of vigilance in larger groups, we are observing the effect of collective vigilance. Although our results suggest that the risk-averseness of semelparity induces semelparous prey to evolve to take advantage of collective vigilance, this selective pressure does not ap- pear to be as strong as the pressure we observed in homogeneous groups. Proof of this observation can be found in the heterogeneous/semelparous treatment, where most group members attempt to cheat their way into collective vigilance by evolving lower levels of vigi- lance behavior than is observed in populations where prey are either forced to forage on their own or in the group (Figure 4). Ultimately, this selfish behavior results in lower fitness than the fitness of prey that are forced to forage in groups (Figure 5), but the constantly-present, short-term benefits of selfishness appear to be too enticing to allow a more advantageous, cooperative behavior to emerge. The breakdown of cooperation in the heterogeneous/semelparous populations suggests that the populations are succumbing to a tragedy of the commons [50, 51]. In our experi- ments, all prey are competing against each other to forage as much food as possible without being captured by the predator. However, because there is an unlimited amount of food, the only depletable group resource is vigilance, which protects the entire group from the preda- tor. As the resulting non-cooperative behavior in the heterogeneous/iteroparous populations demonstrate, absent any major selective pressures for collective vigilance, prey will evolve to selfishly forage 100% of the time. Therefore, group homogeneity and semelparity correspond to two previously-established mechanisms for preventing a tragedy of the commons, namely kin selection and punishment for non-cooperative behaviors, respectively [50]. The relative efficacy of these mechanisms to prevent cheating merits further investigation. For example, does group homogeneity play a larger role than reproductive strategy in the evolution of 18