Coordination and Synchronization of Vigilance in Groups of Prey: The Role of Collective Detection and Predators' Preference for Stragglers

Prey living in groups often partly rely on companions to detect predators. Accordingly, individuals having vigilant neighbors could decrease their own vigilance, engendering a certain level of vigilance coordination in the group. However, when a predator attacks, individuals that spot it react quicker than individuals that follow them and have less chance of being targeted. Individuals with vigilant neighbors may then be particularly exposed, since they risk lagging behind their companions in an attack. Here, we incorporate this effect in a spatially explicit model of vigilance. A first version of the model derives the evolutionarily stable proportion of time allocated to vigilance for each position in the group. A second version considers the real alternation of feeding and vigilance and allows individuals to respond immediately when their neighbors raise their head or resume feeding. The model confirms then that the collective detection effect tends to coordinate vigilance. However, when predators display marked preferences for stragglers, adaptive response by prey generates waves of collective vigilance that may spread and circulate over the group. The overall level of vigilance in the group strongly oscillates, sometimes far away from the evolutionarily stable values, and the stabilizing effect of vigilance coordination is thwarted. These results illustrate real patterns of vigilance, in particular, the fact that group members often synchronize their vigilance.