One of the most fundamental antipredator strategies is to live in large groups, as group life confers a suite of potential antipredator benefits. The most basic benefit, obtained by what is called the dilution effect, is a simple statistical property. If a predator is going to strike at one member of a group of size N, then the odds that any particular individual will be its victim is 1/N, which decrease with group size. The larger the group size, the safer is each individual in the group. Individuals living in groups also obtain benefits with respect to antipredator behaviors because as group size increases, there are more and more individuals vigilant for predators, making all group members safer. This has been dubbed the 'many eyes' benefit of group life. One additional benefit of 'many eyes' is that each group member can spend less time being vigilant (and hence more time doing other activities), since vigilance behavior can be parceled across more individuals as group size increases.
Large groups also provide positional benefits. In large groups, individuals in the center of the group are often safer than those on the periphery (which will often lead to a scramble among prey to get positioned at the center of their group). Living in groups also provides other benefits with respect to safety. Groups can often respond to potential predation threats in coordinated ways which solitary individuals cannot, and the activity levels in large groups may overload a predator's sensory input mechanisms, making a successful strike less likely.
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