Individual animal behavior has the potential to influence the probability of mortality regardless of the evolutionary prediction of the species' survivorship curve. For example, dominance hierarchies or kin selection may improve the survival and fitness of a few individuals. Mortality is reduced in other species by living in groups. Through this behavior, group membership will have increased vigilance for predators (the 'many eyes' hypothesis). Additionally, if group vigilance is higher, single individuals can spend more time feeding (thereby increasing their physiological fitness) rather than scanning for predators. However, as group size increases, aggression over scarce resources will define the upper group size observed. Consequently, an 'optimal group size' exists where mortality is lower than in very small or very large groups (Figure 2).
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