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Which strategy is preferred depends on how costs scale with the level of individual vigilance: Many-eyes strategies are preferred where costs of vigilance rise gently at low levels but become steeper at higher levels (convex; e.g., an open field); sentinel strategies are preferred where costs of vigilance rise steeply at low levels and then flatten out (concave; e.g., environments with vantage points). This same dichotomy emerges whether individuals act selfishly to optimize their own fitness or cooperatively to optimize group fitness. 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