Re-examining safety in numbers: interactions between risk dilution and collective detection depend upon predator targeting behaviour

Many studies document that individuals visually scan for predators less frequently when in the safety of larger groups. This widely replicated effect has generally been explained in terms of distinct predator detection and risk-dilution effects. We show that a strict distinction between detection and dilution disappears when information about attacks is imperfectly shared (as it is in reality). Furthermore, dilution and detection effects change depending on when during an attack the predator targets a particular prey individual for pursuit. Realistic detection and dilution effects probably interact with each other and also with the targeting behaviour of predators. Instead of considering detection and dilution effects on vigilance, it may be more profitable to consider each prey's probability of being targeted during an attack and its probability of escaping if attacked. This perspective emphasizes that a full understanding of safety in numbers requires an understanding of predator targeting strategies.