Confusion of predators does not rely on specialist coordinated behavior

Antipredatory benefits are generally considered important in the evolution and maintenance of animal aggregations. One such benefit is the confusion effect: the reduced ease of prey capture experienced by some predators resulting from an inability to single out and attack an individual prey from a group as a result of cognitive or sensory limitations. Although widely cited, empirical data that do any more than demonstrate the effect are sparse. Here, we use the artificial system of humans attempting to "capture" images on a computer screen using a computer mouse to explore several hypotheses on the properties of the confusion effect. This system has the advantage that we can control the behavior of the prey and eliminate the risk of confounding factors due to differential prey behavior and/or phenotypes in groups of different sizes. One important result of our study is the demonstration that the confusion effect can occur in the absence of these confounding factors and indeed in the absence of complex coordinated behavior between individuals in the prey group (such as are commonly observed in schooling fish). We also demonstrate for the first time that an individual prey item can still benefit from the confusion effect if it is only loosely associated in space with a larger group of similar prey. Both these results suggest that the confusion effect can arise under less specialist circumstances than previously realized, and so the importance of this mechanism in shaping aggregation by prey and predator--prey interactions may be substantially greater than previously considered. Copyright 2007, Oxford University Press.