Evidence for the dilution effect in the selfish herd from fish predation on a marine insect

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It has been proposed that a major factor underlying the evolution of gregarious behaviour is a reduction in the risk of being attacked by predators1–3. One way in which individuals may gain protection from predators by joining a group is through a simple ‘dilution’ effect—for any one predator attack, the larger the group of prey animals, the smaller is the chance that any particular individual will be the victim. We present here field observations of predation on a marine insect, in which it is possible, for the first time, to quantify the magnitude of the dilution effect and distinguish it from other benefits of group living.

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Author information


  1. Department of Zoology, University of Cambridge, Cambridge CB2 3EJ, UK

    • W. A. Foster
    •  & J. E. Treherne


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