Attack Abatement: A Model for Group Protection by Combined Avoidance and Dilution

An evolutionarily-stable-strategy model for the antipredator advantages of group living in animals is described and termed here the attack-abatement effect. The model considers the possible benefits to the individual of joining or not joining a group and is based on the relative fitnesses of these competing strategies in the same population. It is shown that neither avoidance of contact with the predator nor dilution of the effects of its attack once encountered is advantageous on its own, but in combination they reduce the risk to grouped prey. This is so even in the absence of predation at the margins of the group (as is assumed in Hamilton's selfish-herd model) and without the need for active group defense, evasion measures, or confusion of the predator. It is suggested that avoidance and dilution effects of group living cannot be considered separately, but should be regarded as effective only when in combination (the abatement effect).