Causes vs Benefits in the Evolution of Prey Grouping

The presence of predators alters the evolutionary pressures acting on prey populations, often driving them to engage in new behaviors in order to avoid being the target of an attack. One common antipredator behavior is group formation, which can reduce the odds of any individual prey being the target of a given attack, typically scaling inversely with the number of prey in the group. This “dilution effect” is often hypothesized to be the primary driver of prey group formation as an anti-predator strategy. However, groups may help with predator avoidance in other ways as well. For example, prey behaviors or physical characteristics that visually confuse predators may reduce their ability to target an individual and make a kill. Indeed, some have suggested that the “predator confusion” effect alone is sufficient to drive the evolution of grouping in prey. Here we examine coevolving populations of predators and prey using the Avida digital evolution platform. We evaluate the relative importance of these two potential drivers of the evolution of prey grouping and show that the dilution effect, an inherent property of most prey groups, readily creates the pressures necessary for the evolution of prey grouping. In contrast, we found no evidence that predator confusion plays a significant role in prey group formation. Instead, the dilution effect alone is indicated as the primary driver of antipredator prey grouping strategies.