Evolving an optimal group size in groups of prey under predation

Considerable progress has been made in understanding the evolutionary forces underlying animal group-living behavior. Even so, Krause and Ruxton (2002) identified optimal group size as an under-researched area characterized by unwieldy large-predator study systems and simulations based on group rather than individual decision-making. Therefore, we present a simple, flexible simulation of foraging and predation that demonstrates that the evolution of an optimal, evolutionarily stable group size is in fact possible. Prey genomes are evolved in subpopulations, each with a coevolved group size factor. These groups can be either heterogeneous or homogeneous. Genome fitness is determined by placing the subpopulation in the predator simulation used in Olson et al. (2014). Fitness-proportionate selection first acts on entire subpopulations (where a subpopulation’s fitness is the mean fitness of its genomes) and then on genomes within each subpopulation (if the subpopulation is heterogeneous). Group size can also mutate between generations, with selection again choosing candidates for removal when a group shrinks and reproduction when it grows. To study the potential disadvantages of living in large groups (e.g., competition for mates and resources), we apply a grouping penalty to foraging prey proportionate to the size of the group. This penalty is described by the equation: