Intraspecific defense: advantage of social cooperation among paper wasp foundresses.

Foundress associations and high frequencies of conspecific nest usurpation are most common where densities of Polistes metricus are high. Here nest usurpation occurs primarily in single-foundress colonies resulting in multiple-foundress colonies having significantly greater productivities than single-foundress colonies. This is not true at low densities. Conspecific pressures and not predation or parasitism provide an advantage to cooperating wasp foundresses in P. metricus.