Colony founding in Myrmecocystus mimicus wheeler (Hymenoptera: Formicidae) and the evolution of foundress associations

1. Field studies of the honey ant Myrmecocystus mimicus have revealed that colonies are often founded by groups of foundresses ranging in size from 2-9 females, with groups of 2-4 females being most common. Founding nests are also aggregated together in patches which are distant from existing M. minicus colonies. 2. Laboratory experiments have shown that colony founding involves inter-colony raiding: the brood from neighboring incipient colonies is transported by workers from one nest to another. Ultimately all of the brood ends up in a single nest chamber which the workers adopt as their own. Workers frequently abandon their mothers in favor of these nests. We conclude that competition among founding nests favors foundress associations and that inter-colony raiding is a natural consequence of aggregated pleometrotic founding nests. 3. A general theory of the evolution of foundress associations is developed which describes the conditions under which foundress associations will be favored. It is shown that the evolution of foundress groups of particular sizes depends on the shape of the colony productivity function. Increasing relatedness among foundresses will generally result in a larger optimum foundress group size. Production data on foundress groups of different sizes gathered in the laboratory on M. mimicus are analyzed using the theory developed. It is shown that the size of foundress associations expected on the basis of the theory is very near the value observed in nature.