Influence of the social context on division of labor in ant foundress associations

Abstract

Previous studies indicate that division of labor can arise spontaneously in social groups. The comparison between normally social populations and forced associations of solitary individuals allows us to dissect the mechanisms by which tasks are distributed within a group and to ask how selection acts on division of labor during the incipient stages of sociality. In some ant species, newly mated queens form cooperative associations during nest initiation, in which individuals specialize on different tasks. The harvester ant Pogonomyrmex californicus shows geographical variation across populations in colony-founding strategies: solitary founding (haplometrosis) and group founding (pleometrosis). This system provides a unique opportunity to investigate how social context affects division of labor during social evolution. We created groups containing normally solitary, normally group founding, or mixed groups of solitary and social queens to examine how social phenotype affects division of labor. We also examined how group size affects task specialization by comparing pairs of queens with groups of 6 queens. Division of labor arose consistently across all associations. Groups of haplometrotic or pleometrotic queens differentiated into an excavation and a brood care specialist. In mixed groups, the haplometrotic queens took the role of excavator whereas the pleometrotic queens mainly tended brood. Our data also show that the intensity of specialization was greater in larger associations, consistent with current models of group size and division of labor. We discuss these data in the context of how emergence and selection act on the evolution of division of labor within incipient social groups.