The metabolic costs of building ant colonies from variably sized subunits

Ant colonies are superorganisms with emergent traits that, for some species, reflect the combined activity of physically distinct worker castes. Although larger castes have high production costs, they are thought to save their colonies energy by efficiently performing specialized tasks. However, because workers are generally idle until sensing specific stimuli, their maintenance costs may be an important component of colony-level investment. I used metabolic scaling to examine the maintenance costs of dimorphic major and minor Pheidole castes across levels of colony organization (e.g., individual, group, and colony). Majors from three species had lower mass-specific metabolic rates than minors because of allometries at both individual and group levels and subsequently lived longer when starved. Thus, large major castes may offset their production costs in both their idle and active states. The slope scaling metabolic rate from incipient to reproductive colonies of Pheidole dentata (∼colony mass^0.89) fell between the slopes for minor groups (∼group mass^1.04) and major groups (∼group mass^0.79) and appears to reflect developmental shifts in subunit mass and number and their offsetting effects on per capita energy demands. These results highlight how metabolic scaling may help visualize the energetic correlates of emergent behavior and unravel the mechanisms governing colony organization.