Ontogeny of energy use in harvester ant colonies, and the metabolic expense of colony growth

In animals, metabolic rates during ontogeny often scale differently from the way they do in cross-species or population comparisons, with near-isometric scaling patterns more often observed during juvenile growth. In multiple social insect taxa, colony metabolic rate scales hypometrically across species or populations at the same developmental stage, but metabolic patterns during ontogeny have not been examined for any social insect species. We performed the first ontogenetic study of social metabolic scaling in harvester ant colonies (Pogonomyrmex californicus) over 3.5 years as they grew from 52 [Formula: see text] 12 to 767 [Formula: see text] 380 workers. Our data reveal iso- and hypermetric metabolic scaling during the first 20 months of growth, transitioning to hypometric scaling (scaling exponent = 0.93) thereafter. We discovered that the fraction of colony as brood has dual effects on colonial metabolic rate. A higher fraction of the colony that is brood decreases colonial metabolic rate because brood has approximately half the mass-specific metabolic rates of adult ants. Conversely, metabolic rate and activity of adult ants increase as this fraction increases. We further developed a nonlinear composition model, which shows that the maximal colony metabolic rate occurs when 29% of the colony mass is brood, suggesting that demographic changes and colony size may interact to drive the metabolic scaling.