Divergent evolution of colony-level metabolic scaling in ants

Metabolic scaling-the relationship between organismal metabolic rate (R) and body mass (M)-is an important property of life. In general, this relationship has been summarized by the scaling function, R = aM^b. Both the scaling elevation (a) and the scaling exponent (b) have been shown to diverge among taxa and ecological groups. However, it is unclear whether this ecological divergence observed in unitary organisms also occurs at higher levels of biological organization, such as eusocial colonies. We used the published literature to assemble the estimates of the metabolic rate of active colonies and their mass for 51 species of ants, along with three ecologically important traits with available data: trophic level (herbivorous to predaceous), foraging coordination level (solitary to trunk trail) and caste polymorphism (polymorphic vs. monomorphic). Interspecific colony metabolic scaling was steeper (higher b) in species occupying higher trophic levels and in species with polymorphic versus monomorphic workers. Species occupying higher trophic levels also had a higher metabolic level (higher a). These findings are consistent with divergent selection on colony-level metabolic scaling. We conclude that the ecological dependence of metabolic scaling has evolved across levels of biological organization and should be explicitly considered by both metabolic and social evolution theories.