An Experimental Test of Colony Size Effects on Mass‐Specific Metabolic Rate in a Social Insect

The metabolic costs of being an organism generally scale hypometrically, such that per‐gram metabolic rates decrease with increasing size. The drivers of this almost universal feature have yet to be clearly identified. Cohesive social groups, particularly the social insects, also show hypometric scaling with colony size. It is not known whether size is a causal factor or whether this relationship is only a correlation. We experimentally reduced the size of harvester ant colonies ( Pogonomyrmex californicus ), to test whether colony size is causally responsible for large colonies utilizing less energy per gram. We identified three potential parameters, all related to the organization of the work, that are candidate mechanisms for driving hypometric scaling of metabolism. Larger colonies had lower median (but not average) walking speeds, and more ants classified as stationary, potentially reducing locomotory costs. Larger colonies had reduced mass‐specific perimeters of the brood pile, consistent with reduced per‐capita brood rearing, a potentially expensive task. Finally, larger colonies had reduced network density and centrality, consistent with greater division of labor, specialization, and segregation of individuals, which may reduce metabolic costs by improving efficiency. A simulation model suggests that simple behavioral rules may cause reduced network densities that could reduce colonial metabolic rates by promoting specialization in task performance and activity levels. Support or Funding Information This research has been supported by Providence College, the James S. McDonnell Foundation, and the National Science Foundation under Grant No. 1110796.