Metabolic scaling and early colony growth in harvester ants

Colony metabolic scaling provides a window into how costs and potential savings change for social insect colonies as they grow. Diverse studies have found a frequent pattern in which colony per-gram metabolic rates are lower for larger colonies, known as hypometric scaling. We investigate this large scale pattern by closer examination of how colony metabolism changes ontogenetically from very early colony growth through the first years of survival for harvester ant colonies, Pogonomyrmex californicus. Instead of a smooth change in metabolism, we find a segmented change with a breakpoint. In the early growth phase, colony metabolism actually scales isometrically with size. Later, however, it moves towards a hypometric pattern. To understand this shift, we take a closer look at colony demography, especially brood production, and at task allocation to the priorities of brood care versus colony maintenance. Our collective data suggest that the intensive early investment colonies make towards producing new workers is costly, but also necessary for early colony success, and that as colonies age, they increase relative investment toward lower-cost maintenance tasks. Supported by NSF IOS Award 558127 to JHF and JFH.