Evolution of neurometabolic frugality in harvester ants

Despite the importance of neurometabolic cost in brain size evolution, quantitative data on brain metabolic rates (MRs) are lacking. We measured ex vivo brain MRs among species of the ant genus Pogonomyrmex to test three hypotheses: (i) more socially complex species have higher brain MRs; (ii) smaller-bodied species have higher brain mass-specific metabolic rates (MSMRs); and (iii) more recently derived species have evolved more metabolically 'frugal' brains. Worker body size and colony size, a proxy for social complexity vary significantly among Pogonomyrmex species and were positively correlated. Species with larger colonies did not have larger brains or higher brain MRs. Species with smaller workers had relatively larger brains and higher brain MSMR: 38% of resting MR was attributable to brain metabolism in species with the smallest workers, compared to 6% in species with the largest workers. We suggest that smaller worker body sizes may require higher brain mass-specific energetic costs to achieve comparable performance by absolutely smaller brains. More derived species had strikingly lower mass-specific brain metabolic costs, suggesting that increases in worker body size have selected for neurometabolic frugality through reductions in brain MSMR.