Ants are eusocial insects and as such variations in individual abilities are organized based on what benefits the colony as a whole rather than the individual. The authors hypothesized that this colony-level selection may lead to different brain sizes in different castes of ant workers, depending on the cognitive demands placed on them by the function they perform within the colony. The authors compared total brain size against body size for 109 army ant workers and 39 soldiers across eight species and subspecies of Eciton. Examining the ants’ antennal lobes, which receive olfactory information, and their mushroom bodies, higher brain centers involved in learning and memory, the authors also investigated if brain architecture differed between workers and soldiers. They found that although soldiers were larger than workers, their total brain size was not significantly different. They also had relatively smaller antennal lobes and smaller mushroom bodies.
The findings suggest that as brain tissue development and maintenance is costly to a single organism as well as to the ants‘ colonies as a whole, natural selection at the colony level favors reduced investment in brain tissue in soldiers which deal with fewer cognitive demands than other workers. The authors also found that soldiers have relatively large muscles attached to their mandibles—which are used in fighting off attackers—which suggests that brain investment may trade off against muscle development in different kinds of ant workers.