Task-dependent effects of colony size on behavioral specialization and brain anatomy in acacia ants (Pseudomyrmex spinicola)

Colony size has been proposed to increase task-specialization among workers in social animals (task-specialization hypothesis), but life in groups could also impose higher cognitive skills (social brain hypothesis), which in turn affect allocation of neural tissue to different brain regions. The social brain hypothesis predicts that integration centers in the brain should get larger with colony size, while the task-specialization hypothesis predicts an increase in the relative size only of regions relevant to the performed tasks. Here, we studied whether colony size affected task-specialization and brain anatomy of acacia ant workers. We color-marked ants: (1) foraging (leaf-ants); and (2) patrolling and defending the trunk (trunk-ants) on the host tree. Task-specialization was estimated as the proportion of marked ants that were working in the same tasks than where they were originally marked. We assessed whether task-specialization affected the response towards food (storing or discarding) of ants working in different tasks, and studied their brain anatomy (volume of neuropiles). We found that task-specialization positively correlated with colony size, at a higher rate for defense than for foraging. Trunk-ants were less likely to discard food in colonies with higher foraging specialization. We also observed changes in the brain anatomy: as colony size increased, the relative volume of integration centers in the brain (mushroom bodies) decreased in trunk-ants but increased in leaf-ants. These results suggest that trunk-ants in larger colonies may have impaired associative learning. Overall, our results support the task-specialization hypothesis for brain plasticity in social animals.