Worker brain development and colony organization in ants: does division of labor influence neuroplasticity?

Brain compartment size allometries may adaptively reflect cognitive needs associated with behavioral development and ecology. Ants provide an informative system to study the relationship of neural architecture and development because worker tasks and sensory inputs may change with age. Additionally, tasks may be divided among morphologically and behaviorally differentiated worker groups (subcastes), reducing repertoire size through specialization and aligning brain structure with task-specific cognitive requirements. We hypothesized that division of labor may decrease developmental neuroplasticity in workers due to the apparently limited behavioral flexibility associated with task specialization. To test this hypothesis, we compared macroscopic and cellular neuroanatomy in two ant sister clades with striking contrasts in worker morphological differentiation and colony-level social organization: Oecophylla smaragdina, a socially complex species with large colonies and behaviorally distinct dimorphic workers, and Formica subsericea, a socially basic species with small colonies containing monomorphic workers. We quantified volumes of functionally distinct brain compartments in newly eclosed and mature workers and measured the effects of visual experience on synaptic complex (microglomeruli) organization in the mushroom bodies—regions of higher-order sensory integration—to determine the extent of experience-dependent neuroplasticity. We demonstrate that, contrary to our hypothesis, O. smaragdina workers have significant age-related volume increases and synaptic reorganization in the mushroom bodies, whereas F. subsericea workers have reduced age-related neuroplasticity. We also found no visual experience-dependent synaptic reorganization in either species. Our findings thus suggest that changes in the mushroom body with age are associated with division of labor, and therefore social complexity, in ants.