Brain investment under colony-level selection: soldier specialization in Eciton army ants (Formicidae: Dorylinae)

Abstract

BackgroundIn eusocial animal colonies, individuals from different castes often play divergent behavioral roles and confront distinct cognitive demands. Neuroecology theory predicts variation in cognitive demands will correspond to differences in brain investment because brain tissue is energetically expensive. We hypothesized colony-level selection for efficient energy allocation will favor reduced brain investment for castes with reduced cognitive demands. Neotropical army ants (genus Eciton) have morphologically-based worker castes; the specialized soldiers have reduced behavioral repertoires. We asked whether Eciton worker brain size and brain architecture varied with body size, and predicted the specialized soldier caste would have reduced investment in brain tissue.ResultsEciton brain size generally increased with body size, but relative brain size (brain size/body size ratio) decreased sharply with body size. Soldiers were the largest-bodied workers, but their total brain volume overlapped extensively with other workers and was small relative to body size. Furthermore, soldier chemosensory antennal lobes and central-processing mushroom bodies were smaller relative to their brain size.ConclusionsThese patterns suggest colony-level selection on brain investment efficiency has led to adaptive adjustments in brain tissue allocation among Eciton worker castes, with reduced brain investment in the behaviorally specialized soldiers.