Behavioral performance and division of labor influence brain mosaicism in the leafcutter ant Atta cephalotes

Brain evolution is hypothesized to be driven by behavioral selection on neuroarchitecture. We developed a novel metric of relative neuroanatomical investments involved in performing tasks varying in sensorimotor and processing demands across polymorphic task-specialized workers of the leafcutter ant Atta cephalotes and quantified brain size and structure to examine their correlation with our computational approximations. Investment in multisensory and motor integration for task performance was estimated to be greatest for media workers, whose highly diverse repertoire includes leaf-quality discrimination and leaf-harvesting tasks that likely involve demanding sensory and motor processes. Confocal imaging revealed that absolute brain volume increased with worker size and functionally specialized compartmental scaling differed among workers. The mushroom bodies, centers of sensory integration and learning and memory, and the antennal lobes, olfactory input sites, were larger in medias than in minims (gardeners) and significantly larger than in majors ("soldiers"), both of which had lower scores for involvement of olfactory processing in the performance of their characteristic tasks. Minims had a proportionally larger central complex compared to other workers. These results support the hypothesis that variation in task performance influences selection for mosaic brain structure, the independent evolution of proportions of the brain composed of different neuropils.

Division of labour in a lower termite: the majority of tasks are performed by older workers

Division of labour among workers was investigated in the lower termite, Reticulitermes fukienensisWorkers were separated into three age groups based on size, small workers being the youngest, medium workers intermediate and large workers the oldest. Workers were then compared in behavioural assays for the degree to which they would carry out specific tasks, which included: (1) foraging-related tasks; (2) care of eggs, larvae and the queen; and (3) some other important behaviours including burying corpses, alarm-giving and time spent stationary. All tasks were performed by two or all three of the size-groups of workers. Hence evidence does not support the hypothesis of tasks being discretely allocated among different instars in termites and this having evolved towards the extreme of one caste for every task. The oldest workers (i.e. large workers) carried out the highest frequencies of all tasks investigated. This contrasts with the social Hymenoptera, where younger workers specialize in some tasks (especially brood and queen care). The results suggest a new pattern for social insects for division of labour among workers.Copyright 1997 The Association for the Study of Animal Behaviour1997The Association for the Study of Animal Behaviour