Neural variation in the worker subcastes of the carpenter ant Camponotus rufipes: brain volume, antennal lobe development, and implications for labor division

Abstract The ant Camponotus rufipes is facultatively polygynous and has polymorphic castes, in which the worker is composed of two subcastes: the major worker and the minor worker. This study evaluated the whole and relative volumes of the antennal lobe, optic lobe, and calyx of the mushroom body in the subcastes of C. rufipes . The antennal lobe of minor workers is composed of circular glomeruli with smaller volume and little variation in size, while those of larger workers have larger, irregular shapes, larger volume, and greater variation in size. The whole brain volume of C. rufipes was larger in major workers than in minor workers, as was the relative volume of the antennal lobe. However, the relative volume of the optic lobe and mushroom body were similar between minor and major workers. The larger brain volume of the major worker may reflect greater neural processing capacity or the bigger head. The larger antennal lobe in major workers may be due to the number of glomeruli present. None of the neuropils observed were proportionally larger among the C. rufipes subcastes, indicating that the low investment in proportional volumes of the brain regions in larger organs may be a way to avoid excessive energy expenditure. The results indicate that maintaining a similar brain profile may be associated with a flexible division of labor, enabling both subcastes to meet a variety of sensory and cognitive demands, regardless of body size.