Size constraints and sensory adaptations affect mosaic brain evolution in paper wasps (Vespidae: Epiponini)

Abstract

Body size limits brain volume, but size may also differentially constrain the volumes of brain regions. Size variation and compartmentalized brains of swarm-founding paper wasps (Epiponini) make them good models for the study of size-related brain allometry. We analysed the relative volumes of brain regions that process different sensory inputs: the optic lobes (OL, vision), the antennal lobes (AL, chemosensation) and insect forebrain neuropils (MB, mushroom bodies). Species varied 25-fold in head capsule volume. Total brain volume increased with body size. This brain size increase was a non-linear decelerating function of size: relative brain volume was highest for the smallest-bodied species. In contrast, relative MB and AL volumes declined sharply at the smallest wasp body sizes, but relative OL size did not. Different brain regions showed distinct size-related allometries among Epiponinae. Such brain allometry differences should be accounted for in comparative analyses of brain evolution. However, there was significant deviation from size-typical brain architecture in the nocturnal genus Apoica: A. pallens had reduced relative investment in OL but greater relative investment in MB visual processing regions (calyx collars). This suggests that body size effects on brain region investment can be overcome by selection on particular sensory systems when species evolve to occupy novel sensory environments.