Conservation of sensory pathways implies a localized change in the mushroom bodies is associated with cognitive evolution in Heliconius butterflies

Evolution of novel behavior is reflected in changes in sensory investment or integration, but the exact nature of these changes is often unclear. The Neotropical butterfly tribe, Heliconiini, offer an attractive system for studying how behavioral evolution is facilitated by changes in the neural system. Within the Heliconiini tribe, the genus Heliconius possess fourfold larger mushroom bodies, the insect learning and memory center, than closely related Heliconiini. Mushroom body expansion in Heliconius co-occurred with a dietary innovation, and is associated with systematic spatial foraging and extended lifespan. Heliconius' foraging relies on visual scene memories and, indeed, Heliconius have stable visual long-term memory, and evidence of visual specialization in the mushroom bodies. Here, we explore how vision-specific neuroanatomical and behavioral enhancement in Heliconius impacts sensory pathways upstream of the mushroom bodies by assessing investment across the eyes, sensory neuropils, and projection pathways. Despite evidence of refinement in visually based behavior, we found no increased investment in visual structures, brain areas, or pathways. This suggests that the rapid expansion of the Heliconius mushroom body occurred in a context of conserved detection and processing of visual cues, and that a localized shift within integrative brain centers facilitated the evolution of Heliconius' novel behaviors.