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

Evolution of novel behaviour 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 behavioural evolution is facilitated by changes in the neural system. Within the Heliconiini tribe, the genus Heliconius possess 4-fold larger mushroom bodies, the insect learning and memory centre, 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 specialisation in the mushroom bodies. Here, we explore how vision-specific neuroanatomical and behavioural 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 behaviour, 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 localised shift within integrative brain centres facilitated the evolution of Heliconius’ novel behaviours.