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

Acquisition of novel behaviour is often reflected in changes in sensory investment or integration, but the exact nature of these changes is often unclear. Within the Neotropical butterfly tribe, Heliconiini, the genus Heliconius possess 4-fold larger mushroom bodies, an insect learning and memory centre, than closely related outgroup Heliconiini. Mushroom body expansion in Heliconius co-occurred with a dietary innovation, and is associated with systematic spatial foraging and extended lifespans. Heliconiini therefore offer an attractive system for studying how the evolution of behaviour is facilitated by changes in neural systems. Heliconius' foraging likely relies on visual scene memories. Indeed, Heliconius have more 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 the degree of investment across the eyes, sensory structures and projection pathways. Despite evidence of refinement in visual-based behaviour, we found no increased investment in visual structures, brain areas and 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 a largely localised shift within integrative brain centres facilitated the evolution of Heliconius' novel behaviours.