Shifting balances in the weighting of sensory modalities are predicted by divergence in brain morphology in incipient species of Heliconius butterflies

Integrating and weighting sensory perception across modalities is crucial to how animals adapt to their environment. Divergence in brain structure is often in sensory processing regions, suggesting investment reflects the needs of species specific ecologies. Here, we use two parapatric, closely related species, Heliconius erato cyrbia and H. himera, to test the hypothesis that divergence in sensory brain regions affects foraging decisions. These butterflies are isolated across an ecological gradient, which is linked to differences in brain morphology, with H. e. cyrbia investing more in visual centres, and H. himera investing in olfactory centres. Here, we demonstrate that these two species vary in how they associate visual and olfactory cues with positive food rewards. We show that when individuals are trained on paired olfactory and visual stimuli, and are then presented with these stimuli in conflict, they show distinct behavioural responses. H. himera is more likely to favour positive olfactory cues than H. e. cyrbia, which favours visual cues regardless of the paired stimulus. This suggests that these species have diverged in the emphasis placed on these different sensory domains during foraging, in a way which is consistent with observed differences in brain morphology. This result strengthens evidence that speciation initiated by local adaptation is partly facilitated by changes in the neural basis of key behavioural functions.