Sensory weighting reflects changing patterns of visual investment during ecological divergence in Heliconius butterflies

Integrating information across sensory modalities enables animals to orchestrate a wide range of complex behaviours. The relative importance placed on one sensory modality over another reflects the reliability of cues in a particular environment and corresponding differences in neural investment. As populations diverge across environmental gradients, the reliability of sensory cues may shift, favouring divergence in neural investment and sensory weighting. During their divergence across closed forest and forest-edge habitats, Heliconius butterflies H. cydno and H. melpomene evolved distinct brain morphologies, with the former investing more in vision. Molecular and anatomical data suggest selection drove these changes, but their behavioural effects remain uncertain. We hypothesised that divergent investment in neuropils may alter sensory weighting during behavioural tasks. To address this, we trained individuals in an associative learning experiment using multimodal colour and odour cues. When positively rewarded stimuli were presented in conflict pairing positively trained colour with negatively trained odour, and vice-versa , H. cydno prioritised visual cues more strongly than H. melpomene . Hence, differences in sensory weighting may evolve early during divergence and are potentially predicted by patterns of neural investment. These findings, alongside other ecological divergence examples, imply that differences in sensory weighting stem from sensory investment as adaptations to local sensory environments.