Weighting of sensory cues 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 the weight given to different sensory modalities. During
their divergence across closed-forest and forest-edge habitats, closely
related butterflies Heliconius cydno and H. melpomene evolved distinct
brain morphologies, with the former investing more in vision. Quantitative
genetic analyses suggest selection drove these changes, but their
behavioural effects remain uncertain. We hypothesised that divergent
neural investment may alter sensory weighting. 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 favoured visual cues more strongly than
H. melpomene. Hence, differences in sensory weighting may evolve early
during divergence and are predicted by patterns of neural investment.
These findings, alongside other examples, imply that differences in
sensory weighting stem from divergent investment as adaptations to local
sensory environments.