Parallel shifts in flight-height associated with altitude across incipient Heliconius species

Vertical gradients in microclimate, resource availability and interspecific interactions are thought to underly stratification patterns in tropical insect communities. However, only a few studies have explored the adaptive significance of vertical space use during early population divergence. We analysed flight-height variation across speciation events in Heliconius butterflies representing parallel instances of divergence between low and high-altitude populations. We measured flight-height in wild H. erato venus and H. chestertonii, lowland and mountain specialists respectively, and found that H. chestertonii consistently flies at a lower height. We compared these data with previously published results for H. e. cyrbia and H. himera, the latter of which flies lower and, like H. chestertonii, recently colonised high-altitude, dry forests. We show that these repeated trends largely result from shared patterns of selection across equivalent environments, producing parallel trait-shifts in H. himera and H. chestertonii. Although our results imply a signature of local adaptation, we did not find an association between resource distribution and flight-height in H. e. venus and H. chestertonii. We discuss how this pattern may be explained by variation in forest structure and microclimate. Overall, our findings underscore the importance of behavioural adjustments during early divergence mediated by altitude-shifts.