Divergence in Heliconius flight behaviour is associated with local adaptation to different forest structures

Micro-habitat choice plays a major role in shaping local patterns of biodiversity. In butterflies, stratification in flight height has an important role in maintaining community diversity. The speciation in Heliconius butterflies is often associated with strong assortative mating, but ecological isolation and local adaptation is also considered essential. Despite its presumed importance, the role of behavioural shifts in early stages of speciation in response to differences in habitat structure is yet to be established. Here, we investigated variation in flight height behaviour in two closely related species, H. erato cyrbia and H. himera, which produce viable hybrids but are isolated across an environmental gradient, spanning lowland wet forest to high altitude scrub forest. We show that the two species fly at different heights in the wild, and demonstrate that this can be explained by differences in the vertical distribution of plant resources. We subsequently explored whether this divergence in flight height has a genetic component using common garden experiments. In both the wild and captivity, H. himera choose to fly lower and feed at lower positions, mirroring differences in resource availability in the wild. We suggest that this shift in foraging behaviour may reflect local adaptation to divergent forest structures highlighting the role of behaviour during early stages of speciation.