Divergence in brain composition during the early stages of ecological specialization in $\textit{Heliconius}$ butterflies

During speciation across ecological gradients, diverging populations are exposed to contrasting sensory and spatial information that present new behavioural and perceptive challenges. These challenges may be met by heritable or environmentally induced changes in brain function which mediate behaviour. However, few studies have investigated patterns of neural divergence at the early stages of speciation, inhibiting our understanding of the relative importance of these processes. Here, we provide a novel case study. The incipient species pair, $\textit{Heliconius erato}$ and $\textit{H. himera}$, are parapatric across an environmental and altitudinal gradient. Despite ongoing gene flow, these species have divergent ecological, behavioural and physiological traits. We demonstrate that these taxa also differ significantly in brain composition, in particular in the relative levels of investment in structures that process sensory information. These differences are not explained solely by environmentally-induced plasticity, but include heritable, nonallometric shifts in brain structure. We suggest these differences reflect divergence to meet the demands of contrasting sensory ecologies. This conclusion would support the hypothesis that the evolution of brain structure and function play an important role in facilitating the emergence of ecologically distinct species.

We thank the Ministerio del Ambiente del Ecuador for permission to collect butterflies. SHM was supported by research fellowships from the Royal Commission for the Exhibition of 1851 and the Leverhulme Trust, a Royal Society Research Grant (RG110466) and a British Ecological Society Early Career Project Grant. RMM was supported by a Junior Research Fellowship at King’s College, Cambridge, and funding from the Bedford Fund. We are also grateful to Chris Jiggins for providing insectary space and Swidbert Ott for discussions on insect brain evolution.