Hot habitats support light-colored butterfly communities in a tropical rainforest
Ectotherm morphology is a determining factor in thermal adaptation in that it mediates the relationship between body temperature and the environment. For example, previous studies have found that wing darkness of butterflies is positively correlated with environmental temperature across large scale environmental gradients such as latitude and altitude. This is because butterflies with darker wings might gain a thermal advantage in cool environments, by absorbing heat more efficiently, but could be susceptible to overheating in hot environments. At a smaller scale, the color of butterfly communities in different habitats would also be expected to vary due to different thermal regimes between habitats. In this study, we examined darkness variation within butterfly communities across hot and cool habitats in the tropical woodland-rainforest ecosystems of Far North Queensland, Australia. Using a novel thermal imaging technique, we documented the absorption of solar radiation relative to color variation and built a phylogenetic tree based on available sequences to analyze the effects of habitat on phylogenetic and specific components of color darkness.
For all the individuals we examined (n=410), the color darkness of the butterfly assemblage was significantly darker in closed rainforests compared to the assemblage of immediately adjacent open woodlands (p<0.01). These closed rainforests were on average 2.9±0.3°C lower than that of the open woodlands which is just as steep as that created by the broader latitudinal or altitudinal gradient and suggests that color prevalence in habitats is linked to its thermal regime. Our thermal experiments support our field observations as the body temperature of dark butterflies increased significantly faster than light butterflies under standardized sun conditions (p<0.01). Importantly, the phylogenetic and specific components for the ventral wings of butterflies (p<0.01) lends additional support for dark morphologies evolving in cool habitats. Our results show a thermal consequence of butterfly morphology and how it could affect the distribution of butterflies across habitats in the tropics. This morphological pattern along thermal gradients will help us to predict how butterfly distributions might shift with climate change and how thermal refugia might emerge at microclimatic and habitat scales.