Phylogeny not mimicry drives chemical variation in tropical aposematic butterflies

Abstract

Chemical signals serve a vast array of functions for both intra- and interspecific communication and defence. Chemically defended, aposematic insects will advertise their noxiousness to predators with warning signals and it has been suggested that volatile compounds shared between species could send a common signal to deter avian predators, similar to colour pattern mimicry. This is exemplified in the bright, mimetic warning colours of toxic heliconiine and ithomiine butterflies. Besides defence, males in these groups also use chemicals to communicate with potential female mates and male competitors. Here, we characterise chemical profiles of male Heliconiini and Ithomiini species from a range of mimicry rings. We find that chemical distances between species are not associated with mimicry and instead show a strong phylogenetic signal. In addition, we show that the distribution of specific shared compounds does not correlate with mimicry. We propose chemical signals in these groups are primarily used for intraspecific communication and highlight the potential selective mechanisms driving chemical profile evolution, such as selection for species-specific pheromone concentrations and collective male deterrent compounds. Our findings imply that chemical profile variation among butterfly species is the product of shared ancestry. However, more research is required on the behavioural significance of compounds among butterflies and predator perception of olfactory warning signals to identify which selective pressures are exerted on specific chemical traits.