The influence of ultraviolet reflectance differs between conspicuous aposematic signals in neotropical butterflies and poison frogs.

Justin Yeager,James B Barnett

doi:10.1002/ece3.7942

Abstract

Warning signals are often characterized by highly contrasting, distinctive, and memorable colors. Greater chromatic (hue) and achromatic (brightness) contrast have both been found to contribute to greater signal efficacy, making longwave colored signals (e.g., red and yellow), that are perceived by both chromatic and achromatic visual pathways, particularly common. Conversely, shortwave colors (e.g., blue and ultraviolet) do not contribute to luminance perception yet are also commonly found in warning signals. Our understanding of the role of UV in aposematic signals is currently incomplete as UV perception is not universal, and evidence for its utility is at best mixed. We used visual modeling to quantify how UV affects signal contrast in aposematic heliconiian butterflies and poison frogs both of which reflect UV wavelengths, occupy similar habitats, and share similar classes of predators. Previous work on butterflies has found that UV reflectance does not affect predation risk but is involved in mate choice. As the butterflies, but not the frogs, have UV‐sensitive vision, the function of UV reflectance in poison frogs is currently unknown. We found that despite showing up strongly in UV photographs, UV reflectance only appreciably affected visual contrast in the butterflies. As such, these results support the notion that although UV reflectance is associated with intraspecific communication in butterflies, it appears to be nonfunctional in frogs. Consequently, our data highlight that we should be careful when assigning a selection‐based benefit to the presence of UV reflectance.

Highlights

Our contemporary understanding of the evolution of bright and conspicuous color patterns is rooted in the work of the early pioneers of evolutionary themes such as natural selection
We recently described the first example of UV reflectance in poison frogs, from an Ecuadorian population of Oophaga sylvatica
We found that our sample of heliconiian butterflies and poison frogs all reflected detectable quantities of ultraviolet light

Summary

| INTRODUCTION

Our contemporary understanding of the evolution of bright and conspicuous color patterns is rooted in the work of the early pioneers of evolutionary themes such as natural selection. In the Neotropics two independent, and completely unrelated, radiations of bright conspicuous colors have drawn much scientific attention: the heliconiian butterflies (Heliconiinae; Nymphalidae) and the poison frogs (Dendrobatidae: Anura) Both groups are found in similar rainforest habitats, are highly toxic, are at risk from similar predatory taxa, and have become renowned for their high diversity of species and bright colors (Merrill et al, 2015; Stynoski et al, 2015), including the presence of UV in their conspicuous signals (Briscoe et al, 2010; Yeager & Barnett, 2020). Despite many similarities in color diversity, chemical defense, the visual environment, and the predator community, ultraviolet reflective colors are relatively common in heliconiian butterflies but seemingly rare in poison frogs (Briscoe et al, 2010; Bybee et al, 2012; Yeager & Barnett, 2020). We compare the contribution of UV to signal contrast between these two dendrobatid frogs and five species of heliconiian butterflies and discuss the importance of predator versus conspecific vision in light of an extensive literature on heliconiian butterflies, to point to potential explanations for the evolution of UV reflectance in these frogs

| METHODS

| DISCUSSION