Variable utility in the enigmatic presence of ultraviolet reflectance in conspicuous aposematic signals

Abstract

Warning signals are often characterized by highly contrasting,\ndistinctive and memorable colors. Both chromatic (hue) and achromatic\n(brightness) contrast contribute to signal efficacy, making longwave\ncolored signals (red and yellow) that generate both chromatic and\nachromatic contrast common. Shortwave colors (blue and ultraviolet) do\nnot contribute to luminance perception, yet are also common in warning\nsignals. The presence of UV aposematic signals is paradoxical as UV\nperception is not universal, and evidence for its utility is at best\nmixed. We used visual modeling to quantify how UV affects signal\ncontrast in aposematic butterflies and frogs. We found that UV only\nappreciably affected visual contrast in the butterflies. As the\nbutterflies, but not the frogs, have UV-sensitive vision these results\nsupport the notion that UV reflectance is associated with intraspecific\ncommunication, but appears to be non-functional in frogs. Consequently,\nwe should be careful when assigning a selection-based benefit from UV\nreflectance.