Bio-fluorescence—the absorption of light at one wavelength and its re-emission at a longer, lower-energy wavelength—is widespread in marine life but was long thought to be rare in terrestrial vertebrates. However, a recent survey using ultraviolet (UV) light has revealed that bio-fluorescence is common among many species of amphibians, including frogs and salamanders. In these species, the skin or bones glow bright green or orange when exposed to UV light. Researchers hypothesize that this serves as a communication signal that is visible to other amphibians but hidden from predators, many of whom cannot see the fluorescent wavelengths. Critics, however, suggest that the fluorescence may be a non-functional byproduct of the chemical composition of amphibian skin or a way to protect the animal from UV radiation damage. If the fluorescence is merely a secondary effect of skin chemistry, then it played no role in the evolutionary selection of these species' visual systems.