The fluorescent tree frog
Tree-frogs are usually hard to spot because their green colour camouflages them against forest plants. Some have striking colour patches that they use in courtship displays but most of the frog is a shade of green. It turns out that not all green tree-frogs are really green.
The polka dot tree frog (Hypsiboas punctatus) of South America has a mixture of colours, but is mostly green to human eyes. However, under ultra-violet light it is fluorescent, and far from inconspicuous. This frog has three types of fluorescent molecule that have not been found in any other animal so far. The molecules absorb UV light and emit it at a visible wavelength. This would mean that in very low light conditions they would be visible to animals attuned to those wavelengths.
At present we know nothing about their visual sensitivity but it seems probable that polka dot frogs are using this to be locate one another. We are only just starting to appreciate how widespread fluorescence may be. Its function remains unclear though, is illustrated by the recent discovery of fluorescence in hawksbill turtles. Who knows why they do it, or is it just a by-product of another chemical process?
Taboada, C. et al. 2017 Proc. Natl Acad. Sci. USA http://dx.doi.org/10.1073/pnas.1701053114
Gruber & Sparks. 2015. Am. Mus. Novit. 3845, 1-8