Most animals that take on a natural fluorescent glow are found underwater, including a variety of fish and some sea turtles. On land, only parrots are known to fluoresce—that is, until a team of researchers took a closer look at a South American tree frog. María G. Lagorio and Julián Faivovich of the University of Buenos Aires and colleagues discovered that Hypsiboas punctatus produces a class of fluorescent pigments, which they named hyloins, that are derived from dihydroisoquinolone in the skin and lymph glands (Proc. Natl. Acad. Sci. USA 2017, DOI: 10.1073/pnas.1701053114). In full daylight, the frog appears yellow. But in low-light situations, the frog’s fluorescence makes it appear lime green. This twilight fluorescence enhances the brightness of individual frogs, the researchers note. They don’t yet know precisely how the animals use this fluorescence to their benefit, but they point to seven other amphibian species that have similar skin and physiology. They suggest that frog fluorescence may be a widespread phenomenon, despite a long-held belief among some biologists that fluorescence is mostly irrelevant in terrestrial environments.