Frog fluorescence

Biofluorescence, the re-emission of electromagnetic radiation at a wavelength longer than that at which an organism initially absorbed it, occurs widely in nature—in flora and fauna, on land and in the ocean. But among vertebrates, almost all known biofluorescent species—which include sharks, surgeonfish, and sea turtles—awure aquatic; parrots had been the sole terrestrial exception. A team of researchers from Buenos Aires, Argentina, and São Paulo, Brazil, now report the first observation of naturally occurring fluorescence in an amphibian: the polka-dot tree frog, Hypsiboas punctatus. Under white light the frog’s skin appears translucent (bottom photo), but when illuminated with 400 nm UV light, the frog gives off bright blue-green light (top). The team traced that glow to a class of fluorescent compounds located in the frog’s lymph and skin glands, with the emission filtered by pigments in the translucent skin. The polka-dot frog is active primarily between dusk and dawn, so the researchers analyzed the relative contributions of fluorescence and reflection to the frog’s visibility in low-light conditions. They estimate that at night under a full moon, fluorescence accounts for 18% of the light emerging from the frog; at twilight, the contribution approaches 30%. Moreover, the spectrum of the emitted fluorescence is well matched to the peak sensitivity of amphibian night vision, which suggests that the phenomenon plays an important role in the visual perception of fellow frogs in the chromatically complex terrestrial environment. (C. Taboada et al., Proc. Natl. Acad. Sci. USA 114, 3672, 2017, doi:10.1073/pnas.1701053114 .)