Argentine scientists recently found the first amphibian that emits light: the dot-striped tree frog. They glow in the dark, and their brightness changes with the moon's clouds. The results were published in the February issue of the PNAS. The researchers speculate that the phenomenon of fluorescence in animals may be far more common than previously thought.
The polka-dot tree frog, which lives in the Amazon basin, is only 3 centimeters long. The frog is pale green all over and covered with dots of white, yellow and red. They are active at dawn, late afternoon and night.
Juliaán faivovich of the Natural Science Museum of Benardino Rivadavia, Argentina, stumbled upon the fluorescence of frogs while studying a pigment on the polka dot tree frog. Faivovich said that ultraviolet light was used in the experiment, and he was surprised to find that the whole frog began to glow.
Polka-striped tree frog under ultraviolet light (top) and polka-striped tree frog in daylight (bottom)
Faivovich and colleagues found fluorescent components in the frog's lymph and skin glands. It absorbs short-wavelength, high-energy light and converts long-wavelength, low-energy light to radiate out. Not only that, but this ingredient can also manipulate the brightness of fluorescence: frogs increase their brightness by 19% during a full moon and 30% more during a new moon.
Fluorescence spectrum at moonlight, crescent moon, and dusk
Fluorescent terrestrial and aquatic animals are not uncommon. Many species of fish and a small number of turtles, parrots and other animals are fluorescent animals. However, of the more than 7,600 amphibians, the dot-striped tree frog is the first fluorescent animal ever discovered.
Fluorescent animals are distinguished from luminous animals. Do you remember the schools of fish that illuminate the sea in "The Fantasy Drifting of the Young Pi"? The chemical composition of these fish reacts with the air to emit light autonomously, a process that requires energy consumption. Fluorescence, on the other hand, is simply the transformation of light at different wavelengths, and must occur in a light-filled environment. The human eye can only see fluorescence under ultraviolet lamps.
Little is currently known about the field of animal fluorescence. What role can the fluorescence of animals play? Researchers have previously made a lot of speculations, such as communication, light protection, antioxidants, or related to the natural color of animals.
Fluorescence of the back and abdomen of the polka-striped tree frog
In the case of the polka-striped tree frog, faivovich found that the polka-striped tree frog absorbed its own relatively insensitive short-wavelength light and emitted its own more sensitive long-wavelength light, suggesting that the frog's naked eye is likely to be able to detect fluorescence. Therefore, scientists deduce that the fluorescence of frogs may be related to communication between animals. David Blackburn of the University of Florida says the diversity of animal communication is interesting, visual and auditory, and fluorescence communication could be an exciting new field.
Fluorescence comparison of female dotted tree frog (left) and male polka-studded tree frog (right).
Both faivovich and Blackburn believe that little is known about the field of animal fluorescence in humans at present, and this phenomenon may be more common in amphibians than we think. Of the more than 5,000 species of frogs currently found, the dotted tree frog should not be the only frog that glows.
"Most of the features of frogs have commonalities, so fluorescence should be a more common phenomenon." The next question is, what kind of ecological environment will contribute to the production of fluorescence? Is this trait common in tree frogs? Can it appear in other ecological environments? Blackburn said.